# What happens if I have a square conducting wire being permeated by a magnetic field and the field suddenly disappears?

Suppose there is a square conducting wire in a magnetic field. The two vertical branches will have the same emf, and the two horizontal branches will have 0 emf, resulting in a net emf of zero and zero current. Now, suppose the magnetic field disappears. Faraday's law says that there should be an induced emf. My question is, how does that work? Wouldn't the two opposite branches still have the same emf, leading to a net of zero and no current?

I really hope the answer is something that'll make me smack my head, because it's bugging me no end.

Thank you!

• Is the magnetic field time-varying before switch off? Is the loop moving relative to the field? What exactly is the field's orientation relative to the loop? A sketch would be helpful. – WetSavannaAnimal Aug 1 '15 at 15:04
• @WetSavannaAnimalakaRodVance No, it's a constant field; nothing is happening, nothing is moving. I've put up a sketch. I hope that helps. – Gauri Aug 1 '15 at 15:57
• I think it'd be better if you rephrased your question to: "What happens when there is a sudden spike or dip in the Magnetic flux through a closed loop?" (That is what is happening here. The field rapidly changes from a non zero value to 0. The title would then be more generalized.) – Hritik Narayan Aug 1 '15 at 16:06
• @HritikNarayan Do you know what would happen in the scenario I've presented? – Gauri Aug 1 '15 at 16:24
• There is a sudden decrease in the flux in the direction of the magnetic field. Intuitively, I think there should be a sudden current pulse to counter the resulting change in the flux (you can find the direction with Lenz's Law.) I'm not quite sure about this though, so I'm leaving it upto someone else. – Hritik Narayan Aug 1 '15 at 16:40

The situation you posed is an ideal situation or somewhat fantasy in my view.Even though consider a loop with a particular magnetic flux and of course it is not possible for the magnetic field to not be there at the the time it is there so some change in time will be there but infinitely small.This would be the situation when rate of change of magnetic flux would be $$\frac{-d¢}{dt}$$ where ¢ is magnetic flux.This is a large value so you may think emf to be large that the impulse on electrons would be so large possible for the electrons to get thrown tangential to induced electric field lines where possible.A huge amount of heat may be produced to burn the wire.It may not be so easy to create such a situation since by Following lens law you will also have to encounter such a huge force when you are changing flux.To my imagination it may work like seatbelt of a car.