Induced Currents and Magnetic Forces
Introduced| English | Meaning |
|---|---|
| Lenz's law/ˈlentsɪz lɔː/ | An induced current or electromotive force acts in a direction that opposes the change in magnetic flux that produces it. |
| eddy currents/ˈedi ˈkʌrənts/ | Circulating currents induced within a conductor by a changing magnetic flux. |
The induced current always fights the change that made it
- Push a magnet at a coil and the coil pushes back.
- The induced current flows the way that opposes the incoming change.
- This stubborn opposition is Lenz's law 楞次定律.
- It is the minus sign in Faraday's law, and it comes from energy conservation.
Lenz's law: oppose the change
- The induced current makes its own magnetic field.
- That field points so as to fight the change in flux.
- Push a north pole in, and the coil's near face becomes a north pole to repel it.
- Pull it out, and the coil turns south to try to hold it back.
Watch the opposition
Push the magnet in and out and see the induced current reverse to always oppose the change.
The induced current in a coil always flows so that its magnetic field:
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Lenz's law: the induced field always opposes the change.
You push a north pole toward a coil. The coil's near face becomes a:
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To oppose the approach, the near face turns north and repels.
Why: you can't get free energy
- Suppose the induced current helped the magnet instead of opposing it.
- Then the magnet would speed up on its own, making even more current — free energy.
- That would break energy conservation, so nature forbids it.
- You must do work against the opposition to keep the flux changing.
Lenz's law is a direct consequence of the conservation of:
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If the current helped the change, you would get free energy — forbidden.
The opposition feels like a drag
- Because it opposes the motion, the induced current creates a resisting force.
- Move the magnet faster and the drag grows — you push harder, and more current flows.
- The mechanical work you do becomes the electrical energy induced.
- Opposition is exactly what makes induction an energy converter.
The induced current creates a force that opposes the motion causing it (a drag).
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The opposition acts as a drag; your work becomes electrical energy.
Select all true statements about Lenz's law.
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Opposes the change, from energy conservation, acts as a drag. It never helps the change.
Everyday uses of the drag
- Eddy currents 涡电流 induced in a metal disc brake a train smoothly, with no pads.
- Induction cooktops heat a pan by driving eddy currents in its base.
- Metal detectors sense the eddy currents a coin induces.
- Each turns a changing flux into heat or a braking force.
The swirling induced currents used for smooth braking are called ____ currents.
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Eddy currents brake trains and heat induction cooktops.
You push the north pole of a magnet toward a coil.
- By Lenz's law, the coil's near face becomes a north pole to repel it.
- You must do work pushing against that repulsion — that work becomes the induced electrical energy.
The induced current always opposes the change, never helps it. If you ever find an induced current that reinforces the change, you have the direction backward — check with Lenz's law, which guarantees energy conservation.
Lenz's law: the induced current always opposes the change in flux — the minus sign in $\varepsilon = -d\Phi_B/dt$. It follows from energy conservation (no free energy), so the opposition feels like a drag; eddy currents use it for braking and induction heating.
Write from a flux fall of 0.50 weber
- The north pole of a magnet is pulled away from a coil. The flux falls from 0.80 weber to 0.30 weber.
- Find the flux change and say which pole the near face becomes.
A north pole is pulled away from a coil and the flux falls from 0.80 weber to 0.30 weber. What is the flux change, and which pole does the near face become?
0.80 minus 0.30 is 0.50 weber. Lenz's law attracts the departing north pole, so the near face is south.
What happens as this north pole is pulled away?
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The decrease is 0.50 weber, and opposing that decrease makes the near face a south pole.