Electromagnetic Induction
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| electromagnetic induction/ɪˌlektrəʊməɡˈnetɪk ɪnˈdʌkʃn/ | cảm ứng điện từ |
| Faraday's law/ˈfærədeɪz lɔː/ | định luật Faraday |
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:
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
$\varepsilon = -N\,d\Phi_B/dt$ — the rate of change of flux.
A coil in a very strong but steady magnetic field has a large induced EMF.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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).
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
$\varepsilon = \Delta\Phi_B/\Delta t = 0.6/0.3 = 2\ \text{V}$.
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.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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 ____.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
20 times 0.30 divided by 0.50 is 12 volts. Induction needs a changing flux.