CBSE Class 10 Science HOTs Magnetic Effects of Electric Current

Question. H₁ and H₂ are heats produced by two copper wires have the same length and different diameters when they are connected in series and parallel respectively. From the above, we infer what of the following ?
(a) H₁ > H₂
(b) H₁ < H₂
(c) H₁ = H₂
(d) H₁ ≠ H₂

Answer: B

Question. The process shown in the diagram below is: 

(a) Electriomagnetism
(b) Electric generator
(c) Electric Motor
(d) Electric fuse

Answer: A

Question. Strength of the magnetic field at a point in the space surrounding the magnet is measured by:
(a) thickness of the magnet.
(b) number of lines crossing a given point.
(b) resistance of it.
(d) length of the magnet.

Answer: B

Question. A toaster of 4 kW is running in an existing circuit 110 volt that has a stream ranking of 4 A. Find the stream of electrons drawn by the toaster.
(a) 36.36 A
(b) 23.34 A
(c) 14.6 A
(d) 9.06 A

Answer: A

Question. The most important safety method used for protecting home appliance from short circuiting on overloading is:
(a) earthing
(b) use of fuse
(c) use of stablizers
(d) use of fuse electric meter

Answer: B

Question. The direction of force acting on a current carrying conductor placed in a magnetic field can be obtained by:
(a) Fleming’s left hand rule.
(b) Fleming’s right hand rule.
(c) Clock face rule.
(d) Ampere’s swimming rule.

Answer: A

Question. The north pole of a long bar magnet was pushed slowly into a short solenoid connected to a galvanometer. The magnet was held stationary for a few seconds with the North pole in the middle of the solenoid and then withdrawn rapidly. The maximum deflection of the galvanometer was observed when the magnet was:
(a) Moving towards the solenoid
(b) Moving into the solenoid
(c) At rest inside the solenoid
(d) Moving out of the solenoid

Answer: D

Question. Inside the magnet, the field lines move:
(a) from South to North
(b) from North to South
(c) away from North pole
(d) away from South poles

Answer: A

Question. A copper wire is held between the poles of a magnet: 

The current in the wire can be reversed. The pole of the magnet can also be changed over. In how many of the four directions shown can the force act on the wire?
(a) 1
(b) 2
(c) 3
(d) 4

Answer: B

Question. Commercial electric motors do not use:
(a) An, electromagnetic to rotate the armature
(b) Effectively large number of conducting wire in the current carrying coil
(c) A permanent magnet to rotate the armature
(d) A soft iron core on which the coil is wound

Answer: C

Question. An electron moves with a speed v along positive direction of the x-axis. If a magnetic field B acts along the positive y-direction, then the force on the electron will act along:
(a) x-axis
(b) y-axis
(c) – ve z-direction
(d) +ve y-direction

Answer: C

Question. The force exerted on a current carrying wire placed in a magnetic field is zero when the angle between wire and the direction of magnetic field is:
(a) 45°
(b) 60°
(c) 90°
(d) 180°

Answer: D

Question. In the figure shown below, the point A and B are respectively: 

(a) North pole, South pole
(b) South pole, North pole
(c) North pole, North pole
(d) South pole, South pole

Answer: B

Question. The phenomenon of electromagnetic induction is:
(a) the process of charging a body.
(b) the process of generating magnetic field due to a current passing through a coil.
(c) producing induced current in a coil due to relative motion between a magnet and the coil.
(d) the process of rotating a coil of an electric motor.

Answer: C

Question. A rectangular coil of copper wires is rotated in a magnetic field.
The direction of the induced current changes once in each:
(a) two revolutions
(b) one revolution
(c) half revolution
(d) one-fourth revolution

Answer: C

Question. If it takes 520 turns to make a solenoid that is 40 cm long with a radius of 1.2 m that carries a current of 6 A, then determine the magnetic field inside the solenoid.
(a) 7.2 × 10– 3 T
(b) 9.8 × 10– 3 T
(c) 8.4 × 10– 4 T
(d) 10.2 × 10– 4 T

Answer: B

Question. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is:
(a) towards south
(b) towards east
(c) downward
(d) upward

Answer: D

Question. The magnetic field lines inside a solenoid are in the form of:
(a) Curved line
(b) Circular lines
(c) Zig - zag lines
(d) Parallel straight lines

Answer: D

Question. The magnetic field of the current was discovered by:
(a) Maxwell
(b) Fleming
(c) Oersted
(d) Faraday

Answer: C

Question. By removing the inducing magnets, the induced magnetism is:
(a) Finished after sometime
(b) Finished just after
(c) Non-finished for a long time
(d) Not Charged

Answer: B

CASE STUDY BASED QUESTIONS

Answer the following questions 11 to 15 on the basis of your understanding of the following paragraph and the related studied concepts.
Paragraph I: the phenomenon of generation of an electric current in a circuit from magnetic effects, i.e., by changing the magnetic flux linked with it is called electromagnetic induction. This phenomenon is widely used to construct generation which produce large scale electric power for domestic and industrial use.

Question. The magnetic field reparented by Fig. 4.39 (a) is due to:
(a) A source of uniform magnetic field
(b) A source of non-uniform magnetic field
(c) A bar magnet
(d) A straight current-carrying conductor
Answer : D

Question. The magnetic field represented by Fig. 4.39 (b) is/due to:
(a) A circular coil
(b) A solenoid
(c) Uniform
(d) A straight conductor
Answer : A

Question. The magnetic field represented by Fig. 4.39 (c) is/due to:
(a) A bar magnet
(b) Non-uniform magnetic field
(c) A straight current-carrying conductor
(d) Uniform magnetic field
Answer : D

Question. The magnetic field lines Fig. 4.39 (d) represented the magnetic field due to:
(a) A straight current-carrying conductor
(b) A circular coil
(c) A solenoid
(d) A source of uniform magnetic field
Answer : C

Question. Two organs where magnetic field is produced are
(a) Heart and lungs
(b) Heart and brain
(c) Brain and lungs
(d) Heart and Liver
Answer : B

ASSERTION –REASON TYPE QUESTIONS

Each of these questions contain two statements, Assertion (A) and Reason (R). Each of these questions also has four alternative choices, only one of which is the correct answer. You have to select one of the codes (a), (b), (c) and (d) given below. Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(A) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(B) Assertion is correct, reason is incorrect (D)Assertion is incorrect, reason is correct.

Question. Assertion:Deflection in the compass needle placed at a given point near a current carrying copper wire increases on increasing the current through the copper wire.
Reason:Copper is a good conductor of electricity
Answer : B

Question. Assertion: In Fleming’s left hand rule, the direction of magnetic field, magnetic force and current are mutually perpendicular.
Reason: Fleming’s left hand rule is applied to measure the induced current.
Answer : C

Question. Assertion: A current carrying conductor experience a force in the magnetic field
Reason: The force acting on a current carrying conductor in a magnetic field is due to interaction between magnetic fields produced by the conductor and external magnetic field.
Answer : A

Question. Assertion:The magnetic field intensity at the centre of a circular coil carrying current changes, if the current through the coil is doubled.
Reason:The magnetic field intensity is dependent on current in the conductor.
Answer : A

Question. Assertion: Magnetic Field due to a current carrying coil at its centre becomes double if current in the coil is doubled.
Reason: Magnetic Field due to a current carrying coil at its centre is directly proportional to the current 
Answer : A

VSA

Question. What is meant by magnetic field?
Answer : Magnetic field: It is defined as the space surrounding the magnet in which magnetic force can be experienced.

