P4.5.2P4. Electricity and Magnetism
The a.c. generator — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 9 parts Lesson 13 of 19 1 quiz
Lesson summary
generator — COMBINED SCIENCE notes: Describe the structure and operation of a simple a.c. generator and the graph of its output.A.c. generator = a device that converts energy from motion into an alternating electrical outputAn alternating e.m.f.
- P4.5.2 The a.c. generator — COMBINED SCIENCE notes
- Electromagnetic induction
- Alternating output
- Continuous output
- Energy transfer
- Maximum e.m.f
Part 1 of 7Purpose and structure
Purpose and structure
A.c. generator
A.c. generator a device that converts energy from motion into an alternating electrical output
- An alternating e.m.f. is generated and it causes an alternating current to flow.
Parts
- Parts a rotating coil of wire between the poles of a permanent magnet, with slip rings and brushes joined to an external circuit
| Component | Function |
|---|---|
| Permanent magnet | Provides a uniform magnetic field |
| Rotating coil | Cuts the magnetic field as it rotates so that a current is induced |
| Slip rings | Rotate with the coil and keep each end of the coil connected to the same terminal |
| Carbon brushes | Give a good electrical connection between the coil and the external circuit |
Part 2 of 7Operation
1Electromagnetic induction
- Electromagnetic induction the sides of the coil cut the magnetic field lines as it rotates, so an e.m.f. is induced
2Alternating output
- Alternating output the direction of the e.m.f. reverses every half turn
- One side of the coil moves up through the field and then down.
- A centre-zero galvanometer deflects one way, then the other, showing a constantly changing size and direction.
3Continuous output
- Continuous output the e.m.f. continues as long as the coil keeps turning in the same direction
4Energy transfer
- Energy transfer in power stations a turbine turns the coil or magnet, transferring kinetic energy to electrical energy
Part 3 of 7Coil position and e.m.f.
Maximum e.m.f.
- Maximum e.m.f. when the plane of the coil is parallel to the field
- The coil sides move perpendicular to the field and cut the greatest number of field lines.
Zero e.m.f.
- Zero e.m.f. when the plane of the coil is perpendicular to the field
- The coil sides move parallel to the field and cut no field lines.
Part 4 of 7One rotation of the coil
1
Start with the coil vertical and moving parallel to the field; the e.m.f. is zero.
2
After 90° the coil is horizontal and moving perpendicular to the field; the e.m.f. is a maximum.
3
After 180° the coil is vertical again; the e.m.f. is zero.
4
After 270° the coil is horizontal; the e.m.f. is a maximum in the opposite direction to that at 90°.
5
After 360° the coil is back at the start; the e.m.f. is zero and one full cycle is complete.
Part 5 of 7The e.m.f.-time graph
Shape
- Shape a sine or cosine curve of e.m.f. against time or angle of rotation
- Starting from a vertical coil (e.m.f. zero) gives a sine curve.
- Starting from a horizontal coil (e.m.f. maximum) gives a cosine curve.
One rotation
- One rotation gives one complete cycle with a positive peak and a negative peak
Part 6 of 7Factors affecting the output
Factors affecting the output
Faster rotation
- Faster rotation increases the e.m.f. and raises the frequency, giving more cycles per second
More turns on the coil
- More turns on the coil increase the e.m.f.
Stronger magnet
- Stronger magnet increases the e.m.f.
Soft iron core in the coil
- Soft iron core in the coil increases the e.m.f.
Alternative design
- Alternative design a magnet rotating within a coil also works, because the coil still experiences a changing magnetic field
Part 7 of 7More from the specification
1
Lorentz deflection electromotive force
- Lorentz deflection electromotive force relative motion between a magnetic field and wire exerts magnetic force on conduction electrons, generating induced electromotive force.
2
A.c. generator
- A.c. generator a coil rotating in a magnetic field cuts field lines, inducing an alternating voltage.
3
Electromagnetic induction
- Electromagnetic induction an e.m.f. is induced in a conductor when the magnetic flux linking it changes.
4
Factors for voltage
- Factors for voltage a stronger field, more turns and faster rotation all raise the induced voltage.
5
Slip rings
- Slip rings carry current from the rotating coil to the external circuit without reversing it, so the output alternates.
6
Transformer
- Transformer primary and secondary coils on an iron core change voltage in proportion to the turns, using induction in a solenoid.
7
Potential difference
- The work done by a unit charge passing through a component.
8
Step-up transformer
- A transformer with more turns on the secondary coil than the primary, raising the voltage.
9
Step-down transformer
- A transformer with fewer turns on the secondary coil, lowering the voltage.
10
Transformer
- A device with a primary coil and secondary coil on an iron core that changes an alternating voltage.
11
Solenoid
- A coil of wire that produces a magnetic field like a bar magnet when a current flows.
12
Voltage
- A measure of the electrical energy supplied or changed in a circuit.
13
Conductor
- Material that lets current flow.
14
Voltmeter
- Meter that measures voltage, connected in parallel.
15
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 |
|---|---|
| A.c. generator | A device with a coil rotating in a magnetic field that produces an alternating e.m.f. |
| Slip ring | A ring on the coil that keeps each end of the coil connected to the same brush. |
| Brush | A carbon contact pressing on a slip ring to connect the coil to the external circuit. |
| Alternating current | A current that repeatedly reverses direction. |
| Frequency | The number of complete cycles of the output per second. |
| Armature (rotating coil) | The coil that turns in the field and cuts the field lines. |
Avoid these mistakes
1
Confusing the maximum e.m.f. with the coil perpendicular to the field — the e.m.f. is zero then, and maximum when the coil is parallel to the field
2
Thinking the slip rings reverse the current — they only keep the connection, and the output reverses because the coil sides swap direction through the field
3
Confusing a generator with a motor — a generator takes in motion and gives electricity, and a motor takes in electricity and gives motion
4
Thinking faster rotation only raises the peaks — it also raises the frequency
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