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The motor effect turns electricity into movement. The generator effect does the exact opposite: it turns movement into electricity. Move a wire through a magnetic field — or change the magnetic field passing through a coil — and a potential difference appears across the wire, ready to drive a current. This single idea, discovered by Michael Faraday in 1831 and called electromagnetic induction, is how almost all of the world's electricity is generated, whether in a coal power station, a wind turbine or a hydroelectric dam. It also runs the bicycle dynamo, the microphone and the read-head of older hard drives. This lesson, part of Topic P4 (Magnetism and magnetic fields) of OCR Gateway Science A, explains the generator effect, the use of Fleming's right-hand rule, the factors that change the induced potential difference, and the all-important rule that the induced effect always opposes the change that caused it.
By the end of this lesson you should be able to describe electromagnetic induction (the generator effect), use Fleming's right-hand rule, state the factors affecting the size of the induced potential difference, and explain that the induced current opposes the change producing it.
The generator effect, or electromagnetic induction, is the production of a potential difference (p.d.) — and a current, if the circuit is complete — when there is relative movement between a conductor and a magnetic field. There are two equivalent ways to bring this about:
In both cases the key requirement is change: the wire must be cutting field lines, or the field through the coil must be changing. If nothing moves and the field is steady — if the wire sits still in the field, or a magnet rests motionless inside a coil — then no potential difference is induced at all. A changing situation is essential.
If the conductor is part of a complete circuit, the induced potential difference drives an induced current around it. If the circuit is open, a potential difference still appears across the ends, but no current flows.
You can demonstrate the effect by connecting a coil to a sensitive galvanometer (a centre-zero meter) and pushing a bar magnet into the coil: the needle flicks one way as the magnet goes in, returns to zero when the magnet is held still inside, and flicks the other way as the magnet is pulled out.
Exam Tip: Induction needs change: a wire cutting field lines or a changing field through a coil. A wire stationary in a field, or a magnet held still in a coil, induces no potential difference. The word examiners look for is "cutting" the field lines (or a "changing" field).
The direction of the induced potential difference depends on the direction of the movement and the direction of the field:
This is why moving a magnet in and out of a coil repeatedly produces an alternating potential difference — the direction flips each time the motion reverses, which is the basis of the a.c. generator in the next lesson.
For a wire being moved through a field, the direction of the induced current can be found with Fleming's right-hand rule (note: the right hand, not the left). Holding the thumb and first two fingers of the right hand at right angles:
graph TD
A["Right hand: fingers at right angles"] --> B["First finger = Field (N to S)"]
A --> C["Thumb = Motion of the wire"]
A --> D["Second finger = induced Current"]
Exam Tip: Use the right hand for the generator effect (a wire being moved to generate a current) and the left hand for the motor effect. A handy memo: geNerator and Right both feel "natural" together once you have separated them from the motor's left hand.
The size of the induced potential difference can be made larger in several ways. In every case, the bigger or faster the change, the bigger the induced p.d.:
| To increase the induced potential difference… | …because |
|---|---|
| Move the wire or magnet faster | The field lines are cut more quickly, a faster change |
| Use a stronger magnet (greater flux density) | A stronger field means more field lines are cut per second |
| Use more turns on the coil | Each turn adds to the induced p.d., so many turns give much more |
| Increase the area of the coil | A bigger coil links more of the field, a bigger change |
The unifying idea is rate of change: anything that makes the magnetic field through the coil change more, or more quickly, increases the induced potential difference. Moving the magnet faster is the most commonly tested factor — do it slowly and the meter barely flickers; do it quickly and the needle kicks hard.
Exam Tip: To increase the induced p.d.: move faster, stronger magnet, more turns, bigger coil. The common thread is a bigger or faster change in the magnetic field — "rate of cutting field lines".
There is a beautiful and important rule about the direction of the induced current: the induced current always acts to oppose the change that produced it. (This is Lenz's law, though you do not need the name.) The induced current sets up its own magnetic field, and that field always pushes against whatever change is being made.
For example, when you push the north pole of a magnet into a coil, the induced current flows in the direction that makes the near end of the coil also become a north pole — which repels the incoming magnet and so opposes its motion. When you then pull the magnet out, the induced current reverses so that the near end becomes a south pole, attracting the magnet and again opposing its motion (this time the motion away).
This "opposition" is not just a curiosity — it is a consequence of the conservation of energy. Because the induced current opposes the motion, you have to do work to keep the magnet (or wire) moving against that opposing force, and it is precisely this work that is transferred into electrical energy. If the induced current helped the motion instead, you would get electrical energy for free and energy would not be conserved — which is impossible.
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