Question. Draw magnetic field lines around a bar magnet. Name the device which is used to draw magnetic field lines.
Answer : 

Compass needle is used to draw magnetic field lines.

Question. Write one application of Flemings left hand rule.
Answer : Flemings left hand rule is used to find the direction of force on a current carrying conductor placed in a magnetic field acting perpendicular to the direction of current.

Question. What is the function of a galvanometer in a circuit?
Answer : Galvanometer is an instrument that can detect the presence of electric current in a circuit

Question. Why does a compass needle get deflected when brought near a bar magnet?
Answer : A compass needle behaves like a small bar magnet when it is brought near a bar magnet. Its magnetic field lines interact with that of bar magnet. Hence compass needle gets deflected.

Question. Explain why magnetic field lines are closed curves?
Answer : They are continuous closed curves because they diverge from the north pole of a bar magnet and converge to its south pole.

Question. Why don’t two magnetic lines of force intersect each other?
Answer :  No two magnetic field lines intersect each other because if they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.

Question. What type of core is used to make electromagnets?
Answer : Soft Iron

Question. What type of core is used to make electromagnets?
Answer : When the current in the conductor flows perpendicular (90°) to the direction of the magnetic field, maximum force is generated.

Question. A beam of alpha particles enters a chamber moving along the magnetic field. What is the magnetic force experienced by the beam?
Answer :  Zero, it is because beam is moving parallel to the magnetic field.

Question. What is the pattern of field lines inside a solenoid? What do they indicate?
Answer : The magnetic field is in the form of parallel lines. It indicates a uniform magnetic field because magnetic field lines are parallel.

Question. How is magnetic field produced in a solenoid used?
Answer : It is used to magnetise a soft iron bar to form an electromagnet.

Question. What does the direction of thumb indicate in the right-hand thumb rule?
Answer : The thumb indicates the direction of current in the straight conductor held by curved fingers of our hand.

Question. Suggest one way to distinguish a wire carrying current from a wire carrying no current.
Answer : The magnetic compass needle will get deflected near the wire current carrying but not near the wire with no current

Question.  Imagine that you are sitting in a chamber with your back to one wall. An electron beam,moving horizontally from the back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of the magnetic field?
Answer : The direction is vertically downwards

SHORT ANSWER

Question. The given magnet is diavided into three parts A, B and C as: A B C Name the part when the strength of magnetic field is:
(i)maximum, (ii) minimum. How will the density of magnetic field lines differ at these parts?
Answer :
(i) Maximum of magnetic field strength is at ‘A’ and ‘C’
(ii) Minimum of magnetic field strength is at ‘B’.
At ‘A’ and ‘C’ magnetic field lines are crowded whereas these are spread out at ‘B’.

Question. A compass needle is placed near a current-carrying wire. State your observation for the following cases, and give reason for the same in each case.
(a) Magnitude of electric current in the wire is increased. (b) The compass needle is displaced away from the wire.
Answer : (a) Observation: The compass needle is deflected more. Reason: Current carrying wire produces magnetic field, (B α I). (b) Observation: The deflection of magnetic needle decreases.
Reason: The strength of magnetic field decreases with increase in distance from the wire. (B∝ 1/d)

Question. The magnetic field associated with a current carrying straight conductor is in anticlockwise direction. If the conductor was held along the east-west direction, what will be the direction of current through it? Name and state the rule applied to determine the direction of current.
Answer : When the observer observes the direction of magnetic field from west then the direction of current is from east to west and if observer is at east side then the direction of current is from west to east. Right hand thumb rule: If we hold a current carrying conductor in our right hand in a such a way that stretched thumb is along the direction of the current, then curls of fingers around the conductor represents the direction of magnetic field lines.

Question. (a) In a pattern of magnetic field lines due to bar magnet, how can the regions of relative strength be identified?
(b) Compare the strength of magnetic field near the poles and the middle of a bar magnet.
Answer :
(a) The closeness of lines measures the relative strength of magnetic field.
(b) The strength of magnetic field is highest near the poles whereas minimum in the middle of bar magnet.

Question. Why and when does a current carrying conductor kept in magnetic field experiences force?
List the factors on which direction of force will depend.
Answer : The movement of electrons takes place in the conductor in a particular direction when current is passed through it. These charged particles are moving in the magnetic field which experiences force. The current carrying conductor has its own magnetic field, when it superimposes the magnetic field of magnet. Due to this, current carrying conductor experiences a force. Thus, conductor experiences a force when placed in a uniform magnetic field.
Factors on which direction of force depends: (i) The direction of force depends upon the direction of magnetic field.
(ii) It also depends upon the direction of current flowing through the conductor.
(iii) The strength of magnetic field is directly proportional to the strength of current.

Question.  State how the magnetic field produced by a straight current carrying conductor at a point depends on:
(a) current through the conductor.
(b) distance of point from conductor.
Answer :  Strength of magnetic field produced by a straight current-carrying wire at a given point is
(a) directly proportional to the current passing through it.
(b) inversely proportional to the distance of that point from the wire.

Question. Distinguish between a bar magnet and an electromagnet.
Answer : 

Question. Find the direction of magnetic field due to a current carrying circular coil held:
(i) vertically in North – South plane and an observer looking it from east sees the current to flow in anticlockwise direction,
(ii) vertically in East – West plane and an observer looking it from south sees the current to flow in anticlockwise direction,
(iii) horizontally and an observer looking at it from below sees current to flow in clockwise direction.
Answer : According to right hand rule, the direction of magnetic field is
(i) west to east
(ii) north to south
(iii) into the paper.

Question. What is solenoid? Draw the pattern of magnetic field lines of
(i) a current carrying solenoid and
(ii) a bar magnet.
List two distinguishing features between the two fields.
Answer :  (i) Solenoid: A coil of many circular turns of insulated copper wire wrapped in the shape of cylinder is called solenoid.

The pattern of magnetic field lines inside the solenoid indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.
(ii) Magnetic field lines around a bar magnet.

Following are the distinguishing features between the two fields.
(a) A bar magnet is a permanent magnet whereas solenoid is an electromagnet, therefore field produced by solenoid is temporary and stay till current flows through it.
(b) Magnetic field produced by solenoid is stronger than magnetic field of a bar magnet.
(a) When an unduly high electric current flows through the circuit, the fuse wire melts due to joule heating effect and breaks the circuit. Hence, it keeps an eye on the amount of current flowing and also stops the current if exceeds the maximum value. So, fuse acts like a watchman in an electric circuit. (b) (i) A fuse of rating 5A is usually used for lights and fans.
(ii) A fuse of rating 15 A is usually used for appliance of 2 kW or more power.

