CBSE Class 10 Science Magnetic Effects of Electric Current Assignment Set B

Question. An alpha particle is placed in a magnetic field.
Will it experience any force, if: 
(i) it moves in the magnetic field parallel to field lines?
(ii) it moves in the magnetic field perpendicular to field lines? 
Answer : (i) No, because, the force is zero if current and field are in the same direction.
(ii) Yes, because, the force is maximum when current and magnetic field are perpendicular. 

Question. Give reasons for the following:
(i) There is either a convergence or a divergence of magnetic field lines near the ends of a current carrying straight solenoid.
(ii) The current carrying solenoid when suspended freely along a particular direction.
(iii) The burnt out fuse should be replaced by another fuse of identical rating. 
Answer : (i) Divergence or degree of closeness of magnetic field lines near the ends of a current carrying straight solenoid indicates a increase in the strength of the magnetic field near the ends of the solenoid.
(ii) A current carrying solenoid acts as a bar magnet. We know that a freely suspended bar magnet aligns itself in the North-South direction. So, a freely suspended current carrying solenoid also aligns itself in the North-South direction. 
(iii) Burnt out fuse cannot be re-used. Also, a fuse wire works because of its lower melting point. If the fuse with larger rating is used with an appliance, the fuse wire shall not melt and hence would fail to serve the required purpose. So, new fuse of same rating should be used for electrical safety. 

Question. Name, state and explain with an example the rule used to determine the direction of force experienced by a current carrying conductor placed in a uniform magnetic field. 
Answer : Fleming’s Left Hand Rule: The direction of force which acts on the current carrying conductor placed in a magnetic field is given by Fleming’s left hand rule. It states that if the forefinger, thumb and middle finger of left hand are stretched mutually perpendicular and if the forefinger point along the direction of external magnetic field, middle finger indicates the direction of current, then thumb points along the direction of force acting on the conductor. Example: When an electron enters a magnetic field at right angles, the direction of force on electron is
perpendicular to the direction of magnetic field and current according to this rule. 

Question. Can a freely suspended current carrying solenoid stay in any direction ? Justify your answer. What will happen when the direction of current in the solenoid is reversed ? Explain.
Answer : A current carrying solenoid behaves like a magnet. When suspended freely, it will stay in north - south direction.
On reversing current its polarity will be reversed and so it will turn at 180°. 

Question. State any two factors on which the magnetic field produced by a current carrying straight conductor depends.
Mention the rule which helps to find the direction of its magnetic field. 
Answer : Factors on which the magnetic field produced by a current carrying conductor depends:
(i) Strength of current passing through the conductor.
(ii) Distance of the point of measurement from the conductor. 
Right Hand Thumb Rule gives the direction of magnetic field. 

Question. Crosses ⊗ represent a uniform magnetic field directed into the paper. A conductor XY moves in the field toward right side. Find the direction of induced current in the conductor. Name the rule you applied. What will be the direction of current if the direction of field and the direction of motion of the conductor both are reversed ?

Answer : (i) Y to X
(ii) Fleming’s right hand rule.
(iii) The direction of induced current will still be the same i.e., Y to X.

Question. What is meant by solenoid ? How does a current carrying solenoid behave ? Give its main use.
Answer : A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
The field lines around a current-carrying solenoid is similar to that produced by a bar magnet. This means that a current carrying solenoid behaves as a magnet having north pole and south pole.
The strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material like soft iron when placed inside the coil.

Question. What is solenoid? Draw the field lines of the magnetic field produced on passing current through and around a current carrying solenoid.
Answer : Definition: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called solenoid.

Magnetic field lines through and around a current carrying solenoid. 

Question. What are magnetic field lines ? List three characteristics of these lines. Describe in brief an activity to study the magnetic field lines due to a current flowing in a circular coil.
Answer : Activity :
(i) A rectangular cardboard having holes is used. A circular coil is passed through the holes. Coil is kept normal to the cardboard.
(ii) Ends of the coil is connected to a battery through a key.
(iii) Iron filings are sprinkled uniformly on the card board.
(iv) Key is plugged in.
(v) On tapping cardboard gently, the iron filings get arranged in concentric circular loops around the holes on the cardboard indicating the magnetic field lines.

