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Solved problems for the chapter:

d.c. motor

View the theory »

Magnetic Induction

1. Magnetic flux

2. Faraday's law of induction

3. Self induction

4. Direction of induced voltage

5. Forum

↓ Show theory [1] ↓

Detailed solution steps are available at:

https://en.openprof.com/wb/chapter:dc_motor/3653/

Example 1.

In the diagram of the d.c motor shown, the side WX of the coil will move in which direction?

towards the top of the page
to the right
to the left
towards the bottom of the page

Example 2.

The coil in a d.c. motor is connected to a split-ring commutator. What is the purpose of the split-ring commutator?

to ensure that the coil continues to rotate in the same direction
to ensure that the size of the current in the coil remains constant
to ensure that the size of the turning effect on the coil remains constant
to ensure that the turning effect on the coil changes direction

The figure below shows a simple motor.

Explain why the coil starts to rotate when the switch is closed.

A coil carries a current in a magnetic field. The coil experiences a turning effect. Which device uses this effect?

a d.c. motor
an electromagnet
a relay
a transformer

In which device is a split-ring commutator used, and what is its purpose?

	device	purpose
A	a.c. generator	to change the direction of the current in the coil as it turns
B	a.c. generator	to change the output current from d.c. into a.c.
C	d.c. motor	to change the direction of the current in the coil as it turns
D	d.c. motor	to change the input current from a.c. into d.c.

Which device uses a split-ring commutator?

a d.c. motor
a relay
a transformer
an a.c. generator

Which electrical device uses the turning effect produced by a current-carrying coil in a magnetic field?

a.c. generator
d.c. motor
relay
transformer

A simple d.c. electric motor is fitted with a coil that rotates in a magnetic field. A commutator connects the power supply to the coil. What is the purpose of the commutator?

It converts a.c. into d.c. in the coil.
It prevents the current from becoming too great, because the coil has a low resistance.
It reverses the direction of the current in the coil after every 180° rotation of the coil.
It switches the current off momentarily after every 90° rotation of the coil.

Which component forms part of a d.c. motor but not a simple moving coil a.c. generator?

the coil
the brushes
the magnet
the split-ring commutator

10.

The diagram shows a simple d.c. electric motor which is rotating.

Which change makes the motor rotate more quickly?

increasing the number of turns on the coil
removing the magnets
reversing the battery
reversing the polarity of the magnets

11.

A generator uses the principle of electromagnetic induction.

Which change would increase the induced electromotive force (e.m.f.) in the coil?

increasing the number of turns in the coil
placing the magnets further apart
using a coil made from steel wire
reversing one of the magnets

12.

Diagram 1 shows a coil of wire P between the poles of a magnet. The ends of coil P are connected to a battery by slip rings. Diagram 2 shows a coil of wire Q between the poles of a different magnet. The ends of coil Q are connected to a battery by a split-ring commutator.

What happens to coils P and Q?

	coil P	coil Q
A	continuously turns anticlockwise	makes one quarter turn anticlockwise then stops
B	continuously turns clockwise	makes one quarter turn clockwise then stops
C	makes one quarter turn anticlockwise then stops	continuously turns anticlockwise
D	makes one quarter turn clockwise then stops	continuously turns clockwise

13.

Over time, the strength of the magnets in an electric motor decreases. Which row describes two ways to keep the motor running at its original speed?

	current in the coil	number of turns on the coil
A	decrease	decrease
B	decrease	increase
C	increase	decrease
D	increase	increase

14.

The diagram shows a d.c. motor.

Which two changes together will always make the coil spin more slowly?

Decrease the current in the coil and use a magnet of less strength.
Decrease the current in the coil and increase the number of turns on the coil.
Increase the current in the coil and use a magnet of less strength.
Increase the current in the coil and decrease the number of turns on the coil.

15.

A wire is placed between the poles of a magnet.

Which statements are correct?

An a.c. current in the wire causes a changing force on it.
A downward d.c. current in the wire causes a constant force on it.
An upward d.c. current in the wire causes a constant force on it.

1 and 2 only
1 and 3 only
2 and 3 only
1, 2, and 3

16.

A class is designing a d.c. motor. To achieve a greater turning effect, three suggestions are made.

Have a larger current in the coil of the motor.
Have a stronger magnet in the motor.
Put a larger number of turns on the coil.

Which suggestions will help to increase the turning effect?

1, 2 and 3
1 and 2 only
1 and 3 only
2 and 3 only

17.

The diagrams show the coils of two simple direct current (d.c.) motors. Coil P has three turns of wire and coil Q has two turns. Coil P has the same dimensions as coil Q. The coils are in identical magnet fields.

What produces the greatest turning effect?

	coil	current /
A	P	2
B	P	4
C	Q	2
D	Q	4

18.

The coil in a d.c. motor is connected to a split-ring commutator. What is the purpose of the split-ring commutator?

to ensure that the coil continues to rotate in the same direction
to ensure that the size of the current in the coil remains constant
to ensure that the size of the turning effect on the coil remains constant
to ensure that the turning effect on the coil changes direction

19.

What is the purpose of the split-ring commutator in an electric motor?

to ensure that the magnetic field in the motor changes direction every half rotation
to ensure that the magnetic field in the motor stays in the same direction at all times
to ensure that the turning effect on the motor changes direction every half rotation
to ensure that the turning effect on the motor stays in the same direction at all times

20.

What is the function of the split-ring commutator in an electric motor with a single rotating coil?

to enable the motor to function with an a.c. source
to reverse the current in the coil once every revolution
to reverse the current in the coil whenever its plane becomes perpendicular to the magnetic field
to reverse the current in the coil whenever its plane is parallel with the magnetic field

21.

Four positions of a current-carrying coil in a magnetic field, as in a d.c. motor, are shown. In diagrams 2 and 4, the coil is at an angle of to the field lines.

Which row is correct?

	turning effect of the forces in positions 1 and 3	turning effect of the forces in positions 2 and 4
A	different	different
B	different	same
C	same	different
D	same	same

22.

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.

23.

The diagram shows a flat, rectangular coil placed between the poles of a magnet. There is a current in the coil that makes it turn in the direction shown in the diagram.

Which change would make the coil turn in the opposite direction?

decreasing the current in the coil
increasing the number of turns on the coil
reversing both the direction of the current in the coil and the poles of the magnet
reversing only the direction of the current in the coil