LONG ANSWERS

Question. What is a solenoid? Draw a diagram to show field lines of the magnetic field through and
around a current carrying solenoid. State the use of magnetic field produced inside a solenoid.
List two properties of magnetic lines of force.
Answer : Solenoid: A coil of many circular turns of insulated copper wire wrapped in the shape of cylinder is called solenoid. The pattern of magnetic field lines inside the solenoid indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid. Solenoid is used to form strong but temporary magnet called electromagnets. These electromagnets are used in wide variety of instruments and used to lift heavy iron objects. (a) Two magnetic field lines never intersect each other. (b) Magnetic field are closed curves.

Question. (a) A coil of insulated copper wire is connected to a galvanometer. With the help of a labelled diagram state what would be seen if a bar magnet with its south pole towards one face of this coil is
(i) moved quickly towards it,
(ii) moved quickly away from it,
(iii) placed near its one face?
(b) Name the phenomena involved in the above cases.
(c) State Fleming’s right-hand rule.
Answer : (a) If a coil of insulated wire is connected to a galvanometer and a bar magnet with south pole is moved towards one face of the coil then, given situation is shown in the figure.

(i) Moved quickly towards the coil: A current is induced in clockwise direction in the coil with respect to the side facing the north pole of the magnet and needle of galvanometer will deflect in one direction from zero position.

(ii) Moved quickly away from coil: A current is induced in anti-clockwise direction in the coil with respect to the side facing the north pole of the magnet and the needle of the galvanometer will deflect in opposite direction from (i).

(iii) Placed near its one face : No deflection of the needle of galvanometer is observed.
(b) The phenomena involved is called electromagnetic induction.
(c) Fleming’s right-hand rule: Stretch the right hand such that the first finger, the central finger and the thumb are mutually perpendicular to each other. If the first finger points along the direction of the field (magnetic field) and the thumb points along the direction of motion of the conductor, then the direction of induced current is given by the direction of the central finger.

Question. A current carrying conductor is placed in a magnetic field now answer the following.
(i) List the factors on which the magnitude of force experienced by conductor depends.
(ii) When is the magnitude of this force maximum?
(iii) State the rule which helps in finding the direction of motion of conductor. (iv) If initially this force was acting from right to left, how will the direction of force change if: (a) direction of magnetic field is reversed?
(b) direction of current is reversed?
Answer :
(i) When a current carrying wire is placed in a magnetic field, it experiences a magnetic force that depends on (a) current flowing in the conductor (b) strength of magnetic field (c) length of the conductor (d) angle between the element of length and the magnetic field. (ii) Force experienced by a current carrying conductor placed in a magnetic field is largest when the direction of current is perpendicular to the direction of magnetic field. (iii) The rule used in finding the direction of motion of the conductor placed in a magnetic field is Fleming’s left-hand rule.
Fleming’s left-hand rule is as follows: Stretch out the thumb, the forefinger, and the second (middle) finger of the left hand so that these are at right angles to each other. If the forefinger gives the direction of the magnetic field (N to S), the second (middle) finger the direction of current then the thumb gives the direction of the force acting on the conductor. (iv) (a) Direction of force will be reversed when direction of magnetic field is reversed, i.e., now force on conductor will act from left to right. (b) Direction of force will be reversed, if the direction of current is reversed, i.e., the force on the conductor will act from left to right.

Question. Describe an activity with labelled diagram to show that a force acts on current carrying conductor placed in a magnetic field and its direction of current through conductor. Name the rule which determines the direction of this force.
A small aluminium rod suspended horizontally from a stand using two connecting wires. Place a strong horseshoe magnet in such a way that the rod lies between the two poles with the magnetic field directed upwards. For this, put the north pole of the magnet vertically below and south pole vertically above the aluminium rod.

Answer : 

Connect the al uminium rod in series with a battery, a key and a rheostat. Pass a current through the aluminium rod from one end to other (B to A). The rod is displaced towards left. When the direction of current flowing through the rod is reversed, the displacement of rod will be towards right. Direction of force on a current carrying conductor is determined by Fleming’s left-hand rule.

Question. PQ is a current carrying conductor in the plane of the paper as shown in the figure below.

(i) Find the directions of the magnetic fields produced by it at points R and S?
(ii) Given r1 > r2, where will the strength of the magnetic field be larger? Give reasons
Answer :  (iii) If the polarity of the battery connected to the wire is reversed, how would the direction of the magnetic field be changed?
(iv) Explain the rule that is used to find the direction of the magnetic field for a straight current carrying conductor.
(i) The Magnetic field lines produced is into the plane of the paper at R and out of it at S.
(ii) Field at S > Field at P. Magnetic field strength for a straight current carrying conductor is inversely proportional to the distance from the wire.
(iii) The current will be going from top to bottom in the wire shown and the magnetic field lines are now in the clockwise direction on the plane which is perpendicular to the wire carrying current.
(iv) Right hand thumb rule. The thumb is aligned to the direction of the current and the direction in which the fingers are wrapped around in wire will give the direction of the magnetic field.

HOTS Questions and Answers

Q.1 A straight wire carrying electric current is moving out of plane of paper and is perpendicular to it. What is the direction and type of induced magnetic field?
A.1 Induced magnetic field will be in the form of concentric circles in the plane of paper.

Q.2 How can it be shown that magnetic field exist around a wire carrying current?
A.2 By using magnetic compass which, shows deflection.

Q.3 How can a solenoid be used to magnetise a steel bar.
A.3 By inserting the steel bar inside the solenoid and switching on electric current.

Q.4 Why can’t two magnetic field lines ever intersect?
A.4 If so then at the point of intersection there will be two different directions of magnetic field which is not possible.

Q.5 Can a 5 A fuse be used in wire carrying 15 A current? Why?
A.5 No, because both of them would then be ineffective in controlling the amount of current flowing.

Q.6 Give the factors that affect strength of magnetic field at a point due to a straight conductor carrying current.
A.6 Magnitude of electric current, perpendicular distance between that point and conductor.

Q.7 Where do we connect a fuse: with live wire or with neutral wire?
A.7 It is always connected with live wire.

Q.8 Give two uses of electromagnets.
A.8 (i) It is used in cranes for lifting heavy loads. 
(ii) used in electric bells.

Q.9 Name any two devices which use permanent magnets.
A.9 Loudspeakers, Galvanometer, voltmeter.

Q.11 A current-carrying straight conductor is placed in the east-west direction. What will be the direction of the force experienced by this conductor due to earth’s magnetic field? How will this force get affected on? (a) reversing the direction of floe of current (b) doubling the magnitude of current.
A.11 The direction of earth’s magnetic field is from G-south to G-north. Let current is from west to east. Therefore force is vertically upwards.
(a) By reversing the direction of current, the direction of will be reversed i.e. vertically downwards. 
(b) The magnitude of the force is doubled.

