Electromagnetic Induction
Introduced| English | Meaning |
|---|---|
| electromagnetic induction/ɪˌlektrəʊməɡˈnetɪk ɪnˈdʌkʃn/ | The production of an electromotive force when magnetic flux linkage changes. |
| Faraday's law/ˈfærədeɪz lɔː/ | The induced electromotive force equals the negative rate of change of magnetic flux linkage. |
Move a magnet near a coil and you make electricity from nothing
- Push a magnet into a coil and a current appears — with no battery.
- Pull it out and the current reverses.
- A changing magnetic flux drives the current: electromagnetic induction 电磁感应.
- This single effect powers almost every generator on Earth.
Faraday's law
- The induced EMF equals how fast the flux changes: $\varepsilon = -N\dfrac{d\Phi_B}{dt}$.
- $N$ is the number of turns; the faster $\Phi_B$ changes, the bigger the EMF.
- A steady flux ($d\Phi_B/dt = 0$) induces nothing.
- This is Faraday's law 法拉第定律 — the heart of the unit.
Induce a current
Move the magnet faster or slower and watch the induced current grow and reverse.
Faraday's law says the induced EMF depends on:
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$\varepsilon = -N\,d\Phi_B/dt$ — the rate of change of flux.
A coil in a very strong but steady magnetic field has a large induced EMF.
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False. Induced EMF depends on the rate of change of flux. A steady field, however strong, gives no change in flux and so no EMF.
A single loop's flux changes by $0.6\ \text{Wb}$ in $0.3\ \text{s}$. Find the average EMF (in V).
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$\varepsilon = \Delta\Phi_B/\Delta t = 0.6/0.3 = 2\ \text{V}$.
What makes a bigger EMF
- More turns $N$ in the coil — each loop adds its own EMF.
- A stronger magnet — more flux to change.
- A faster motion — a quicker change of flux.
- Slow, weak, or few-turn setups give only a tiny EMF.
Select all the ways to get a bigger induced EMF.
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More turns, faster motion, stronger field all raise the EMF. Holding still gives zero.
Many ways to change the flux
- Move a magnet toward or away from the coil.
- Move the coil through a field.
- Rotate the coil in the field (this makes AC generators work).
- Change a nearby current, whose field threads the coil.
Which of these does not induce an EMF in a coil?
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A steady field with no motion gives no flux change — no EMF.
This is how generators work
- Spin a coil in a magnetic field and its flux changes continuously.
- That induces an alternating EMF — the mains electricity in your home.
- Wind, water, and steam turbines all just spin such coils.
- Induction turns motion into electrical energy.
A device that spins a coil in a field to make electricity is a ____.
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A generator uses induction to turn motion into electricity.
A single loop's flux changes by $0.4\ \text{Wb}$ in $0.2\ \text{s}$. Find the average EMF.
- $\varepsilon = \dfrac{\Delta\Phi_B}{\Delta t} = \dfrac{0.4}{0.2}$.
- $\varepsilon = 2\ \text{V}$ (magnitude).
It is the rate of change of flux that matters, not the flux itself. A coil sitting in a huge but steady field has a large flux and yet zero induced EMF. No change, no EMF.
Electromagnetic induction: a changing magnetic flux drives an EMF, given by Faraday's law $\varepsilon = -N\,d\Phi_B/dt$. A bigger EMF comes from more turns, a stronger field, or faster change. A steady flux induces nothing — this is how generators make electricity.
Write from an induced EMF of 12 volts
- A coil of 20 turns has its flux change by 0.30 weber in 0.50 seconds.
- Find the average EMF magnitude. Say what a steady flux induces.
A 20-turn coil has a flux change of 0.30 weber in 0.50 seconds. What is the average induced EMF, and what EMF does a steady flux induce?
20 times 0.30 divided by 0.50 is 12 volts. Faraday's law needs a changing flux.
What is the average induced EMF?
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20 times 0.30 divided by 0.50 is 12 volts. Induction needs a changing flux.