CBSE Class 10 Science Magnetic Effects of Electric Current VBQs

Question. For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the incorrect statement is :
(a) The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid.
(b) The strong magnetic field produced inside the solenoid can be used to magnetize a piece of magnetic material like soft iron, when placed inside the coil.
(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet.
(d) The N and S-poles exchange position when the direction of current through the solenoid is reversed.
Answer : B

Question. Which of the following properties of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
(a) Mass
(b) Speed
(c) Velocity
(d) Momentum
Answer : C

Question. Choose the incorrect statement from the following regarding magnetic l ines of field
(a) The direction of magnetic field at a point is taken to be the directionin which the north pole of a magnetic compass needle points.
(b) Magnetic field lines are closed curves.
(c) If magnetic field lines are parallel and equidistant, they represent zero field strength.
(d) Relative strength of magnetic field is shown by the degree of closeness of the field lines.
Answer : C

Question. The strength of magnetic field inside a long current carrying straight solenoid is :
(a) More at the ends than at the centre
(b) Minimum in the middle
(c) Same at all points
(d) Found to increase from one end to the other
Answer : C

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. Which of the following correctly describes the magnetic field near a long straight current carrying 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 centered on the wire.
Answer : D

Question. What is SI unit of magnetic field strength:
(a) Pascal
(b) Nm2
(c) Tesla
(d) Nounit
Answer : C

Question. A plotting compass is placed near the south pole of a bar magnet. The pointer of plotting compass will:
(a) point away from the south pole
(b) point parallel to the south pole
(c) point towards the south pole
(d) point at right angles to the south pole
Answer : C

Question. What does the crowding of iron filings at the end of the magnet indicate?
(a) Magnetic field is strongest near the poles of the magnet.
(b) Magnetic field is weakest near the poles of the magnet.
(c) There is no significant magnetic field at the poles of the magnet.
(d) The significance of polarity.
Answer : A

Question. Two magnetic field lines:
(a) Intersect at neutral point
(b) Never intersect each other
(c) Intersect near north-pole or south pole
(d) Intersect at the midpoint of the magnet
Answer : C

Question. What should be the core of an electromagnet?
(a) Soft iron
(b) Hard iron
(c) Rusted iron
(d) None of above
Answer : A

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. No force acts on a current carrying conductor when it is placed-
(a) Perpendicular to the magnetic field
(b) Parallel to the magnetic field
(c) Far away from the magnetic field
(d) Inside a magnetic field
Answer : B

Fill in The Blank

Question. Closeness of field lines indicate .......... magnetic field strength.
Answer : high

Question. In right hand thumb rule the thumb points in the direction of ..........
Answer : current

Question. An e.m.f. is induced in a coil when .......... linked with it changes.
Answer : The magnetic flux

Question. Red colour insulation is used for ......... wire.
Answer : live

Question. A generator converts mechanical energy into ........ energy. It works on the basis of ........
Answer : Electrical, Electromagnetic induction.

Question. The force that a magnetic field exerts on a current is always perpendicular to the ......... and to the ..........
Answer : Field, Current

Question. You are looking into a solenoid, at its S-pole, along its axis. From your view point, the direction of the current in the solenoid is .........
Answer : Clockwise

Question. Larger the number of turns in the solenoid, greater will be the ......... produced.
Answer : magnetic field

Question. To produce DC, the output of a generator must be fed through a ..........
Answer : Commutator

Question. SI unit of magnetic field strength is ..........
Answer : tesla

Question. The force between currents is called the .......... force.
Answer : Magnetic

Question. The unit of self-inductance in SI system is ......... .
Answer : Henry

True/False

Question. There is no change in the energy of a charged particle moving in a magnetic field although a magnetic force is acting on it.
Answer : False

Question. A current-carrying conductor when placed in a magnetic field always experiences a force.
Answer : False

Question. We can use either a two pin (plug and socked) or a three pin (plug and socket) while working with an electric iron.
Answer : False

Question. Two magnetic lines of force never intersect each other.
Answer : True

Question. A positive charge projected along the axis of a current carrying solenoid moves undeviated from its original path.
Answer : True

Question. While replacing a ‘fuse wire’. the electrician must use a fuse wire of correct rating.
Answer : True

Question. No net force acts on a rectangular coil carrying a steady current when suspended freely in a uniform magnetic field.
Answer : True

Question. It is always good habit not to touch an electric switch with wet hands.
Answer : True

Question. In a DC electric motor a pair of split rings is used as commutator.
Answer : True

Assertion and Reason

DIRECTION : In the following questions, a statement of assertion (A) is followed by a statement of reason (R). Mark the correct choice as:
(a) Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
(b) Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
(c) Assertion (A) is true but reason (R) is false.
(d) Assertion (A) is false but reason (R) is true.
(e) Both Assertion and Reason are false.