Q.12 An electron enters a magnetic field at right angles to it as shown in fig. The direction of the force acting on the electron will be: 
(a) to the right          (b) to the left            (c) out of the page           (d) into the page

A.12 When a conductor carrying current is placed perpendicular to the direction of magnetic field, the acting on it is given by Fleming’s left hand rule. Since the direction of current is the same as that of the motion of a positive charge, the d irection of force acting on it when moving perpendicular to the direction of magnetic field is the same as that acting on a current-carrying conductor placed perpendicular to the direction of magnetic field. Obviously, the force acting on an electron is opposite to that. Therefore in this case it is into the page.

Q.13 Why is the earth pin thicker and longer than the live and the neutral pins?
A.13 It is thicker so that it does not enter into the live or neutral sockets. It is made longer so that it gets connected to the earth terminal earlier than the live and neutral pins. This ensures the safety of the user.

Q.14 A coil of insulated copper wire is connected to a galvanometer. What would happen if a bar magnet is
(i) Pushed into the coil?
(ii) Withdrawn from inside the coil? 
(iii) Held stationary inside the coil?
A.14 (i) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards right).
(ii) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards left opposite to that in case one).
( ii i) As it is stationary no change in magnetic flux linked with coil, so galvanometer shows no deflection.

More Questions for Practice

Q.1 The magnetic field inside a long straight solenoid carrying current:
(a) is zero
(b) decreases as we move towards its end
(c) is same at all points.
(d) Increases as we move towards its end

Q.2 Which of the following properties of proton can change while it moves freely in a magnetic field?
(a) mass
(b) speed
(c) velocity
(d) momentum.

Q.3 How do we think the displacement of rod AB will be affected if
(i) current in a rod AB is increased
(ii) a stronger horse-shoe magnet is used
(iii) length of rod AB is increased ?(Figure 13.12 Page number 230).

Q.4 A positively-charged particle (alpha particle) projected towards west is deflected towards north by magnetic field. The direction of magnetic field is :
(a) towards south
(b) towards east
(c) downward
(d) upward

Q.5 What is the role of the split-ring in an electric motor?

Q.6 What will be the frequency of an A.C if its direction changes after every .01 s?

Q.7 An A.C has a frequency of 50 Hz. How many times does it change its direction in one second?

Q.8 A student performs an experiment to study the magnetic effect of current around a current carrying straight conductor. He reports that
(i) The direction of deflection of the north pole of a compass needle kept at a given point near the conductor remains unaffected even when the terminals of the battery sending current in the wire are inter changed.
(ii) for a given battery, the degree of deflection of a N-pole decreases when the compass is kept at a point farther away from the conductor.
Which of the above observations of the student is incorrect and why?

Q.9 Draw the pattern of magnetic field lines of a current carrying solenoid. What does the pattern of field lines inside the solenoid indicate? Write one
application of magnetic field of current carrying solenoid.

Q.10 Sketch magnetic field lines around a current carrying straight conductor.

Q.11 Why does a current carrying conductor kept in a magnetic field experience force?
On what factors does the direction of this force depend? Name and state the rule used for determination of direction of this force.

Q.5 Two circular coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

Q.6 Explain what is short-circuiting and overloading in an electric supply?

Q.7 What is the function of an earth wire? Why is it necessary to earth the metallic appliances?

Q.8 (a) What is an electromagnet? What does it consist of ?
(b) Name one material in each case which is used to make a (i) permanent magnet (ii) Temporary magnet.

Q.9 Draw a sketch of the patten of field lines due to a (i) current flowing in a circular coin (ii) current carrying solenoid.

Q.10 A circuit has a fuse of 5A. What is the maximum number of 100W, 220V bulbs that can be safely used in the circuit.

Q.1 A student draws three magnetic field lines 1,2 and 3 of a bar magnet with the help of a compass needle as shown in figure

(a)Is this configuration possible?

(b)If not what is wrong in figure and why?

Ans. (a)No

       (b) (i) Two field lines cannot intersect (ii) direction of field lines ‘3’ is wrong.

 

Q-2 Suppose you are sitting in a room facing one of the wall. An electron beam moving horizontally from your back goes towards the wall in front you and is deflected to your left, what is the direction of magnetic field in the room?

 

Ans. Vertically upward.

 

Q-3 A current through a horizontal power line flows in north to south direction.What is the direction of magnetic field (i)at a point directly below it and (ii)at a point directly above it?

 

Ans. (i) West to East (ii) East to West

 

Q-4 Electric appliances like electric –press, toaster, fans etc are connected to electric mains through three-pin plug. Why ?

 

Ans. Electric appliances are connected to three pin plug because heavy appliances require earth wire,so that in case of leakage of any current it goes to earth and user will not get shock.

 

(Three Marks Questions)

 

Q-1 Consider a circular wire lying in the plane of the table and the direction of current in it is anticlock wise.

(i) Draw the magnetic field lines produced around it.

(ii) Why does magnetic field at the center of current carrying circular loop appear straight? Explain with diagram.

Ans. (i)

(ii) Because of large curvature of magnetic field lines at centre.

 

Q-2If we place a compass needle near straight conductor carrying current

(a)What happens to the deflection of the compass needle if the direction of current is reversed .

(b)What change will you notice in the compass needle if it is moved away from conductor but the current through the conductor remains the same?

 

Ans. (a) Direction of deflection will reverse

(b) Deflection will decrease

 

Q-3 A magnet is moving towards a coil as shown in figure

(1)Which phenomenon is shown in figure.

(2) Which physical quantity is set up in the coil when there is a relative motion between magnet and coil?

(3) What may be the cause of production of that physical quantity?

 

Ans. (1) Electromagnetic induction

(2)Induced current

(3)Change in magnetic lines of forces through coil

 

Q-4 Suppose your science teacher asks you to demonstrate the phenomena Of EMI with following materials:

(a)Two different coils 1and 2 of copper wire having large no. of turns 50 and 100 respectively.

(b)A non conducting cylinder.

(c)A battery

(d) A plug key

(e) A galvanometer

(i) Draw a labeled diagram of your demonstration setup.

(ii)How will you prove the phenomena of EMI.

Ans.(ii)When key is closed, there is deflection in galvanometer

MAGNETIC EFFECTS OF ELECTRIC CURRENT

KEY CONCEPTS & GIST OF THE LESSON

◊ Magnet: (i) is an object that attracts objects made of iron, cobalt & nickel.

                 (ii) Comes to rest in North-South direction, when suspended freely.

◊  Magnets are used: (i) In radio & stereo speakers,

                                  (ii) In refrigerator doors,

                                  (iii) in audio & video cassettes players,

                                  (iv) On hard discs & floppies of computers &

                                  (v) in children‘s toys.

◊  Magnetic field: The area around a magnet where a magnetic force is experienced is called a magnetic field. It is a quantity that has both direction & magnitude.

◊  Magnetic field lines: Magnetic field is represented by field lines. They are lines drawn in a Magnetic field along which a North magnetic pole moves. Magnetic field lines are called as Magnetic lines of force.

Refer to figure 13.3 & 13.4 page no. 225 of N.C.E.R.T Text book)

◊ Properties of Magnetic field lines:

(i) They do not intersect each other.

