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## Physics library

### Course: Physics library>Unit 13

Lesson 4: Magnetic flux and Faraday's law

# Lenz's Law

Lenz's Law - seeing that the magnetic field induced by a current induced by a change in magnetic flux (Faraday's Law) counteracts the change in flux.

## Want to join the conversation?

• at u hav shown that using right hand rule we get magnetic field in the opp direction of flux. but this production of field will not only be inside in the loop but also outside the loop i.e. there will be circular path of field around the sides of rectangle. what will happen to this extra field?
• The portion of the field that exists outside the loop will not affect the magnetic flux inside the loop, which is why Sal didn't discuss it. It will 'co-exist' without having any effect.
• Does the induced magnetic flux just 'try' and decrease the increase in flux or is it actually able to negate the entire change.
• Is the magnetic field created by the current the same magnitude as the original magnetic field, ie does it cancel it entirely?

I'm also a little confused by what the overall outcome is with this setup. If the current creates a counteracting magnetic field that would mean a change in flux, which would induce a new current going the opposite direction, which would then create another change in flux... and on and on. Does this back-and-forth go on forever, or does it reach some static equilibrium eventually?
• It reaches equilibrium almost immediately
You know how the lights dim a little bit when your refrigerator or air conditioner turns on?
That's because a lot of current flows while the motor gets going
Once it gets going, Lenz' law says it will create a back EMF that will oppose the original.
The sum of that back EMF and the original lead to a lower total voltage to the motor once it reaches speed, and less current flowing through the motor, and then the lights get bright again.
• How the current will go on increasing in the counter-clockwise situation? (Reason)
and should I consider it as an exception for right hand thumb rule?
• Hello Yashi,

Regarding the situation shown at . Here Sal is exploring what would happen if the current was to flow in a a counter clockwise direction. This was a though experiment - not real life. Such a situation could never happen in the real world as it would violate the laws of thermodynamics. It would lead to perpetual motion machines and free energy - nice ideas but they will never happen....

To answer your question, no. this is not an exception to the right hand rule. It is just an exploration that lead to the correct answer - current flows in opposition to that which caused it aka Len's Law.

Regards,

APD
• Won't a decrease in flux make the current decrease which would make the flux decrease even more. Doesn't this violate the conservation of energy?
• Lenz' law says that the direction of the induced current will oppose the change in flux that created it. It's not a matter of increasing and decreasing, it's a matter of direction. A decreasing flux creates an EMF that creates a current and that current will be in whatever direction it has to be to try to keep the flux from decreasing. An increase in flux will lead to a current that will try keep the flux from increasing. In both cases, it's consistent with conservation of energy. If it worked the other way, it would violate conservation of energy because an increasing flux would cause a current that would cause the flux to increase more, which would cause a current that would cause the flux to increase more...
• At the end, is the flux going to reach its original value?
• Could you make me understand the query better? Especially what you mean by 'origibal value'?
• In the second scenario Sal discussed, wouldn't the decrese in magnetic flux reverse the direction of current in the loop?
• Hello Kartikeya,

Correct!

The current will do whatever is necessary to oppose the change in magnetic flux. As Sal pointed out, to do otherwise is to violate the conservation of energy.

Regards,

APD
• wish it was elaborated what current would do if flux decreases
• At , Sal states that the magnetic field is additive...is this because the fingers loop clockwise (using RHR-1) ?
• It is using right hand rule #2, which relates current in a wire with magnetic field. The magnetic field wire due to the current in the wire will be directed upward, in the same direction as the original magnetic field that caused the current in the wire.
(1 vote)
• My textbook states that Len's law refers to induced currents and not to induced emf...what does this actually mean?........... Whereas in the topic of mutual induction it is stated that induced emf in the secondary coil is such that it opposes the time rate if change of current in the the primary coil..... Why they are using emf in the mutual induction instead of current then?
• Hello Sumaira,

Perhaps the answer is the context. I suspect your textbook talks about an ideal transformer with no load attached. I would be very confusing to talk about current in this situation! EMF in this unloaded circuit is considerably easier to understand.