Electromagnetism — Practice

Question 1

Which of the following converts electrical energy into mechanical energy?

Accepted Answer

motor

Question 2

Which of the following converts mechanical energy into electrical energy?

Accepted Answer

generator

Question 3

The magnetic force on a current carrying wire placed in a uniform magnetic field, if the wire is oriented perpendicular to the magnetic field, is

Accepted Answer

ILB

Question 4

One tesla equals

Accepted Answer

newton per ampere-metre

Question 5

The SI unit of magnetic flux is

Accepted Answer

weber

Question 6

No force acts on a conductor carrying electric current when it is kept

Accepted Answer

parallel to the magnetic field

Question 7

An 8 N force acts on a rectangular conductor 20 cm long placed perpendicular to a magnetic field. Determine the magnetic field induction if the current in the conductor is 40 A.

Accepted Answer

1 tesla.

Question 8

Why does the picture appear distorted when a bar magnet is brought close to the screen of an old CRT television?

Accepted Answer

Because the picture is drawn by electrons striking the screen, and a magnetic field exerts a force on moving charges. The magnet's field deflects the electrons from their intended paths, so they land in the wrong places and the image is distorted.

Question 9

Are magnetic field lines closed? Explain.

Accepted Answer

Yes. The field lines of a straight current-carrying wire are seen to be circles, which are closed. For a solenoid the lines outside run from north to south and are continuous with the lines inside, which run from south to north, so each line returns on itself. Since a solenoid's field resembles a bar magnet's, the bar magnet's lines are closed too.

Question 10

A bar magnet with its north pole facing a coil moves towards the coil. What happens to the magnetic flux passing through the coil, and what is the direction of the induced current?

Accepted Answer

The flux through the coil increases. By Lenz's law the induced current opposes that increase, so it flows anticlockwise as seen from the magnet, making the coil's near face a north pole that repels the approaching magnet.

Question 11

A coil of 400 turns has the flux through each single turn increase by 0.001 Wb in 0.1 second, after which the flux stays constant from t = 0.1 s to t = 0.3 s. Find the maximum induced emf, and the emf between 0.1 s and 0.3 s.

Accepted Answer

Maximum induced emf is 4 V; between 0.1 s and 0.3 s the induced emf is zero.

Question 12

Find the length of a conductor moving at 10 m/s perpendicular to a magnetic field of induction 0.8 T, if it induces an emf of 8 V between its ends.

Accepted Answer

1 metre.

Question 13

Both a coil and a bar magnet move in the same direction with the same speed. What happens to the induced current in the coil?

Accepted Answer

No current is induced. Induction depends on the RELATIVE motion between the magnet and the coil. If both move together in the same direction at the same speed, their separation does not change, so the flux linked with the coil does not change, and by Faraday's law no emf and no current are generated.

Question 14

A coil is kept perpendicular to the page. At P the current flows into the page and at Q it comes out of the page. What is the direction of the magnetic field due to the coil?

Accepted Answer

Apply the right hand rule: curl the fingers of the right hand along the direction of the current in the coil, and the thumb gives the direction of the field. With the current going into the page at P and out at Q, the field inside the coil is directed along its axis, perpendicular to the plane of the coil.

Question 15

How do you verify experimentally that a current-carrying conductor experiences a force when kept in a magnetic field?

Accepted Answer

Fix two wooden sticks with split tops on a wooden plank and pass a copper wire through the splits, connecting its ends to a 3 V battery through a switch. Close the switch so current flows, then bring a horseshoe magnet near the wire. The wire deflects. Reverse the poles of the magnet and the deflection reverses; reverse the current and it reverses again. The direction each time agrees with the right hand rule.

Question 16

A metal ring levitates over an AC solenoid but only jumps once when DC is used. Explain both observations.

Accepted Answer

With AC, the current in the solenoid changes magnitude and direction continuously, so the flux linked with the ring changes continuously. A current is induced in the ring in such a direction that like poles face each other, and the resulting repulsion balances the ring's weight, so it levitates. With DC, flux is linked to the ring only at the instant the switch is closed - that change lifts the ring once. After that the flux is steady, there is no change, no induced current, and the ring falls. Switching off is another change, so it lifts once more.

Practice — Electromagnetism

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