(ii) It is taken by convention that magnetic field lines emerge from North pole and merge at the South pole. Inside the magnet, their direction is from South pole to North pole. Therefore magnetic field lines are closed curves.  Magnetic field lines due to a current through a straight conductor (wire)- consist of series of concentric circles whose direction is given by the Right hand thumb rule.

◊  Right hand thumb rule: If a current carrying straight conductor is held in your right hand such that the thumb points towards the direction of current, then the wrapped fingers show the direction of magnetic field lines.

(Refer to figure 13.7, page no. 228 of N.C.E.R.T Text book)  Magnetic field lines due to a current through a circular loop

(Refer to figure 13.8, page no. 228 of N.C.E.R.T Text book)  The strength of the magnetic field at he centre of the loop(coil)depends on:

(i) The radius of the coil- The strength of the magnetic field is  inversely proportional to the radius of the coil. If the radius increases, the magnetic strength at the centre decreases.

(ii) The number of turns in the coil: As the number of turns in the coil increase, the magnetic strength at the centre increases, because the current in each circular turn is having the same direction, thus the field due to each turn adds up.

(iii) The strength of the current flowing in the coil: as the strength of the current increases, the strength of thee magnetic fields also increases.

◊  Solenoid: (Refer to figure 13.10, page no. 229 of N.C.E.R.T Text book)  (i) A coil of many turns of insulated copper wire wrapped in the shape of a cylinder is called a Solenoid.

(ii) Magnetic field produced by a Solenoid is similar to a bar magnet.

(iii) The strength of magnetic field is proportional to the number of turns & magnitude of current.

◊  Electromagnet: An electromagnet consists of a long coil of insulated copper wire wrapped on a soft iron core.

(Refer to figure 13.11, page no. 229 of N.C.E.R.T Text book)

◊  Fleming’s Left hand rule: Stretch the thumb, forefinger and middle finger of left hand such that they are mutually perpendicular. Forefinger points in the direction of magnetic field and centre finger in the direction of current, then the thumb gives the direction of force acting on the conductor.

(Refer to figure13.13, page no. 231 13.13 of N.C.E.R.T Text book)

◊  Electric motor: A device that converts electric energy to mechanical energy. (Refer to figure 13.15, page no. 232 of N.C.E.R.T Text book)

◊  Principle of Electric motor: When a rectangular coil is placed in a magnetic field and a current is passed through it, force acts on the coil, which rotates it continuously. With the rotation of the coil, the shaft attached to it also rotates.

◊  Electromagnetic induction: Electricity production as a result of magnetism (induced current) is called Electromagnetic induction.

◊  Fleming’s Right hand rule: gives the direction of induced current. Stretch the thumb, forefinger and middle finger of right hand such that they are mutually perpendicular. Forefinger points in the direction of magnetic field and centre

finger in the direction of induced current, then the thumb gives the direction of motion of the conductor.

◊  Electric generator: A devise that converts mechanical energy to electric energy. (Refer to figure 13.19, page no. 236 of N.C.E.R.T Text book)

lectric generator is of two types- (i) A.C generator (ii) D. C generator

◊  Principle of Electric generator: Electromagnetic induction  Domestic electric circuits: (Refer to figure 13.20, page 238 of N.C.E.R.T Text book)

◊  We receive electric supply through mains supported through the poles or cables. In our houses we receive AC electric power of 220V with a frequency of 50Hz.

The 3 wires are as follows-

(i) Live wire- (Red insulated, Positive)

(ii) Neutral wire- (Black insulated, Negative)

(iii) Earth wire- (Green insulated) for safety measure to ensure that any leakage of current to a metallic body does not give any serious shock to a user.

◊  Short circuit: is caused by touching of live wires and neutral wire  Fuse: is a protective device used for protecting the circuits from short circuiting and over loading

◊  Important diagrams-

1. Magnetic field lines around a bar magnet.

2. Right hand thumb rule

3. Magnetic field lines through and around a current carrying solenoid.

4. An electromagnet.

5. A simple electric motor

6. Electric generator

◊  Important activities-

1. Magnetic field lines around a bar magnet

2. Direction of electric current in a simple electric circuit.

3. Direction of Magnetic field lines depends on the direction of electric current.

HOTS QUESTIONS (SOLVED)

1. On what effect of an electric current does an electromagnet work?

Ans. Magnetic effect of electric current

2. What is the frequency of AC (Alternating Current) in India?

3. 50 Hz

4. On what effect of an electric current does a fuse work?

Ans. Heating effect of electric current

5. An Alternating Current has a frequency of 50 Hz. How many times it changes direction in one second?

Ans. Since Alternating Current changes its direction twicw in a cycle, it will change its direction 100 times (50x2) in one second as its frequency is 50Hz.

6. What is short circuiting in an electric circuit?

Ans. Short circuiting in an electric circuit is a situation in which the live wire touches the neutral wire.

7. What kind of quantity is magnetic field?

Ans. It is a vector quantity as it has both direction & magnitude.

8. How is Solenoid different from a circular coil?

Ans. Solenoid is different from a circular coil in the sense that the length of the solenoid is much greater than its diameter

9. On what factor the strength of magnetic field of a solenoid depends?

Ans The strength of magnetic field of a solenoid is proportional to the number of turns of the coil and the magnitude of the current.

10. What kind of motor is used in: (i) A fan (ii) A battery operated toy

Ans. (i) A fan has AC motor (ii) A battery operated toy has DC motor.

Important Questions for NCERT Class 10 Science Magnetic Effects of Electric Current

Question 1: Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire
(b) The field consists of straight lines parallel to the wire
(c) The field consists of radial lines originating from the wire
(d) The field consists of concentric circles centred on the wire 
Answer : D

Question 2: The phenomenon of electromagnetic induction is:
(a) the process of charging a body
(b) the process of generating magnetic field due to a current passing through a coil
(c) producing induced current in a coil due to relative motion between a magnet and the coil 
(d) the process of rotating a coil of an electric motor.
Answer : C

Question 3: The device used for producing electric current is called a:
(a) generator
(b) galvanometer
(c) ammeter
(d) motor.
Answer : A

Question 4: The essential difference between an AC generator and a DC generator is that:
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.
Answer : D

Question 5: At the time of short circuit, the current in the circuit:
(a) reduces substantially
(b) does not change
(c) increases heavily 
(d) vary continuously.
Answer : C

State whether the following statements are true or false.

(a) An electric motor converts mechanical energy into electrical energy
Answer : False

(b) An electric generator works on the principle of electromagnetic induction
Answer : True

(c) The field at the centre of a long circular coil carrying current will be parallel straight lines
Answer : True

(d) A wire with a green insulation is usually the live wire of an electric supply
Answer : False

Very Short Answer Type Questions :

Question. State Fleming's Left Hand Rule. 
Answer: According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular to each other. If the first finger points in the
direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

Question. What is an electric motor?
Answer: An electric motor is a device which converts the electrical energy into mechanical energy.

Question. What do you mean by ‘magnetic field’ of a magnet?
Answer: The space or region around a magnet in which the force of attraction or repulsion due to the magnet can be detected is called the magnetic field.