Question. Assertion : A direction current flows through a metallic rod, produced magnetic field only outside the rod.
Reason : There is no flow of charge carriers inside the rod.
Answer : B

Question. Assertion : The magnitude of the magnetic field at a point on the axis of a current carrying solenoid is inversely proportional to the current flowing through the solenoid.
Reason : The magnitude of the magnetic field at a point on the axis of a current carrying solenoid is directly proportional to the number of turns per unit length of a solenoid.
Answer : D

Question. Assertion : On freely suspending a current - carrying solenoid, it comes to rest in N-S direction just like a bar magnet.
Reason : One end of current carrying straight solenoid behaves as a North pole and the other end as a South pole.
Answer : A

Question. Assertion : A compass needle is placed near a current carrying wire. The deflection of the compass needle decreases when the magnitude of an electric current in the wire is increased.
Reason : Strength of a magnetic field at a point near the conductor increases on increasing the current.
Answer : D

Question. Assertion : Electric appliances with metallic body have three connections, whereas an electric bulb has two pin connections.
Reason : Three pin connections reduce heating of connecting wires.
Answer : B

Question. Assertion : Basic difference between an electric line and magnetic line of force is that former is discontinuous and the later is continuous or endless.
Reason : No electric lines of force exist inside a charged body but magnetic lines do exist inside a magnet.
Answer : A

Question. Assertion : Force experienced by moving charge will be maximum if direction of velocity of charge is perpendicular to applied magnetic field.
Reason : Force on moving charge is independent of direction of applied magnetic field.
Answer : C

Question. Assertion : There is no change in the energy of a charged particle moving in a magnetic field although a magnetic force is acting on it.
Reason : Work done by centripetal force is always zero.
Answer : A

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

Question. Assertion : The magnetic field is stronger at a point which is nearer to the conductor and goes on decreasing on moving away from the conductor.
Reason : The magnetic field B produced by a straight current carrying wire is inversely proportional to the distance from the wire.
Answer : A

Question. Assertion : When two long parallel wires, hanging freely are connected in series to a battery, they come closer to each other.
Reason : Wires carrying current in opposite direction repel each other
Answer : B

Question. Assertion : Magnetic field lines never intersect.
Reason : At a particular point magnetic field has only one direction.
Answer : A

Question. Assertion : The strength of the magnetic field produced at the centre of a current carrying circular coil increases on increasing the current flowing through the coil.
Reason : Magnetic field strength is inversely proportional to the current flowing in the coil.
Answer : C

Question. Assertion : In a conductor, free electrons keep on moving but no magnetic force acts on a conductor in a magnetic field.
Reason : Force on free electrons due to magnetic field always acts perpendicular to its direction of motion.
Answer : C

Question. Assertion : Two bar magnets attract when they are brought near to each other with the same pole.
Reason : Unlike poles will attract each other.
Answer : D

Case Based Questions

Question1.
A solenoid is a long helical coil of wire through which a current is run in order to create a magnetic fiel(d) The magnetic field of the solenoid is the superposition of the fields due to the current through each coil. It is nearly uniform inside the solenoid and close to zero outside and is similar to the field of a bar magnet having a north pole at one end and a south pole at the other depending upon the direction of current flow. The magnetic field produced in the solenoid is dependent on a few factors such as, the current in the coil, number of turns per unit length et(c) The following graph is obtained by a researcher while doing an experiment to see the variation of the magnetic field with respect to the current in the solenoi(d) The unit of magnetic field as given in the graph attached is in milli-Tesla (mT) and the current is given in Ampere.

Question. What type of energy conversion is observed in a linear solenoid?
(a) Mechanical to Magnetic
(b) Electrical to Magnetic
(c) Electrical to Mechanical
(d) Magnetic to Mechanical
Answer : B

Question. What will happen if a soft iron bar is placed inside the solenoid?
(a) The bar will be electrocuted resulting in short-circuit.
(b) The bar will be magnetised as long as there is current in the circuit.
(c) The bar will be magnetised permanently.
(d) The bar will not be affected by any means.
Answer : B

Question. The magnetic field lines produced inside the solenoid are similar to that of …
(a) a bar magnet
(b) a straight current carrying conductor
(c) a circular current carrying loop
(d) electromagnet of any shape
Answer : A

Question. From the graph deduce which of the following statements is correct.
(a) For a current of 0.8A the magnetic field is 13 mT
(b) For larger currents, the magnetic field increases non-linearly.
(c) For a current of 0.8A the magnetic field is 1.3 mT
(d) There is not enough information to find the magnetic field corresponding to 0.8A current.
Answer : A

Very Short Answer Type Questions

Question. Why does the bulk of iron fillings stick to the ends of a bar magnet and not at its centre?
Answer : Because at the ends magnetic strength is maximum and at centres magnetic strength is least.