Question. What is an electromagnet ? List any two uses.
(i) Draw a labelled diagram to show an electromagnet is made.
(ii) State the purpose of soft iron core used in making an electromagnet.
(iii) List two ways of increasing the strength of an electromagnet if the material of the electromagnet is fixed. 
Answer : Electromagnet: Magnet formed by producing magnetic field inside a solenoid.
Uses of Electromagnet: Inside TVs, sound speakers and radios.
(i) Labelled diagram to show how an electromagnet is made:

(ii) Soft iron rod increases the magnetism of solenoid by a thousand fold. When the solenoid current is switched off, the magnetism is effectively switched off since the soft iron core has low retentivity.
(iii) Ways to increase the strength of an electromagnet if the material of the electromagnet is fixed are:
(a) By increasing the amount of current flowing in the solenoid.
(b) By increasing the number of turns in the solenoid.

Question. (a) What are magnetic field lines ? How is the direction of magnetic field at a point in a magnetic field determined using field lines ?
(b) Two circular coils ‘X’ and ‘Y’ are placed close to each other. If the current in the coil ‘X’ is changed, will some current be induced in the coil ‘Y’ ? Give reason.
(c) State ‘Fleming’s right hand rule”.
Answer : (a) Magnetic field line: Path along which a hypothetical free north pole would tend to move. 
Direction of magnetic field are a point is determined by drawing a tangent to the magnetic field line at that point 
(b) Yes. 
With change in current in the coil X, the magnetic field associated with it also changes around the coil Y placed near it. This change in magnetic field induces a current in the coil Y. 
(c) Fleming’s right hand rule
Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of the conductor, then the middle finger will show the direction of induced current in the conductor. 

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?
Given r₁ > r₂, where will the strength of the magnetic field be larger? Give reasons.
(ii) Field strength at which point will be greater?
(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. 
Answer : (i) The magnetic field lines produced is into the plane of the paper at position R and out of the paper at position 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 the wire will give the direction of the magnetic field. 

Question. (a) Magnetic field lines of two bar magnets A and B are as shown below. Name the poles of the magnets facing each other.

(b) Two magnetic field lines never intersect each other. Why ?
(c) How does the strength of the magnetic field at the centre of a current carrying circular coil depend on the:
(i) Radius of the coil,
(ii) Number of turns in the coil, and
(iii) Strength of the current flowing in the coil ?
Answer : (a) North poles.
(b) Intersection of magnetic field lines at a point means two tangents can be drawn at that point and there will be two direction of a magnetic field which is not possible.
(c) (i) Inversely proportional; more radius, less strong magnetic field.
(ii) Directly proportional; more turns, more strong magnetic field.
(iii) Directly proportional; more strength of current, more strong magnetic field.

Question. What is a 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 : A coil of many turns of insulated copper wire wrapped closely in the shape of a cylinder. 

Question. (i) With the help of an activity, explain the method of inducing electric current in a coil with a moving magnet. State the rule used to find the direction of electric current thus generated in the coil.
(ii) Two circular coils-I and coil-II are kept close to each other as shown in the diagram. Coil-I is connected to a battery and key and coil-II with a galvanometer. State your observation in the galvanometer:

(a) When key k closed ;
(b) When key k is opened;
Give reason for you observations.
Answer : (i) Activity
• Take two different coils of copper wire having large number of turns (say 50 and 100 turn respectively). Insert them over a nonconducting cylindrical roll.
• Connect the coil-1, having larger number of turns, in series with a battery and a plug key. Also connect the other coil-2 with a galvanometer as shown.
• Plug in the key. Observe the galvanometer. There is a deflection in its needle. You will observe that the needle of the galvanometer instantly jumps to one side and just as quickly returns to zero, indicating a momentary current in coil-II.
• Disconnect coil-I from the battery. You will observe that the needle momentarily moves, but to the opposite side. It means that now the current flows in the opposite direction in coil-II.
(ii) Same as CBSE Marking Scheme Answer.