Question. Why steel is not used for making electromagnets ?
Answer: The steel does not lose all its magnetism when the current is stopped and becomes a permanent magnet. That’s why it is not used for making electromagnets.

Question. What is the principle of an electric motor?
Answer: Electric motor works on the principle that ‘when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously. Thus, when the coil rotates, the shaft attached to it also rotates converting the electrical energy supplied to the motor to the mechanical energy of rotation.

Question. What is a galvanometer?
Answer: A galvanometer is an instrument which can detect the presence of electric current in a circuit.

Question. What is the function of a galvanometer in a circuit ?
Answer: Galvanometer is a device that detects the presence of current in a circuit. It is also used for measuring the amount of current in the circuit.

Question. Name the two factors that completely define a magnetic field at a point.
Answer: The strength and the direction of magnetic field at the given point.

Question. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: The metallic body of electric appliances is connected to the earth by means of earth wire so that any leakage of electric current is transferred to the ground. This prevents any electric shock to the user. That is why earthing of the electrical appliances is necessary

Question. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Answer: When the direction of current is perpendicular to the direction of the magnetic field.

Question.: Name some devices in which electric motors are used?
Answer: (a) Water pumps (b) Electric fans (c) Electric mixers (d) Washing machines etc.

Question. Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Answer: Yes, when the current in coil A is changed, the magnetic field associated with it also changes. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it.

Question. When does an electric short circuit occur?
Answer: When the insulation of live and neutral wires undergoes wear and tear and then touches each other, the current flowing in the circuit increases abruptly. Hence, a short circuit occurs.

Question. List three sources of magnetic fields.
Answer: Three sources of magnetic fields are as follows: (a) Current-carrying conductors (b) Permanent magnets (c) Electromagnets.

Question. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer: By Fleming’s left hand rule magnetic field inside the chamber will be downward.

Question. What is a permanent magnet? Give one use of it.
Answer: A permanent magnet is a magnet made from steel such that once magnetized, it does not lose it magnetism easily.

Question. What is the role of the split ring in an electric motor?
Answer: The split ring in the electric motor acts as a commutator. The commutator reverses the direction of
current flowing through the coil after each half rotation of the coil. Due to this reversal of the current, the coil continues to rotate in the same direction.

Question. Name two safety measures commonly used in electric circuits and appliances.
Answer: Electric fuse and Earthing.

Question. Explain different ways to induce current in a coil.
Answer: (a) If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an electric current isinduced in the coil.
(b) If a magnet is moved relative to a coil, then an electric current is induced in the coil.

Question. Which sources produce alternating current?
Answer: AC generators, power plants, etc., produce alternating current.

Question. State the principle of an electric generator.
Answer: An electric generator works on the principle of electromagnetic induction. It states that whenever there is a change in magnetic flux of a coil, an emf or current is induced in the coil.

Question. Name some sources of direct current.
Answer: Some sources of direct current are cell, DC generator, etc.

Question. Define a compass.
Answer: A compass is a device used to show magnetic field direction at a point. It consists of a tiny pivoted magnet usually in the form of a pointer which can turn freely in the horizontal plane.

Short Answer Type Questions :

Question. Where do we connect a fuse; with alive wire or with a neutral wire. ? what happens if the fuse wire is connected to the neutral wire?
Answer: Fuse is always connected to a live wire. If fuse wire is connected to neutral wire instead of live wire then even when the fuse burns out the appliance remains connected to the live wire and the current supplied will not be disrupted due to overloading.

Question. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand thumb rule to find out the direction of the magnetic field inside and outside the loop.
Answer: Inside the loop = Pierce inside the table,
Outside the loop = Appear to emerge out from the table.

Question. An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Answer: Power = 2 kW = 2000 W
Potential difference = V = 220 V
Current drawn by oven = I = P/V 2000/220 = 9.09 A
As oven draws current 9.09 A but safety limit (rating) of current in the circuit is 5 A, So fuse will melt and circuit will break.

Question. In Activity 13.7, how do we think the displacement of rod AB will be affected if
(i) current in rod AB is increased;
(ii) a stronger horse-shoe magnet is used; and
(iii) length of the rod AB is increased?
Answer: Displacement of rod AB will increase in all the three cases, because force experienced by a conductor placed in magnetic field is directly proportional to current, Length of conductor and strength of Magnetic field.
{ F = ILB }

Question. Draw magnetic field lines around a bar magnet.Answer:

Question. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer:

Question. What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer: (a) Too many appliances should not be connected to a single socket.
(b) Too many appliances should not be used at the same time.
(c) Faulty appliances should not be connected in the circuit.
(d) Fuse should be connected in the circuit.

Question. In a three pin plug, the earth pin is thicker and longer than the live and neutral pins. Why?
Answer: The earth pin is thicker so that it doesnot enter into the live or neutral sockets. It is longer so that the earth connection is made first. This ensures the safety of the users.

Question. A coil of insulated copper wire is connected to a galvanometer. What would happen if a bar magnet is
(i) Pushed into the coil?
(ii) Withdrawn from inside the coil?
Answer: (i) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards right).
(ii) Due to change in magnetic flux linked with coil, the galvanometer shows deflection (say towards left opposite to that in case one).
(iii) As it is stationary no change in magnetic flux linked with coil, so galvanometer shows no deflection

Question. What are the factors on which magnetic field produced by a current carrying straight conductor depend? State the rule which gives the direction of magnetic field.
Answer: The factors are:
i) The strength of current
ii) Distance of point from the conductor.
Right hand thumb rule: When a current carrying straight conductor is holded in the right hand, the thumb points towards the direction of current and the fingers wrap around the conductor in the direction of the field lines of the magnetic field.

Question. A straight wire carrying electric current is moving out of plane of paper and is perpendicular to it. What is the direction and type of induced magnetic field?
Answer: Induced magnetic field will be in the form of concentric circles in the plane of paper.

Question. i) How can it be shown that magnetic field exist around a wire carrying current?
ii) How can a solenoid be used to magnetise a steel bar.
iii) Why can’t two magnetic field lines ever intersect?
Answer: i) By using magnetic compass which, shows deflection.
ii) By inserting the steel bar inside the solenoid and switching on electric current.
iii) If so then at the point of intersection there will be two different directions of magnetic field which is not possible.

Question. A current-carrying straight conductor is placed in the east-west direction. What will be the direction of the force experienced by this conductor due to earth’s magnetic field? How will this force get affected on?
(a) reversing the direction of flow of current
(b) doubling the magnitude of current.
Answer: The direction of earth’s magnetic field is fron G- south to G-north. Let current is from west to east, force is vertically upwards.
(a) By reversing the direction of the current, the direction of the force will be reversed i.e. vertically downwards.
(b) The magnitude of the force is doubled.

Question. An electron enters a magnetic field at right angles to it . The direction of the force acting on the electron will be:
(a) to the right (b) to the left (c) out of the page (d) into the page
Answer: When a conductor carrying current is placed perpendicular to the direction of magnetic field, the acting on it is given by Fleming’s left  and rule. Since the direction of current is the same as that of the motion of a positive charge, the direction of force acting on it when moving perpendicular to the direction of magnetic field is the s.