Question. If the frequency of A.C. is 50 Hz. Then how many times it is changing its direction in 1 second?
Answer : - 100 Times.

Question. What is the pattern of the magnetic field lines around a straight conductor carrying current?
Answer : Concentric circles

Question. If the current is flowing in the direction of advancement of screw, then what is the direction of magnetic field lines?
Answer : In the direction of rotation of screw.

Question. How can you say that the magnetic field is uniform inside the solenoid.
Answer : Because field lines are parallel inside the solenoid.

Question. Which property of a proton will change while it moves freely in a magnetic field?
Answer : Momentum or Velocity.

Question. According to Flemings right hand rule, which part of right hand indicate the movement of conductor?
Answer : Thumb

Question. If the no. of turns of a circular current carrying coil are doubled, then how will the magnetic field produced by it changes?
Answer : Doubled

Question. In which position the force on conductor is maximum when it is placed in uniform magnetic field?
Answer : When conductor is Perpendicular to field

Question. Which sources produce alternating current?
Answer : AC generator, Thermal power station, Hydro-electric station.

Question. State two ways by which the speed of rotation of an electric motor can be increased.
Answer : The speed of rotation of an electric motor can be increased by
(i) increasing the strength of the current
(ii) increasing the number of turns in the coil.

Question. State the effect of a magnetic field on the path of a moving charged particle.
Answer : A charged particle moving in a magnetic field may experience a force in the direction perpendicular to direction of magnetic field and direction of motion of particle. This force
deflects the charged particle from its path.

Question. State the direction of magnetic field in the following case.
Answer : Using Fleming’s left hand rule, the direction of magnetic field is into the plane of paper.

Question. Name the factors on which strength of an electromagnet depend.
Answer : The strength of an electromagnet depends on the following factors :
(a) Number of the turns in the coil
(b) Strength of the current
(c) Nature of the core material.

Question. What is the role of the split ring in an electric motor ?
Answer : Split ring facilitates the contacts with the ends of the rectangular coil to keep the rotation continuous and not a reversal after every 180°.

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 gets deflected when brought near a bar magnet because magnetic force is exerted by the bar magnet on the compass needle, which is itself a tiny pivoted magnet.

Question. A charged particle enters at right angle into a uniform magnetic field as shown. What should be the nature of charge on the particle if it begins to move in a direction pointing vertically out of the page due to its interaction with the magnetic field? 
Answer : By using Fleming’s left hand rule if the direction of motion of charged particle is vertically out of the page, then the charged particle must be positive in nature.

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. A circular coil is carrying a current of 100 A in the anti clockwise when seen from upward direction. Find the pole of magnetic field when seen from 1.0 m below.
Answer : The pole of magnetic field will be south pole below the power line.

Question. Why does a current-carrying wire move when placed in magnetic field ?
Answer : A current-carrying wire produces a magnetic field around it. When placed in a magnetic field, the two magnetic fields interact with each other and the wire moves.

Question. Write any one method to induce current in a coil.
Answer : By keeping the magnet in a fixed position and moving the coil towards and away from the magnet, we can induce current in the coil.

Question. If a current carrying straight conductor is placed is east-west direction, then find the direction of the force experienced by the conductor due to earth’s magnetic field.
Answer : The force will act in upward direction perpendicular to both, the direction of current as well as to the field.

Short Answer Type Questions

Question. 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?
Answer : (a)No
(b) (i) Two field lines cannot intersect (ii) direction of field lines ‘3’ is wrong.

Question. 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?
Answer : Vertically upward.

Question. 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?
Answer : (i) West to East (ii) East to West

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

Question. It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving (i) alpha particles, (ii) neutrons?
Answer : (i) Yes, alpha particles being positively charged constitutes a current in the direction of motion.
(ii) No, the neutrons being electrically neutral constitute no current.

Question. What does the divergence of magnetic field lines near the ends of a current carrying straight solenoid indicate?
Answer : The divergence, that is, the falling degree of closeness of magnetic field lines indicated the fall in strength of magnetic field near and beyond the ends of the solenoid.

