Lenz's law
The induced current flows in the direction that opposes the change in flux that made it.
Last updated
Analogy
A flywheel pushing back when you try to change its speed.
Example
Magnet falls in.Induced current opposes it.It falls slower than free fall.
In practice
Dropping a magnet through a copper tube slows it, because induced currents push back.
Practice
Flashcards
- What does Lenz's law say?Induced current opposes the change in flux that causes it.
Interview questions
- Explain how a generator makes electricity using Faraday's and Lenz's laws.
- A coil rotates in a magnetic field
- Flux changes so emf is induced
- Lenz's law explains why more work is needed when current flows
Questions people ask
Why does Lenz's law have a minus sign?
The minus sign shows that the induced emf opposes the change in flux. If it helped the change instead, the effect would grow without limit and create energy from nothing, breaking energy conservation. The opposing effect means you must do work to induce current.
How do magnetic brakes work?
A moving magnet or metal disc induces eddy currents in nearby metal. By Lenz's law these currents create fields that oppose the motion, turning kinetic energy into heat without touching. Roller coasters and some trains use this for smooth, wear-free braking.