23 + Interview Questions in ELECTRO MAGNETISM IN GENERAL AWARENESS Page 1 InterviewSolution

1.	State the unit of magnetic field in terms of the force experienced by a current carrying conductor placed in a magnetic field
Answer» Unit is: Newton/ampere x meter (or NA^-1m^-1).

Discussion

2.	Name and state the law which is used to determine the direction of force on a current carrying conductor placed in a magnetic field.
Answer» Fleming's left hand rule: Stretch the forefinger, middle finger and the thumb of your left hand mutually perpendicular to each other. If the forefinger indicates the direction of magnetic field and the middle finger indicates the direction of current, then the thumb will indicate the direction of motion of conductor.

Discussion

3.	How is the magnetic field due to a straight current carrying wire affected if current in wire is (a) decreased, (b) reversed?
Answer» (a) On decreasing the current the magnetic field lines become rarer. (b) The direction of magnetic field lines will get reversed.

3.

How is the magnetic field due to a straight current carrying wire affected if current in wire is (a) decreased, (b) reversed?

Answer»

(a) On decreasing the current the magnetic field lines become rarer.

(b) The direction of magnetic field lines will get reversed.

Discussion

4.	State a law, which determines the direction of magnetic field around a current carrying wire.
Answer» Right hand thumb rule determines the direction of magnetic field around a current carrying wire. It states that if we hold the current carrying conductor in right hand such that the thumb points in the direction of flow of current, then the fingers encircle the wire in the direction of the magnetic fields lines.

Discussion

5.	Suggest two ways in an a.c. generator to produce a higher e.m.f.
Answer» Two ways in an a.c generator to produce a higher e.m.f. are: (1) By increasing the speed of rotation of the coil. (2) By increasing the number of turns of coil.

5.

Suggest two ways in an a.c. generator to produce a higher e.m.f.

Answer»

Two ways in an a.c generator to produce a higher e.m.f. are:

(1) By increasing the speed of rotation of the coil.

(2) By increasing the number of turns of coil.

Discussion

6.	In an a.c. generator the speed at which the coil rotates is doubled. How would this affect (a) the frequency of output voltage, (b) the maximum output voltage is doubled.
Answer» (a)In an a.c generator, if the speed at which the coil rotates is doubled, the frequency is also doubled. (b) Maximum output voltage is also doubled.

6.

In an a.c. generator the speed at which the coil rotates is doubled. How would this affect (a) the frequency of output voltage, (b) the maximum output voltage is doubled.

Answer»

(a)In an a.c generator, if the speed at which the coil rotates is doubled, the frequency is also doubled.

(b) Maximum output voltage is also doubled.

Discussion

7.	The following diagram in Fig. 10.44 shows a coil X connected to a sensitive centre –zero galvanometer G and a coil P connected to a battery through a switch S.(a) Describe the observation when the switch S is (i) closed suddenly, (ii) then kept closed, (iii) finally opened.(b) Name and state the law which explains the above observations.
Answer» (a) (i) When switch is closed suddenly, the galvanometer needle deflects for a moment. (ii) If switch is kept closed then galvanometer needle returns to zero. (iii) If switch is opened again then galvanometer needle deflects again but in opposite to the direction of deflection in case (a). (b) This can be explained by Faraday's law which states that whenever there is change in the magnetic flux linked with a coil, an e.m.f. is induced. The induced e.m.f. lasts as long as there is a change in the magnetic flux linked with the coil.

7.

The following diagram in Fig. 10.44 shows a coil X connected to a sensitive centre –zero galvanometer G and a coil P connected to a battery through a switch S.(a) Describe the observation when the switch S is (i) closed suddenly, (ii) then kept closed, (iii) finally opened.(b) Name and state the law which explains the above observations.

Answer»

(a)

(i) When switch is closed suddenly, the galvanometer needle deflects for a moment.

(ii) If switch is kept closed then galvanometer needle returns to zero.

(iii) If switch is opened again then galvanometer needle deflects again but in opposite to the direction of deflection in case (a).

(b) This can be explained by Faraday's law which states that whenever there is change in the magnetic flux linked with a coil, an e.m.f. is induced. The induced e.m.f. lasts as long as there is a change in the magnetic flux linked with the coil.

Discussion

8.	What determines the frequency of a.c. produced in a generator?
Answer» The number of rotations of the coil in one second or the speed of rotation of the coil.

Discussion

9.

State two dis-similarities and one similarity between a d.c. motor and an a.c. generator.

Answer»

Two dissimilarities between D.C. motor and A.C. generator:

A.C. Generator	D.C. Motor
A generator is a device which converts mechanical energy into electrical energy	A D.C. motor is a device which converts electrical energy into mechanical energy.
A generator works on the principle of electromagnetic induction.	A D.C. works on the principle of force acting on a current carrying conductor placed in a magnetic field.