Question. magnetic field produced by a current carrying straight conductor depend? State the rule which gives the direction of magnetic field.
Answer: The factors are:
i) The strength of current
ii) Distance of point from the conductor. Right hand thumb rule: When a current carrying straight conductor is holded in the right hand, the thumb points towards the direction of current and the fingers wrap around the conductor in the direction of the field lines of the magnetic field.

Question. Answer the following question:
(a) A straight wire conductor passes vertically through a piece of cardboard sprinkled with iron filings. Copy the diagram and show the setting of iron filings when a current is passed through the wire in the upward direction and the cardboard is tapped gently. Draw arrows to represent the direction of the magnetic field lines.
(b) Name the law which helped you to find the direction of the magnetic field lines. 

Answer: (a)
(b) Right hand thumb rule.

Question. State five differences between an electromagnet and a permanent magnet.
Answer: 

Question. State the rule to determine the direction of a :
(a) Magnetic field produced around a straight current carrying conductor.
(b) Force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it.
(c) Current induced in a coil due to its rotation in a magnetic field.
Answer: (a) Right hand thumb rule or Maxwell’s Corkscrew rule.
(b) Fleming’s left hand rule.
(c) Fleming’s right hand rule.

Question. Answer the following questions:
(a) What do you mean by Overloading?
(b) Define an electromagnet.
(c) What is a galvanometer?
Answer: (a) Overloading is the process of overheating of a wire due to excess current drawn by all the appliances than the permitted limit for that wire.
(b) An electromagnet is a magnet consisting of a long coil of insulated copper wire wrapped around a soft iron core that is magnetized only when electric current is passed through the
coil.
(c) A galvanometer is an instrument which can detect the presence of electric current in a circuit.

Question. A metallic wire loop is suspended freely and a bar magnet is brought near it as shown in the diagram. What will be the direction of induced current in the wire loop when the magnet is moved towards it? 

Answer: Anticlockwise from the side of a magnet. As when magnet is brought near it the magnetic flux increases so the induced current will flow in the direction so as to oppose the current. So, current will be anticlockwise.

Long Answer Type Questions :

Question. A coil of insulated copper wire is connected to a galvanometer.
What will happen if a bar magnet is :
(a) pushed into the coil ?
(b) held stationary inside the coil ?
(c) withdrawn from the coil ?
Answer: (a)When a bar magnet is pushed into the coil, a momentary deflection is observed in the galvanometer.
(b) When the bar magnet is held stationary inside the coil, there is no deflection in the galvanometer.
(c) When the bar magnet is withdrawn from the coil, the deflection in the galvanometer is in the opposite direction.

Question. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Answer: An electric motor converts electrical energy into mechanical energy. It works on the principle of the magnetic effect of current. A current-carrying coil rotates in a magnetic field. Direction of rotation can be determined by using Fleming’s left hand rule. The following figure shows a simple electric motor.

Working- When current is made to flow through the coil ABCD by closing the switch, the coil starts to rotate in the anticlockwise direction. This is due to the downward force acting on the length AB and simultaneously upward force acts along the length CD. As a result of which the coil rotates in the anticlockwise direction.
Function of split ring- The split ring in the electric motor acts as a commutator. The commutator reverses the direction of current flowing through the coil after each half rotation of the coil. Due to this reversal of the current, the coil continues to rotate in the same direction.

Question. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.
Answer: A solenoid is a long coil of circular loops of insulated copper wire. Magnetic field lines are produced around the solenoid when a current is allowed to flow through it.
The magnetic field produced by it is similar to the magnetic field of a bar magnet. End of solenoid having current in clockwise direction behaves as South pole, so it will attract North pole of a bar magnet. On the other hand the end of solenoid having current in anticlockwise direction behaves as North pole, so it will repel the North pole of a bar magnet.

Question. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Answer: An electric generator converts mechanical energy into electrical energy. An electric generator works on the principle of electromagnetic induction. It states that whenever there is a change in magnetic flux of a coil, an emf or current is induced in the coil. The following figure shows a simple AC generator. 

When the axle is rotated clockwise, AB moves upwards while CD moves downward. Therefore, according to Fleming’s right hand rule, the direction of the induced current will be from A to B along the length AB.
Similarly, the direction of the induced current will be from C to D along the length CD. Hence, galvanometer shows a deflection in a particular direction. After half a rotation, length AB starts moving downwards while the length CD starts moving upwards. Now, the direction of the induced current reverses to DCBA. Since the direction of the induced current reverses every half rotation, the current induced is known as alternating current.
Function of Brushes- Brushes help in transferring current from coil to the external circuit.

Question. A student wound an insulated copper wire around a soft iron rod.
He then connected one end to the rheostat and the other free end to the battery via a key. He closed the key and observes the deflection in the magnetic needle placed nearby. Now he altered the current using by reversing the connections of the battery and again noted the change in the deflection of the needle.
(a) Why do the student perform this activity ?
(b) What did the student observe ?
(c) Comment on the statement “a material in the middle of a current carrying coil gets magnetised”.
Answer: (a) The student conducted this activity to make an electromagnet.
(b) The electrical current flowing through a coil will create a uniform magnetic field. This magnetic field causes the needle to turn. Reversing, the connections to the battery, reverses
the direction of the current flow and the needle will point in the opposite direction.
(c) When an iron rod is placed along the axis of a current carrying coil, it gets magnetised under the influence of the magnetic field produced by the coil through induction. But this
magnetism lasts as long as the current supply is not withdrawn.

Question. Two coils A and B are placed close to each other. If the current in coil A is changed, will some current be induced in the coil B ? 

Given reason.
Answer: When we switch on current in coil A, it becomes an electromagnet and produces a magnetic field around coil B. So, an induced current flows in coil B for a moment. When the
current in coil A becomes steady, its magnetic field also becomes steady and the current in coil B stops. When we switch off the current in coil A, then the magnetic field in coil B
stops quickly and in this case an induced current flows in coil B in the opposite direction.

Question. The diagram shows a current carrying coil passing through a cardboard sheet. Draw three magnetic lines of force on the board. State two factors on which magnitude of magnetic field at the centre depends. 

Answer: Figure shows the magnetic lines of force due to current carrying coil. 

The magnitude of magnetic field at the centre of coil depends on :
(a) the strength of current in the coil, and (b) the number of turns in the coil.

Question. Answer the following question:
(a) A coil of insulated wire is connected to a galvanometer.
What would be seen if a bar magnet with its south pole towards one face of the coil is :
(i) moved quickly towards it
(ii) moved quickly away from it
(iii) placed near its one face ?
These activities are then repeated with north pole of the magnet. What will be the observations ?
(b) Name and define the phenomenon involved in above activities.
(c) Name the rule which can determine the direction of current in each case.
Answer: (a) A coil of insulated wire is connected to a galvanometer and if a bar magnet with its south pole towards one face of the coil is
(i) Moved quickly towards it, the galvanometer is deflected towards the left.
(ii) Moved quickly away from it, the galvanometer is deflected towards the right.
(iii) If the magnet is held stationary inside the coil, the deflection of the galvanometer is zero as no change in flux. 

