How does a changing magnetic field induce a voltage, and how do generators and transformers use this?
Electromagnetic induction and the generator effect, the factors that affect the induced potential difference, how a generator produces a.c., and how step-up and step-down transformers change the voltage.
A focused answer to OCR Gateway GCSE Physics A topic P4 on electromagnetic induction, covering the generator effect, the factors that affect the induced potential difference, how a generator produces alternating current, and how step-up and step-down transformers change the voltage.
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What this topic is asking
OCR wants you to explain electromagnetic induction (the generator effect), the factors affecting the induced potential difference, how a generator produces alternating current, and how step-up and step-down transformers change the voltage. This is part of topic P4.2 of the OCR Gateway Physics A (J249) specification.
Electromagnetic induction (the generator effect)
The induced potential difference reverses if the magnet's motion reverses (moving it out instead of in), which is why pushing a magnet in and pulling it out gives opposite induced currents.
Factors affecting the induced potential difference
The size of the induced potential difference is increased by:
- moving the magnet (or coil) faster, so the field changes more quickly,
- using a stronger magnet, giving a bigger field change, and
- using a coil with more turns, so more of the wire is in the changing field.
This is why a dynamo on a bicycle wheel gives a brighter light when you pedal faster: the faster rotation changes the field more quickly and induces a larger voltage.
The generator
A generator (alternator) rotates a coil in a magnetic field (or rotates a magnet near a coil). As the coil turns, the magnetic field through it changes continuously, inducing a potential difference. Because the coil's sides move up through the field on one half-turn and down on the next, the induced voltage reverses every half turn, producing an alternating current (a.c.). This is how power stations generate the mains electricity supply.
Transformers
Transformers are essential to the national grid. A step-up transformer raises the voltage for transmission, which (for a given power, ) lowers the current. Because the power wasted heating the cables is , a smaller current wastes far less energy, so transmission is efficient. Step-down transformers then lower the voltage again to safe values for homes and businesses.
Try this
Q1. State three ways to increase the potential difference induced when a magnet is moved into a coil. [3 marks]
- Cue. Move the magnet faster, use a stronger magnet, use a coil with more turns.
Q2. State why a transformer does not work with a direct current. [1 mark]
- Cue. A steady d.c. gives a steady (unchanging) field in the core, so no voltage is induced in the secondary.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksDescribe how a potential difference can be induced by moving a magnet into a coil of wire, and state three ways to increase the size of the induced potential difference.Show worked answer →
A P4 question worth four marks on the generator effect. When a magnet is moved into (or out of) a coil, the magnetic field through the coil changes, which induces a potential difference across the coil (and a current if the circuit is complete) (1 mark). To increase the induced potential difference you can: move the magnet faster, use a stronger magnet, or use a coil with more turns (3 marks, one for each valid factor). Markers reward the changing field inducing a potential difference and any three of moving faster, a stronger magnet, and more turns. A common error is to say a stationary magnet induces a voltage; the field must be changing.
OCR 20214 marksA step-up transformer at a power station increases the voltage for transmission. Explain how a transformer works, and explain why increasing the transmission voltage reduces the energy wasted in the cables.Show worked answer →
A P4 question worth four marks. A transformer has two coils (primary and secondary) wound on an iron core. An alternating current in the primary coil produces a changing magnetic field in the core, which induces an alternating potential difference in the secondary coil (2 marks for the changing field in the core inducing a voltage in the secondary). A step-up transformer has more turns on the secondary, so it increases the voltage. For a given power, , so a higher voltage means a lower current (1 mark). Because the power wasted heating the cables is , a smaller current wastes far less energy as heat, so transmission is more efficient (1 mark). Markers reward the transformer mechanism, lower current at higher voltage, and the reduced heating loss.
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