Similarity: Both in A.C generator and D.C motor, a coil rotates in a magnetic field between the pole pieces of a powerful electromagnet.

Discussion

10.	The following diagram shows a spiral coil wound on a hollow carboard tube AB. A magnetic compass is placed close to it. Current it switched on by closing the key. (a) What will be the polarity at the ends A and B? (b) How will the compass needle be affected? Give reason.
Answer» (a) A - North pole, B - South pole. (b) The north pole of compass needle will deflect towards west. Reason: The end A of the coil behaves like north pole which repels north pole of compass needle towards west.

10.

The following diagram shows a spiral coil wound on a hollow carboard tube AB. A magnetic compass is placed close to it. Current it switched on by closing the key. (a) What will be the polarity at the ends A and B? (b) How will the compass needle be affected? Give reason.

Answer»

(a) A - North pole, B - South pole.

(b) The north pole of compass needle will deflect towards west.

Reason: The end A of the coil behaves like north pole which repels north pole of compass needle towards west.

Discussion

11.	By reversing the direction of current in a wire, the magnetic field produced by it:(a) gets reversed in direction(b) increases in strength(c) decreases in strength(d) remains unchanged in strength and direction
Answer» By reversing the direction of current in a wire, the magnetic field produced by it gets reversed in direction. Hint: On reversing the direction of current in a wire, the polarity of the faces of the wire also reverses. Thus, the direction of magnetic field produced by it also gets reversed.

11.

By reversing the direction of current in a wire, the magnetic field produced by it:(a) gets reversed in direction(b) increases in strength(c) decreases in strength(d) remains unchanged in strength and direction

Answer»

By reversing the direction of current in a wire, the magnetic field produced by it gets reversed in direction.

Hint: On reversing the direction of current in a wire, the polarity of the faces of the wire also reverses. Thus, the direction of magnetic field produced by it also gets reversed.

Discussion

12.	State one advantage of using a.c. over d.c.
Answer» The voltage of a.c. can be stepped up by the use of step-up transformer at the power generating station before transmitting it over long distances. It reduces the loss of electrical energy as heat in the transmission line wires. On the other hand, if d.c. is generated at the power generating station, its voltage cannot be increased for transmission, and so due to passage of high current in the transmission line wires, there will be a huge loss of electrical energy as heat in the line wires.

Discussion

13.	What will happen to a compass needle when the compass is placed below a wire and a current is made to flow through the wire? Give a reason to justify your answer.
Answer» A current carrying conductor produces a magnetic field around it and the magnetic needle in this magnetic field experience a torque due to which it deflects to align itself in the direction of magnetic field.

Discussion

14.	The adjacent diagram shows a coil would around a soft iron bar XY. (a) State the polarity at the end X and Y as the switch is pressed. (b) Suggest one way increasing the strength of electromagnet so formed.
Answer» (a)X-north pole, Y –south pole. (b)By reducing resistance of circuit by mean of rheostat to increase current.

14.

The adjacent diagram shows a coil would around a soft iron bar XY. (a) State the polarity at the end X and Y as the switch is pressed. (b) Suggest one way increasing the strength of electromagnet so formed.

Answer»

(a)X-north pole, Y –south pole.

(b)By reducing resistance of circuit by mean of rheostat to increase current.

Discussion

15.	Name and state the rule by which polarity at the ends of a current carrying solenoid is determined.
Answer» Right hand thumb rule: If we hold the current carrying conductor in right hand such that the thumb points in the direction of flow of current, then the fingers encircle the wire in the direction of the magnetic fields lines.

Discussion

16.	Why does a current carrying freely suspended solenoid rest along a particular direction?
Answer» A current carrying freely suspended solenoid at rest behaves like a bar magnet. It is because a current carrying solenoid behaves like a bar magnet.

Discussion

17.	A wire, bent into a circle, carries current in an anticlockwise direction. What polarity does this face of the coil exhibit?
Answer» Face of the coil exhibit North polarity.

Discussion

18.	What is the direction of magnetic field at the centre of a coil carrying current in (i) clockwise, (ii) anticlockwise direction?
Answer» (i) Along the axis of coil inwards. (ii) Along the axis of coil outwards.

18.

What is the direction of magnetic field at the centre of a coil carrying current in (i) clockwise, (ii) anticlockwise direction?

Answer»

(i) Along the axis of coil inwards.

(ii) Along the axis of coil outwards.

Discussion

19.	State two factors on which the magnitude of induced e.m.f. depend.
Answer» Magnitude of induced e.m.f depend upon: (i) The change in the magnetic flux. (ii) The time in which the magnetic flux changes.

