Chapter 20 Electromagnetic Forces

A straight wire carrying a current produces a magnetic field.

Which diagram shows the correct shape of the field?

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The diagrams show a straight wire carrying a current into the paper.

Which diagram shows the magnetic field pattern due to this current?

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When the electric current in wire XY is in the direction shown, there is an upward force on the wire.

If the north and south poles of the magnet exchange positions, in which direction will the force on the wire act?

• downwards

• upwards

• to the left

• to the right

A current-carrying coil in a magnetic field experiences a turning effect.

How can the turning effect be increased?

• increase the number of turns on the coil

• reduce the size of the current

• reverse the direction of the magnetic field

• use thinner wire for the coil

The diagram shows an electrical device. What is this electrical device?

• a d.c. motor

• an a.c. generator

• a magnetising coil

• a transformer

A wire perpendicular to the page carries an electric current in a direction out of the page. There are four compasses near the wire.

Which compass shows the direction of the magnetic field caused by the current?A

• A

• B

• C

• D

The diagram shows a horizontal wire between the two magnetic poles of a U-shaped magnet.

The U-shaped magnet is on a balance.

When the switch closed, the reading on the balance decreases.

The experiment is carried out three more times with the following changes made.

1 Only the current is reversed.

2 Only the magnetic field is reversed.

3 Both the current and the magnetic field are reversed at the same time.

Which changes cause an increase in the reading on the balance?

• 1 only

• 2 only

• 3 only

• 1 and 2 only

The diagram shows a coil of wire between the poles of a magnet.

The coil consists of 20 turns of insulated wire.

The coil is connected to a variable resistor and a power supply.

How can the turning effect on the coil be increased?

• by moving the poles of the magnet closer to the coil

• by reducing the number of turns on the coil while keeping the current constant

• by increasing the resistance of the variable resistor

• by reversing the terminals of the power supply

There is an electric current in a wire. The wire is placed in a magnetic field. A force acts on the

wire due to the current.

Which statement is correct?

• The magnetic field must be produced by a permanent magnet and not by an electromagnet.

• The wire must be made from a magnetic material.

• When both the current and the magnetic field are reversed, the direction of the force is

  unchanged.

• When the current is reversed, but not the magnetic field, there will be no force on the wire.

A current-carrying conductor placed in a magnetic field experiences a force.

Which changes result in reversing the direction of the force?

• Decrease the current, keeping the field direction constant.

• Increase the current and reverse the field direction.

• Increase the current, keeping the field direction constant.

• Reverse the current and reverse the field direction.