7.2.4Magnetism and Electromagnetism
Electric Motors — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 10 parts Lesson 9 of 9
Part 1 of 8Principle of the d.c. motor
Motor effect
- Motor effect a current-carrying coil in a magnetic field experiences forces that make it turn.
Direction of turning
- Direction of turning the force on a coil causes it to rotate in a single direction.
Energy transfer
- Energy transfer electrical energy is transferred to kinetic energy, with some thermal energy from the resistance of the wire.
Part 2 of 8Parts of a simple motor
Parts of a simple motor
Coil
- Coil a rectangular coil of insulated wire, free to rotate on an axle.
Permanent magnets
- Permanent magnets opposite poles face each other to give a nearly uniform field across the coil.
Split-ring commutator
- Split-ring commutator a ring cut in two halves, each connected to one end of the coil, rotating with it.
Carbon brushes
- Carbon brushes two brushes touch the commutator halves and connect them to the d.c. supply.
Part 3 of 8How the coil turns
Opposite currents
- Opposite currents current flows in opposite directions in the two sides of the coil.
Opposite forces
- Opposite forces by Fleming's left-hand rule one side is pushed up and the other down, giving a turning effect.
Turning effect
- Turning effect the greater the force, the greater the turning effect and the faster the coil turns.
Part 4 of 8Action of the split-ring commutator (Extended)
1
The coil is horizontal and current flows from the brushes through the split ring into the coil.
2
Forces act upward on one side and downward on the other, so the coil rotates.
3
At the vertical position the brushes touch the gaps, no current flows and no force acts.
4
The momentum of the coil carries it past the vertical.
5
The ring halves swap brushes, which reverses the current in the coil.
6
The forces on each side reverse, so the coil keeps turning in the same direction.
- Without a commutator
- the coil would swing back and forth and stop.
Part 5 of 8Changing the motor
1Increase turning effect and speed
- Increase turning effect and speed a larger current, a stronger magnet, more turns on the coil or a larger coil area.
2Reverse the rotation
- Reverse the rotation reverse the current or reverse the magnetic field by swapping the magnet poles.
- Reversing both leaves the direction unchanged.
Part 6 of 8Practical motors
Several coils
- Several coils wound on a soft-iron core at different angles for a smooth turning effect.
Electromagnets
- Electromagnets some motors use electromagnets instead of permanent magnets.
Stalled motor
- Stalled motor the coil cannot turn, so it can overheat and be damaged.
Back e.m.f.
- Back e.m.f. the turning coil cuts field lines, inducing an e.m.f. that opposes the supply, so a running motor draws less current than at start-up.
Part 7 of 8Motor compared with generator
| Device | Input | Output | Ring type |
|---|---|---|---|
| d.c. motor | Electricity | Motion | Split-ring commutator |
| a.c. generator | Motion | Electricity | Slip rings |
Part 8 of 8More from the specification
1
Electric motor mechanical power
- Electric motor mechanical power electromagnetic torque spinning the armature delivers useful mechanical power, equal to angular velocity multiplied by motor torque.
2
The Motor Effect and DC Electric Motors
- Opposite magnetic forces on a current-carrying coil create a turning couple.
- A split-ring commutator reverses current every half-turn for continuous rotation.
3
Electromagnetic Induction in a Solenoid
- When a magnet is moved into a coiled conductor (solenoid), the moving magnetic field induces an electromotive force (voltage) across the coil ends, detected by a sensitive voltmeter or galvanometer.
4
Step-Up and Step-Down Transformers
- Alternating primary voltage creates alternating magnetic flux in the iron core.
- Mutual induction induces secondary voltage obeying Vp / Vs = Np / Ns.
5
Step-up transformer
- Transformer with more turns on the secondary, raising the voltage.
6
Step-down transformer
- Transformer with fewer turns on the secondary, lowering the voltage.
7
Electromagnetic induction
- Production of an e.m.f. by relative movement of a conductor and a magnetic field.
8
Transformer
- Device that changes the size of an alternating voltage.
9
Secondary
- Output coil.
10
Primary
- Input coil.
11
Solenoid
- Long coil of wire carrying a current
12
Conductor
- Material that allows current to flow.
13
Voltmeter
- Instrument that measures p.d., connected in parallel.
14
Alternating current
- Current that continually reverses direction.
15
Potential difference
- Work done by a unit charge passing through a component.
16
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
Key terms
| Term | Meaning |
|---|---|
| d.c. motor | Device that uses the motor effect to turn electrical energy into rotation. |
| Split-ring commutator | Ring cut in two that reverses the coil current every half turn. |
| Brushes | Carbon contacts that connect the supply to the commutator. |
| Motor effect | Force on a current-carrying conductor in a magnetic field. |
| Turning effect | Moment that makes the coil rotate. |
Avoid these mistakes
1
Saying the commutator keeps the current constant — it reverses the current in the coil every half turn.
2
Confusing the commutator (d.c. motor) with slip rings (a.c. generator).
3
Forgetting that the forces on the two sides of the coil are in opposite directions.
4
Assuming reversing both the current and the field reverses the motor — it does not.
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