P4.5.5P4. Electricity and Magnetism

The d.c. motor — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 10 parts Lesson 16 of 19 1 quiz

📚 Combined Science 0653 - Physics Hub / Topical Revision

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Lesson summary

motor — COMBINED SCIENCE notes: Describe the structure and action of a simple d.c. motor and the factors affecting its turning effect.D.c.

P4.5.5 The d.c. motor — COMBINED SCIENCE notes
Forces on the coil
Greatest turning effect
Zero turning effect
Motor and generator
Without a commutator

Part 1 of 8Structure

Structure

D.c. motor

D.c. motor a device that uses the motor effect to turn a current-carrying coil in a magnetic field

Parts

Parts a coil free to rotate between the poles of a permanent magnet, with a split-ring commutator and brushes joined to a d.c. source

Component	Function
Coil	Carries the current and experiences the turning force
Permanent magnet	Provides the external magnetic field
Split-ring commutator	Reverses the current in the coil every half turn
Brushes	Maintain contact with the commutator without tangling the wires

Part 2 of 8Turning effect

Forces on the coil

Forces on the coil the current flows in opposite directions in the two sides of the coil
Fleming's left-hand rule gives forces in opposite directions on the two sides.
The two forces act on opposite sides of the axle, so they form a turning effect.

Greatest turning effect

Greatest turning effect when the plane of the coil is parallel to the field

Zero turning effect

Zero turning effect when the coil is vertical, as the forces act through the axle

Motor and generator

Motor and generator a motor takes in electricity and gives motion, whereas a generator takes in motion and gives electricity

Part 3 of 8One half turn of the motor

With the coil horizontal, the split-ring is in contact with the brushes and a current flows through the coil.

One side of the coil is forced up and the other is forced down, so the coil rotates.

After a 90° rotation the coil is vertical and the split-ring is not in contact with the brushes; no current flows and no force acts.

The momentum of the coil carries it past the vertical position.

The split-ring reconnects to the brushes the opposite way round, reversing the current in the coil.

The forces on the two sides now keep turning the coil in the same direction.

Part 4 of 8Need for the commutator

Without a commutator

Without a commutator the coil would rock back and forth and stop in the vertical position, because the forces would then turn it back

With the commutator

With the commutator the current reverses every half turn, so the coil rotates continuously in one direction

Part 5 of 8Increasing the speed and force

Larger current

Larger current increases the force and the speed of rotation

Stronger magnet

Stronger magnet increases the force and the speed of rotation

More turns on the coil

More turns on the coil increases the force

Part 6 of 8Changing the direction of rotation

Reverse the direction of the current supply

Reverse the direction of the magnetic field by swapping the poles of the magnet

Reversing both

Reversing both leaves the direction of rotation unchanged

Part 7 of 8Practical motors

1Many coils at different angles

Many coils at different angles give a smoother and larger turning effect

2Curved magnets and a soft iron core

Curved magnets and a soft iron core improve the turning effect

Part 8 of 8More from the specification

Step-up and step-down transformer induction

Step-up and step-down transformer induction primary ac coils induce alternating voltage in secondary windings across laminated iron cores to transmit electrical power efficiently.

Electromagnetic induction in solenoid circuits

Electromagnetic induction in solenoid circuits moving a magnet inside a helical solenoid cuts magnetic flux lines, driving induced current through closed circuit conductors.

Torque on current-carrying conductor loops

Torque on current-carrying conductor loops magnetic field lines exert opposite Lorentz forces on opposite sides of a conductor loop, rotating d.c. motor armatures.

Operating Principles of the D.C. Motor

A current-carrying coil placed in a magnetic field experiences equal and opposite forces on opposite sides of the conductor loop,
producing a continuous turning effect.

Split-Ring Commutator Function

The split-ring commutator reverses the direction of current in the coil every half-turn, ensuring the magnetic torque acts in a constant rotational direction.

Electromagnetic Induction in a Solenoid

Moving a magnet into a solenoid cuts magnetic field lines, creating an induced electromotive force (EMF) that drives current in a complete circuit.

Transformers and Power Transmission

In an ideal transformer, electrical power is conserved; step-up transformers raise primary voltage to reduce current and resistive transmission losses.
- (V_p I_p = V_s I_s)

Potential difference

The work done by a unit charge passing through a component.

Step-up transformer

A transformer with more turns on the secondary coil than the primary, raising the voltage.

Step-down transformer

A transformer with fewer turns on the secondary coil, lowering the voltage.

Electromagnetic induction

The production of an e.m.f. in a conductor when there is relative movement between the conductor and a magnetic field.

IpVp

IsVs

What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil

Transformer

A device with a primary coil and secondary coil on an iron core that changes an alternating voltage.

Solenoid

A coil of wire that produces a magnetic field like a bar magnet when a current flows.

Voltage

A measure of the electrical energy supplied or changed in a circuit.

Conductor

Material that lets current flow.

Voltmeter

Meter that measures voltage, connected in parallel.

Field line

A line showing the direction of the force on a north pole.

Alternating current

A current that continuously changes direction.

Induced magnet

A magnetic material that becomes a magnet only while in a magnetic field.

Key terms

Term	Meaning
D.c. motor	A device in which a current-carrying coil in a magnetic field is turned by the motor effect.
Split-ring commutator	A ring split into two halves that reverses the current in the coil every half turn.
Brush	A carbon contact that presses on the commutator to carry current to the rotating coil.
Turning effect	The tendency of the equal and opposite forces on the coil to rotate it about the axle.
Motor effect	The force on a current-carrying conductor in a magnetic field.
Armature	The rotating coil of the motor.

Avoid these mistakes

Saying the commutator makes the current alternate in the supply — the supply is d.c. and the commutator only reverses the current in the coil

Saying the forces on the two sides act in the same direction — they are opposite and form a turning effect

Thinking the turning effect is greatest when the coil is vertical — it is greatest when the coil is parallel to the field and zero when vertical

Mixing up motors and generators — a motor converts electrical energy to motion

Last updated: 18 October 2026