19.

State two factors on which the magnitude of induced e.m.f. depend.

Answer»

Magnitude of induced e.m.f depend upon:

(i) The change in the magnetic flux.

(ii) The time in which the magnetic flux changes.

Discussion

20.	What is an electromagnet? Name two factors on which the strength of magnetic field of an electromagnet depends and state how does it depend on the factors stated by you.
Answer» An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet. The strength of magnetic field of an electromagnet depends on: (i) Number of turns: The strength of magnetic increases on increasing the number of turns of winding in the solenoid. (ii) Current: The strength of magnetic field increases on increasing the current through the solenoid.

20.

What is an electromagnet? Name two factors on which the strength of magnetic field of an electromagnet depends and state how does it depend on the factors stated by you.

Answer»

An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet.

The strength of magnetic field of an electromagnet depends on:

(i) Number of turns: The strength of magnetic increases on increasing the number of turns of winding in the solenoid.

(ii) Current: The strength of magnetic field increases on increasing the current through the solenoid.

Discussion

21.	Why is soft iron used as the core of the electromagnet in an electric bell?
Answer» The soft iron bar acquires the magnetic properties only when an electric current flows through the solenoid and loses the magnetic properties as the current is switched off. That's why soft iron is used as the core of the electromagnet in an electric bell.

Discussion

22.	Why is it more difficult to move a magnet towards a coil which has large number of turns?
Answer» When a coil has a large number of turns, then magnitude of induced e.m.f. in the coil become more and then by Lenz's law it will oppose more.

Discussion

23.	In which of the following cases does the electromagnetic induction occur?(i) A current is started in a wire held near a loop of wire(ii) The current is stopped in a wire held near a loop of wire(iii) A magnet is moved through a loop of wire(iv) A loop of wire is held near a magnet.
Answer» (1) Yes. (2) Yes. (3) Yes. (4) No.

23.

In which of the following cases does the electromagnetic induction occur?(i) A current is started in a wire held near a loop of wire(ii) The current is stopped in a wire held near a loop of wire(iii) A magnet is moved through a loop of wire(iv) A loop of wire is held near a magnet.

Answer»

(1) Yes.

(2) Yes.

(3) Yes.

(4) No.

Discussion

Explore topic-wise InterviewSolutions in .

State the unit of magnetic field in terms of the force experienced by a current carrying conductor placed in a magnetic field

Name and state the law which is used to determine the direction of force on a current carrying conductor placed in a magnetic field.

How is the magnetic field due to a straight current carrying wire affected if current in wire is (a) decreased, (b) reversed?

State a law, which determines the direction of magnetic field around a current carrying wire.

Suggest two ways in an a.c. generator to produce a higher e.m.f.

In an a.c. generator the speed at which the coil rotates is doubled. How would this affect (a) the frequency of output voltage, (b) the maximum output voltage is doubled.

What determines the frequency of a.c. produced in a generator?

State two dis-similarities and one similarity between a d.c. motor and an a.c. generator.

The following diagram shows a spiral coil wound on a hollow carboard tube AB. A magnetic compass is placed close to it. Current it switched on by closing the key. (a) What will be the polarity at the ends A and B? (b) How will the compass needle be affected? Give reason.

By reversing the direction of current in a wire, the magnetic field produced by it:(a) gets reversed in direction(b) increases in strength(c) decreases in strength(d) remains unchanged in strength and direction

State one advantage of using a.c. over d.c.

What will happen to a compass needle when the compass is placed below a wire and a current is made to flow through the wire? Give a reason to justify your answer.

The adjacent diagram shows a coil would around a soft iron bar XY. (a) State the polarity at the end X and Y as the switch is pressed. (b) Suggest one way increasing the strength of electromagnet so formed.

Name and state the rule by which polarity at the ends of a current carrying solenoid is determined.

Why does a current carrying freely suspended solenoid rest along a particular direction?

A wire, bent into a circle, carries current in an anticlockwise direction. What polarity does this face of the coil exhibit?

What is the direction of magnetic field at the centre of a coil carrying current in (i) clockwise, (ii) anticlockwise direction?

State two factors on which the magnitude of induced e.m.f. depend.

What is an electromagnet? Name two factors on which the strength of magnetic field of an electromagnet depends and state how does it depend on the factors stated by you.

Why is soft iron used as the core of the electromagnet in an electric bell?

Why is it more difficult to move a magnet towards a coil which has large number of turns?

In which of the following cases does the electromagnetic induction occur?(i) A current is started in a wire held near a loop of wire(ii) The current is stopped in a wire held near a loop of wire(iii) A magnet is moved through a loop of wire(iv) A loop of wire is held near a magnet.