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School Physics GCSE level Notes: Electricity-magnetism 11.5 The AC motor

GCSE level Physics exam revision notes: Electromagnetism

Motor effect of electric current: 11.5 How a simple a.c. AC electric motor works and uses of electric motors

[Author © Dr Phil Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics [electromagnetism - motor effect page updated Feb 28th 2026 *]

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INDEX physics notes: motor effect of an electric current & applications

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11.5 A simple a.c. electric motor

How does a simple a.c. AC electric motor work?

diagram of how an a.c. electric motor works gcse physics igcse slip rings brushes coil commutator magnet

Diagram of a simple a.c. electric motor

The a.c. power supply is connected to the rotating coils of the armature by two slip rings and carbon brush contacts.

Note the difference - two slip rings in an a/c. motor, instead of a split-ring commutator in a d.c. motor.

As the coil rotates, the direction of the current passing through the coil reverses direction.

When the coil has moved through half a turn, the direction of the current has reversed.

This ensures the force, produced by the interaction of the two magnetic fields, always operates in the same net direction, so the rotation of the coil is always in the same direction (clockwise or anticlockwise, depending on the poles of the permanent magnet).

 

The usefulness of electric motors (d.c. or a.c.)

Where do we start!

There are hundreds of uses in the home, in industry and transport systems.

They speed up many processes once done by manual labour, and increase the efficiency of machines once powered by coal (eg steam locomotives) or wind (eg windmills).

Electric motors power semi- or fully-automated production lines e.g. packaging of food, car assembly lines etc.

You can build electric motors of any required power from little motors in toys to powerful machines in heavy industry.

Powerful electric motors (ac or dc) drive electric trains.

Electric cars are slowly replacing fossil fuelled cars.

Appliances in the home powered by electric motors:

microwave cooker, food mixer, hair dryer, toothbrush, CD player,

we would be rather lost without all our electrically powered 'gadgets'!

An a.c. AC electric motor powered locomotive

This British Railways 'INTERCITY' electric locomotive has a maximum speed of 140 mph (225 km/hour) derived from a 4700 kW a.c. electric motor powered from 25 kV overhead power lines. Note that electric motors can be a.c. or d.c. current driven.

 

INDEX physics notes: motor effect of an electric current and applications

Key points about electromagnetism - motor effect of electric current - simple AC motor

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

Here's a structured and exam-board-aligned summary on how a simple a.c. (alternating current) motor works and the factors controlling its power, tailored for IGCSE/GCSE Physics students across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

How a Simple A.C. Motor Works

Basic Principle of a simple a.c. motor

An a.c. motor converts electrical energy into mechanical (rotational) energy using the motor effect, but it is powered by alternating current, which reverses direction periodically.

Key Components of a simple a.c. motor

  • Coil of wire: Placed in a magnetic field and carries alternating current.
  • Permanent magnets or electromagnets: Provide a magnetic field.
  • Slip rings: Maintain continuous electrical contact while allowing the coil to rotate freely.
  • Carbon brushes: Press against the slip rings to transfer current to the rotating coil.

Step-by-Step Operation of a simple a.c. motor

  1. A.C. current flows through the coil, creating a magnetic field.
  2. This field interacts with the magnetic field from the magnets.
  3. The motor effect causes a force on each side of the coil (use Fleming’s Left-Hand Rule).
  4. The coil rotates due to the opposing forces on either side.
  5. As the current alternates, the direction of the force also alternates, but the coil continues to rotate in the same direction due to momentum and the design of the motor.

Factors Controlling A.C. Motor Power

Factor Effect on Power/Speed
Current (I) Higher current = stronger magnetic field = greater force = faster rotation
Magnetic field strength (B) Stronger magnets increase the force on the coil
Number of coil turns More turns = greater total force
Frequency of a.c. supply Higher frequency can increase torque and smoothness of rotation
Voltage of power supply Higher voltage increases current (if resistance is constant)
Coil resistance and friction Higher resistance or friction reduces efficiency and speed

Typical Exam Board Syllabus content on a simple a.c. motor

Key Content
Focuses on d.c. motors; a.c. motors may be referenced in extension or comparison
Covers motor effect and rotating coils; a.c. motors may be discussed in context
Emphasises electromagnetic applications; a.c. motors may be included in higher-tier
May include a.c. motors in practical applications or comparison with d.c. motors
Covers motor effect and rotating coils; a.c. motors may be referenced in context
Includes rotating coils in magnetic fields and slip ring operation

Note: A.C. motors are less commonly examined than d.c. motors at GCSE level, but understanding the contrast is useful for higher-tier and IGCSE students.

Student tips on a simple a.c. motor

  • Use Fleming’s Left-Hand Rule to determine force direction at any moment.
  • Understand the difference between slip rings (a.c. motors) and split-ring commutators (d.c. motors).
  • Draw and label diagrams of both motor types to compare structure and function.
  • Explore simulations to visualise how alternating current affects rotation.
  • Practice explaining how the motor continues to rotate even as current reverses.

Keywords, phrases and learning objectives on the motor effect in a simple a.c. AC motor

Be able to interpret a diagram to explain how an a.c. AC electric motor works.

Be able to describe some uses of an a.c. AC electric motor.

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INDEX physics notes: motor effect of an electric current and applications

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