If this activity is repeated with north pole of the magnet :
(i) If the magnet is pushed into the coil, the galvanometer is deflected towards the right.
(ii) If the magnet is withdrawn from the coil, the galvanometer is deflected towards the left.
(iii) If the magnet is held stationary inside the coil, the deflection of the galvanometer is zero.
(b) The phenomenon involved in this activity is electromagnetic induction. The production of electric current by moving a magnet inside a fixed coil of wire is called electromagnetic induction.
(c) The direction of induced current is determined by ‘Fleming’s right hand rule’.

Question. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clock-wise. Apply the right hand rule to find out the direction of the magnetic field inside and outside the loop.
Answer: Since, the current is flowing clockwise through a circular loop. The direction of magnetic field around the conductor can be found by using the right hand thumb rule. As the figure shows, the magnetic field would be towards the plane of the paper when it is inside the loop. On the other hand, the magnetic field would be away from the paper when it is outside the loop. 

Question. When an iron bar is placed inside a solenoid carrying current, it becomes a magnet as long as current flows through the solenoid.
Such a magnet is known as electromagnet. In fact, the magnetic field inside the solenoid magnetises the soft iron bar placed in it, which acts as an electromagnet.
(a) What type of core is used to make an electromagnet?
(b) State two ways by which the strength of an electromagnet can be increased.
(c) State one use of electromagnet.
(d) Basically electromagnet is a :
(i) Magnet
(ii) Solenoid
(iii) Wire
(iv) Coil 136
Answer: (a) Soft iron core.
(b) 1. By increasing the number of turns in the winding on the soft iron core.
2. By increasing the strength of the current through the winding.
(c) Electromagnet are used to lift heavy iron pieces.
(d) (ii) Solenoid

Question. Draw the pattern of magnetic field lines produced around a current carrying straight conductor passing perpendicularly through a horizontal cardboard. State and apply right-hand thumb rule to mark the direction of the field lines. How will the strength of the magnetic field change when the point where magnetic field
is to be determined is moved away from the straight conductor? Give reason to justify your answer.
Answer: Maxwell’s Right Hand Thumb rule states that if current carrying wire is imagined to be held in the right hand so that thumb points in the direction of current, then the direction in which fingers encircle the wire will give the direction of magnetic field lines around the wire. If we hold the current carrying straight wire so that thumbs in upward direction points the direction of current, the direction of magnetic field lines will be anticlockwise. The strength of magnetic field is inversely proportional to the distance of the point of observation from the wire. So, as we move away from the wire the strength of magnet decreases. 

Reasoning Based Questions :

Question. Why does a magnetic needle show a deflection when brought close to a current carrying conductor?
Answer: A current carrying conductor produces a magnetic field around it and the magnetic needle in this magnetic field experiences a torque due to which it deflects to align itself in the direction of magnetic field.

Question. Why steel is not used for making electromagnets?
Answer: The steel does not lose all its magnetism when the current is stopped and becomes a permanent magnet. That’s why it is not used for making electromagnets

Question. Explain why, an electromagnet is called a temporary magnet?
Answer: An electromagnet is called a temporary magnet because as we keep on passing electric current it will work as magnet, if we stop passing electric current, it will no longer work as magnet.

Question. Why does a compass needle get deflected when brought near a bar magnet?
Answer: When a compass needle is brought near a bar magnet, the compass needle experiences a deflection due to the interaction of magnetic fields of the compass needle and the bar magnet.

Question. Why don’t two magnetic lines of force intersect each other?
Answer: The tangent drawn at any point on the magnetic field line gives the direction of magnetic field at that point. Hence, if two magnetic field lines would intersect each other, it would result in two tangents at one point which in turn would result in two directions at one point which is impossible. That is why two magnetic field lines never intersect each other.

Question. Why is soft iron generally used as the core of the electromagnet?
Answer: Soft iron is generally used for making electromagnets because it can easily gain magnetic properties when current is passed around the core and quickly loses when current is stopped.

Creating Based Questions :

Question. For experimenting purpose, Ram made two electromagnets by wrapping a few turns of wire on one nail and doubled the number of turns of wire for the other nail and let the same amount of electric current passed through it. From his inference, which one tends to have larger magnetic strength ?
Answer: The number of turns of the wire wrapped over the two iron nails is in the ratio of 2 : 1. Electromagnetic strength has a direct relationship with the number of turns wrapped over it. The strength of electromagnet increases with increase in a number of turns of the wire wrapped over the nail and the current passing through them. Hence, the one with more number of turns tends to have more magnetic strength.

Question. What would be the inference made by Prashant about the magnetic strength when current passed through a circular coil produces a magnetic field ?
Answer: Magnetic field lines form in concentric circles around a cylindrical current-carrying conductor, such as a length of wire.
The strength of the magnetic field at the centre of a circular coil carrying current is inversely proportional to the radius of the circular coil i.e., the field strength reduces as the radius of the coil increases.

Question. Selena measures the magnetic field produced by an infinitely long wire, a rectangular loop, a solenoid of finite length, a circular loop where all the four carries the same amount of current. After her experiment, she tends to notice that the magnetic field produced by certain cases is similar to the magnetic field produced by the bar magnet. Find out the cases in which both the magnetic fields are equal ?
Answer: Solenoid is the only thing which is tightly-packed and wound in terms of close loops. If current is passed inside a solenoid which is of finite length, the closely packed loops inside it
produce a magnetic field which resembles the magnetic field of a bar magnet. Other than this, the circular or rectangular loop doesn’t produce much magnetic field as that of a bar magnet.

Question. Using the following informations form a pathway to determine the direction of the motor in an electric motor. And also include informations that are not mentioned below to complete it. Motion of the conductor, Direction of current, Three fingers, Magnetic field, Index finger, motion of the conductor.
Answer: Three fingers → In left-hand → Index finger → Middle finger → Thumb → At right angle → Middle finger represents the direction of the current → Index finger represents the direction of the magnetic field → Thumb represents the direction of the motion of the conductor → Used to define the direction of the motion of the conductor in electric motor → Also known as motor rule.

Question. Blair wants to measure magnetic field. Suggest her a better instrument which would measure magnetic field approximately.
Answer: Blair can use Flux meter to measure the magnetic field since it can be used to predict the flux amount produced in the permanent magnet due to its low controlling torque and its heavy electromagnetic damping. It is better than a ballistic galvanometer since it has high torque and its accuracy is less.

Question. Using the following informations form an instruction to draw magnetic lines. And also include informations that are not mentioned below to complete it.
Magnetic compass, repel, board, Bar magnet, Needle, attract, Merge, Emerge.
Answer: Place a board → Place a bar magnet in the middle → Mark the boundary of the bar magnet → Place the magnetic compass near the North Pole of the bar magnet → North side of the needle points away from the north side of the magnet → Same poles repel each other → different poles attract each other → Now place the pin in the direction the needle points → Move the compass to new position where south pole points the previous position of the north pole → Repeat the procedure → Magnetic lines emerge at north pole → Magnetic lines merge at south pole → This forms concentric magnetic lines around bar magnet.