Physics 13.2 - Electromagnetic induction
A changing magnetic field can induce a voltage across a coil. Compare what changes its size and polarity, then explain why the magnetic effect of an induced current resists the original change. This is Higher-tier content.
A changing magnetic situation
Imagine a bar magnet beside a coil of wire. A stationary magnet can produce a magnetic field,
but that alone does not produce a continuous induced potential difference. The magnetic field
passing through the coil must change. This can happen when the magnet and coil move relative
to one another, when the field strength changes, or when the coil's orientation to the field
changes.
Induced potential difference
An induced potential difference is a voltage produced across a conductor because the magnetic
field through or around it is changing.
The potential difference can exist across the ends of an open coil. An induced current flows
only when those ends are joined in a complete circuit. Keeping voltage and current separate here
will also make transformers much easier to understand.
Size and direction of the induced potential difference
The size of the induced potential difference depends on how much the magnetic situation changes,
how quickly it changes, and how many turns of wire experience that change.
| Change to the arrangement | Effect on the induced potential difference |
|---|---|
| Move the magnet and coil faster relative to each other | Larger, because the field through the coil changes more quickly |
| Use a stronger magnetic field | Larger, because the change in field can be greater |
| Use more turns on the coil | Larger, because the change acts across more turns |
| Increase coil area while the same uniform changing field passes through the whole area | Larger, because the change links a larger area |
Increasing the diameter of a coil around a small bar magnet does not automatically increase the induced voltage: the extra area may contain little field or a returning field. The area comparison only applies under the stated uniform-field condition.
These statements compare arrangements in which other relevant factors are kept the same. If
several factors change together, use each one to build the comparison rather than guessing from
only one.
The direction of the induced potential difference is its polarity. If a complete circuit is
connected, this sets the direction of induced current.
- Reversing the relative motion reverses the induced potential difference.
- Reversing the magnetic field, for example by turning the magnet around, also reverses it.
- Reversing both gives two reversals, so the induced direction stays the same.
Worked example
A north pole is pushed into a coil and the centre-zero voltmeter deflects to the right. The same
north pole is then pulled out at the same speed. Predict the second reading. Then predict what
happens if the north pole is pushed in faster than before.
Solution
Pulling the same pole out reverses the relative motion, so the potential difference reverses and
the voltmeter deflects to the left. At the same speed and with the same field and coil, the ideal
comparison gives a similar size of deflection. Pushing the pole in faster keeps the original
direction but produces a larger rightward deflection because the field through the coil changes
more quickly.
Continue this lesson.
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