Question. In what respect does a wire carrying a current differ from a wire, which carries no current?
Answer : A current carrying wire produces a magnetic field. When current is flowing through a wire, the electrons move inside it along a definite direction. On the other hand, if no current is flowing through a wire, the electrons inside this wire are in random motion, their average thermal velocity is zero. Such a wire does not produce any magnetic field.

Question. An electron enters a magnetic field at right angle to it as shown in the figure. What is the direction of force acting on the electron?
Answer : The direction of force is at right angle to the direction of magnetic field and current (opposite to the direction of motion of electrons). Applying Fleming’s left-hand rule, the force is at right angle into the page.

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 currentcarrying 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. In Faraday’s experiment if instead of moving the magnet towards the coil we move the coil towards the magnet. Will there be any induced current? Justify your answer. Compare the two cases.
Answer : Yes, these will be an induced current in both the cases as there is a change in the number of magnetic field lines associated with the coil or we can say that there is a motion of a magnet with respect to the coil. Same current will be induced and the direction of flow of current will also be the same in the two cases.

Question. (a) State Right Hand Thumb rule to find the direction of the magnetic field around a current carrying straight conductor.
(b) How will the magnetic field be affected on:
(i) increasing the current through the conductor
(ii) reversing the direction of flow of current in the conductor?
Answer : (a) It states that you are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your finger will wrap around the conductor in the direction of the field lines of the magnetic field.
(b) (i) If the current is increased, the magnetic field strength also increases.
(ii) If the direction of current is reversed, the direction of magnetic field also get reversed.

Question. Diagram shows the lengthwise section of a current carrying solenoid. ⊗ indicates current entering into the page, indicates current emerging out of the page. Decide which end of the solenoid A or B, will behave as north pole. Give reason for your answer. Also draw field lines inside the solenoid.
Answer : 24
Using right hand thumb rube we can draw the magnetic field lines around the conductor as shown.
From figure, end A of solenoid act as north pole and end B will act as south pole. Inside the solenoid field lines are in the form of parallel straight lines.

Question. What is the direction of the force that a vertical magnetic field, directed upward, will exert on an electron travelling eastward in it?
Answer : Applying Fleming’s left hand rule point your fore finger upward. Since you are dealing with a negative charge, rotate your hand until the middle finger points east. Your thumb will point northward, and that is the direction of the force.

Question. Find the direction of magnetic field at centre of 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) 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) vertically upwards.

Question. Two circular coils P and Q are kept close to each other, of which coil P carries a current. What will you observe in the galvanometer connected across the coil Q
(a) if current in the coil P is changed?
(b) if both the coils are moved in the same direction with the same speed?
Give reason to justify your answer in each case.
Answer : (a) When the amount of current in the coil P is changed, an induced current will induce in the coil Q due to change in magnetic field lines i.e., magnetic flux.
(b) If both the coils are moved in the same direction with the same speed, then there is no net change in magnetic flux.
Hence there will be no deflection in the galvanometer.

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 the magnetic field?
Answer : The direction of current I is opposite to the direction electron beam as shown in figure. Since the beam is deflected to the right side, the force, F acting on the beam is as shown. Applying Fleming’s left hand rule, it is found that magnetic field, B is acting vertically downwards (i.e., perpendicular to the plane of the paper and directed inwards) as shown by ⊗. 

Question. What is a solenoid? Draw the pattern of magnetic field lines of a solenoid through which a steady current flows. What does the pattern of field lines inside the solenoid indicate?
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.

Question. A rectangular coil ABCD is placed between the pole pieces of a horse-shoe magnet as shown in figure.
(i) What is the direction of force on each arm?
(ii) What is the effect of the forces on the coil?
(iii) How is the effect of force on the coil changed if the terminals of the battery are interchanged ?
Direction : Read the passage and answer the following questions given below:
The strength of the magnetic field produced by a current-carrying circular coil (or circular wire) depends on (i) Current flowing through the coil.
(ii) Radius of the circular coil. (iii) Number of turns of wire in the circular coil.
Answer : (i) In figure, the current in the coil is in direction DCBA. By Fleming’s left hand rule, in the arm AB, the force is outward at right angle to the plane of the coil. On the arm BC no force acts. On the arm CD, the force is inwards perpendicular to the plane of the coil. On the arm DA, no force acts.
(ii) The force on the arms AB and CD are equal in magnitude, but opposite in direction. They form a clockwise couple. So the coil will rotate clockwise with the arm AB coming out and the arm CD going in.
(iii) On interchanging the terminals of the battery, the coil will rotate anticlockwise. 

