4.5.5Electricity and Magnetism
The D.c. Motor — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 9 parts Lesson 60 of 65 1 quiz
Lesson summary
Motor — IGCSE PHYSICS notes: Explain why a current-carrying coil in a magnetic field experiences a turning effect and describe the d.c.
- 4.5.5 The D.C. Motor — IGCSE PHYSICS notes
- Opposite forces
- Greatest and zero
- Split-ring commutator
- Reversing the rotation
- Faster rotation and stronger force
Part 1 of 7Turning effect of a coil
Opposite forces
- Opposite forces current flows in opposite directions in the two sides of a rectangular coil,
- so the forces are opposite, up on one side and down on the other.
Turning effect
- Turning effect the forces produce a turning effect (moment) on the coil.
Greatest and zero
- Greatest and zero the turning effect is greatest when the coil plane is parallel to the field and zero when it is vertical.
Part 2 of 7Increasing the turning effect
1
More turns on the coil.
2
A larger current.
3
A stronger magnetic field.
4
A larger coil area.
Part 3 of 7Structure of the d.c. motor (Extended)
Structure of the d.c. motor (Extended)
Coil
- Coil on an axle between the poles of a permanent magnet.
Split-ring commutator
- Split-ring commutator a metal ring split in two, with each half connected to one end of the coil.
Brushes
- Brushes fixed carbon brushes press on the rotating commutator and connect to the d.c. supply.
Part 4 of 7One rotation of a d.c. motor (Extended)
1
Current flows through the brushes and commutator into the coil.
2
Opposite currents in the two sides give opposite forces (left-hand rule), producing a turning effect.
3
The coil rotates, with the largest turning effect when the plane is parallel to the field.
4
At the vertical position the brushes touch the gap and no current flows, and momentum carries the coil on.
5
The commutator reverses the current in the coil, so the forces keep turning it the same way.
6
The cycle repeats every half turn, giving continuous rotation.
Part 5 of 7Changing the motor
Changing the motor
Reversing the rotation
- Reversing the rotation reverse the current or the field.
Faster rotation and stronger force
- Faster rotation and stronger force a larger current, a stronger magnet or more turns.
Direction of rotation
- Direction of rotation found by applying Fleming's left-hand rule to each side of the coil.
Part 6 of 7Practical motors and energy
Several coils
- Several coils a practical motor uses many coils at different angles and curved poles or a soft-iron core for a smoother, stronger turning effect.
Energy transfer
- Energy transfer electrical to kinetic, with some wasted as thermal energy in the coil.
Uses
- Uses fans, drills and cars.
Part 7 of 7More from the specification
1
D.C. motor
- D.C. motor a coil in a magnetic field carrying current experiences opposite forces on its sides, making it rotate.
2
Fleming's left-hand rule
- Fleming's left-hand rule gives the direction of force from the field, current and motion directions.
3
Commutator
- Commutator a split ring that reverses current every half turn so the coil keeps turning in one direction.
4
Induced voltage
- Induced voltage a conductor cutting magnetic field lines has an e.m.f. induced, which is the principle of generators.
5
Transformer
- Transformer primary and secondary coils on an iron core change a.c. voltage in the ratio of their turns, with power conserved ideally.
6
D.C. motor
- D.C. motor a coil in a magnetic field carrying current experiences opposite forces on its sides, making it rotate.
7
Fleming's left-hand rule
- Fleming's left-hand rule gives the direction of force from the field, current and motion directions.
8
Commutator
- Commutator a split ring that reverses current every half turn so the coil keeps turning in one direction.
9
Induced voltage
- Induced voltage a conductor cutting magnetic field lines has an e.m.f. induced, which is the principle of generators.
10
Transformer
- Transformer primary and secondary coils on an iron core change a.c. voltage in the ratio of their turns, with power conserved ideally.
11
Generator
- Device that turns kinetic energy into electrical energy.
12
Field lines
- Lines showing the direction and strength of a magnetic field.
13
Induced magnet
- Material temporarily magnetised in a magnetic field
14
Potential difference
- Work done by a unit charge passing through a component.
15
Step-up transformer
- Transformer with more turns on the secondary, raising the voltage.
16
Step-down transformer
- Transformer with fewer turns on the secondary, lowering the voltage.
17
Electromagnetic induction
- Production of an e.m.f. by relative movement of a conductor and a magnetic field.
18
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
19
Transformer
- Device that changes the size of an alternating voltage.
20
Secondary
- Output coil.
21
Primary
- Input coil.
22
Solenoid
- Long coil of wire carrying a current
23
Conductor
- Material that allows current to flow.
24
Voltmeter
- Instrument that measures p.d., connected in parallel.
Key terms
| Term | Meaning |
|---|---|
| d.c. motor | Device that turns electrical energy into rotation. |
| Commutator | Split ring that reverses the coil current every half turn. |
| Brushes | Fixed carbon contacts on the commutator. |
| Turning effect | Moment that rotates the coil. |
| Armature coil | Coil that rotates in the motor. |
Avoid these mistakes
1
Saying the coil stops at the vertical position because of friction — the commutator gap cuts the current and momentum carries it on.
2
Forgetting that the commutator reverses the current every half turn.
3
Thinking the forces on the two sides are in the same direction — they are opposite.
4
Mixing up a motor with a generator — a motor takes in electricity and gives motion.
Marked practiceQuizzes for this lesson
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