Electromagnetic Induction
Introduced| English |
|---|
| electromagnetic induction/ɪˌlektrəʊməɡˈnetɪk ɪnˈdʌkʃn/ |
| Faraday's law/ˈfærədeɪz lɔː/ |
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:
$\varepsilon = -N\,d\Phi_B/dt$ — the rate of change of flux.
A coil in a strong but steady magnetic field has zero induced EMF.
No change in flux means no induced EMF, however strong the field.
A single loop's flux changes by $0.6\ \text{Wb}$ in $0.3\ \text{s}$. Find the average EMF (in V).
$\varepsilon = \Delta\Phi_B/\Delta t = 0.6/0.3 = 2\ \text{V}$.
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.
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?
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 ____.
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.