Question. A coil of insulated wire is connected to a galvanometer. What would be seen if a bar magnet with its north pole towards one face of the coil is
(i) moved quickly towards it,
(ii) placed near its one face?
Answer : If a coil of insulated wire is connected to a galvanometer and a bar magnet with north 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 anti-clockwise direction in the coil with respect to the side facing the south pole of the magnet and needle of galvanometer will deflect in one direction from zero position. 
(ii) Placed near its one face : No deflection of the needle of galvanometer is observed.

Question. What type of curves we get, between magnetic field and distance along the axis of a current carrying circular coil ?
Answer : At smaller distances, the magnetic field will be described by concentric circles around the wire. As the distance increases, the circles become larger and larger. At the centre of the loop/ coil, the magnetic field will appear as straight line.

Question. In the diagram, A and B represent two straight wires carrying equal currents in a direction at right angle to the paper and downward into the paper. Sketch separately the magnetic lines of force produced by each current carrying conductor. Give reason why the magnetic field at K will be zero. 
Answer : Figure shows the sketch of magnetic lines of force produced by current in wires A and B. The point K is equidistant from the wires A and B, the wires A and B carry equal current, so the magnetic fields at K due to wires A and B are equal in magnitude but opposite in direction. Magnetic field due to the wire A is downward to the plane of paper, while due to the wire B it is upward to the plane of paper. So the net magnetic field at K is zero.

Question. 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.
Answer : (i)

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

Question. If 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?
Answer : (a) Direction of deflection will reverse
(b) Deflection will decrease

Question. 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?
Answer : (1) Electromagnetic induction
(2)Induced current
(3)Change in magnetic lines of forces through coil

Question. 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.
Answer :

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

Long Answer Type Questions

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.
Answer : 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. 

Connect the aluminium 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. 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. 

Case Based Questions :

Read the passage given below and answer the following questions from An electric motor is a rotating device that converts electrical energy into mechanical energy. Electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players, etc. 

An electric motor consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively, as shown in the figure. Commercial motors use an electromagnet in place of a permanent magnet.

Question. When current is switched ON, an electric fan converts 
(a) mechanical energy to chemical energy
(b) electrical energy to mechanical energy
(c) chemical energy to mechanical energy
(d) mechanical energy to electrical energy.
Answer : B

Question. In an electric motor, device that makes contact with the rotating rings and through them current is supplied to coil is 
(a) axle
(b) brushes
(c) coil
(d) split rings.
Answer : B

Question. Choose incorrect statement from the following regarding split rings. 
(a) Split rings are used to reverse the direction of current in coil.
(b) Split rings are also known as commutator.
(c) Split ring is a discontinuous or a broken ring.
(d) Both (a) and (b)
Answer : D

Question. Which of the following has no effect on the size of the turning effect on the coil of an electric motor? 
(a) The amount of the current in the coil.
(b) The direction of the current in the coil.
(c) The number of turns in the coil.
(d) The strength of the magnetic field.
Answer : B

Read the passage given below and answer the following questions from Electric Charges moving in a magnetic field experience a force, while there is no such force on static charges. This fact was first recognized by Hendrik Antoon Lorentz, a great Dutch physicist, nearly a century ago. Thus, a charge moving in a magnetic field experience a force, except when it is moving in a direction parallel to it. The magnitude of force experienced depends on the charge, velocity (v), strength of magnetic field (B), and sine of the angle between v and B. If the direction of velocity is perpendicular to the direction of magnetic field, direction of magnetic force is given by Fleming’s left hand rule. 

Question. A uniform magnetic field exists in the plane of paper pointing from left to right as shown in figure. In the field an electron and a proton move as shown. The electron and the proton experience 
 
(a) forces both pointing into the plane of paper
(b) forces both pointing out of the plane of paper
(c) forces pointing into the plane of paper and out of the plane of paper, respectively
(d) force pointing opposite and along the direction of the uniform magnetic field respectively.
Answer : A

Question. An neutron beam enters a magnetic field at right angles to it as shown in the figure. Due to magnetic field, neutron beam will deflect 
(a) to the left
(b) to the right
(c) into the page
(d) no deflection.
Answer : D

Question. If an electron is travelling horizontally towards east. A magnetic field in vertically downward direction exerts a force on the electron along 
(a) east
(b) west
(c) north
(d) south.
Answer : D

Question. If a charged particle is moving along a magnetic field line. The magnetic force on the particle is 
(a) along its velocity
(b) opposite to its velocity
(c) perpendicular to its velocity
(d) zero.
Answer : D