Question. (i) Describe an activity to determine the direction of magnetic field produced by a current carrying straight conductor. Also show that the direction of the magnetic field is reversed on reversing the direction of current.
(ii) An α-particle, (which is a positively charged particle) enters a uniform magnetic field at right angles to it as shown below. Stating the relevant principle explain in which direction will this α-particle move. 

Answer : (i) Take a battery (12 V), a variable resistance (or a rheostat), an ammeter (0 - 5A), a plug key and a long straight thick copper wire. Insert the thick wire through the centre, normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide up or down.
Connect the copper wire vertically between the points X and Y, in series with the battery, a plug and key. Sprinkle some iron filings uniformly on the cardboard. Keep the variable of the rheostat at a fixed position. Close the key, so that current flows through the wire. Ensure that the copper wire placed between the points X and Y remains vertically straight. Gently tap the cardboard for a few times. Observe the pattern of the iron filings. You would find that the iron filings align themselves showing a pattern of concentric circles around the copper wire. This represents the magnetic field around the current-carrying conductor. The direction of magnetic field changes on reversing the direction of current.

(ii) The alpha particle will move in a circular path. This is because a centripetal force acts on the particle due to the movement of particle in the magnetic field.

Question. (a) State Fleming’s left hand rule.
(b) Write the principle of working of an electric motor.
(c) Explain the function of the following parts of as electric motor.
(i) Armature (ii) Brushes (iii) Split ring
Answer : (a) Fleming’s left-hand rule: Stretch the forefinger, middle finger and thumb of left hand in such a way that they are mutually perpendicular to each other. If the forefinger points in the direction of magnetic field, middle finger points in the direction of current then the thumb shows the direction of force or motion on the current carrying conductor. 
(b) Principle of working of electric motor: A coil carrying electric current placed in an external magnetic field experiences a force. 
(c) (i) Function of armature : It is a rectangular iron core wrapped by the copper coil through which electricity passes and due to magnetic field it experiences a force and rotates.
(ii) Function of brushes: It helps in easy transfer of charge between the coil and the external circuit.
(iii) Function of split rings: It reverses the direction of current after each half rotation of the coil so that the coil can keep rotating continuously.

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 : Pattern of magnetic field lines produced around a current carrying straight conductor:

Right-hand thumb rule: If we are holding a current carrying straight conductor in right hand such that the thumb points towards the direction of current, then, the fingers will wrap around the conductor in the direction of the field lines of the magnetic field. 

As the compass is placed farther, deflection in the needle decreases. Thus, the magnetic field produced by given current decreases as the distance from it increases. The concentric circles around the wire become larger as we move away from it.

Question. (i) Explain with the help of the pattern of magnetic field lines the distribution of magnetic field due to a current carrying a circular loop.
(ii) Why is it that the magnetic field of a current carrying coil having n turns, is ‘n’ times as large as that produced by a single turn (loop) ?
Answer : Magnetic field due to current through a circular Loop: It can be represented by concentric circle at every point. Circles become larger and larger as we move away. Every point on wire carrying current would rise to magnetic field appearing as straight line at centre of the loop. The direction of magnetic field inside the loop is same.

Magnetic field is directly proportional to number of turns (n) in the coil. As the number of turns (n) 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. 

Question. (i) A coil of insulated copper wire is connected to a galvanometer. What happens if a bar magnet is:
(a) Pushed into the coil ?
(b) Withdrawn from inside the coil ?
(c) Held stationary inside the coil ? Give reasons for your observation.
(ii) Mention one more method of inducing current in a coil. 
Answer : (i) (a) When a bar magnet is pushed into the coil of insulated copper wire connected to a galvanometer, an induced current is set-up in the coil due to charge of magnetic field through it. As a result, galvanometer gives a deflection (say towards left) and returns to original position. 
(b) When the bar magnet is withdrawn from inside the coil, again an induced current is set up in the coil due to charge of magnetic field through it. As a result galvanometer gives a deflection in the reverse direction (say towards right) and returns to original position. 
(c) If the bar magnet is held stationary inside the coil, then there is no induced current in the coil, because there is no change in magnetic field through it. As a result, galvanometer does not show any deflection. 
(ii) By changing current in another coil placed near it.