# EMF induced via change in Area

• Just_some_guy

#### Just_some_guy

I have been studying electromagnetism this year and we have spoken about Faradays law of electromagnetic induction and eventually how the emf induced is equal to the negative time rate of change of magnetic flux

I noticed however that all examples include a time varying magnetic field, which produces a current and hence electric field and hence an emf

I was wondering however, if we had a uniform magnetic field and varied the area dA of some surface would this produce an emf. As no current or electric field would be produced? But there would be a change in flux?

This has perplexed me a little and I was hoping someone could shed some light on my trivial (or not) problem

Cheers

Never mind Mr. Walter Lewin has solved my issue

if we had a uniform magnetic field and varied the area dA of some surface would this produce an emf

Yes. I've done a classroom demonstration of this, by holding a loop of wire attached to a galvanometer, between the poles of a large electromagnet. By "squishing" the loop from a circle to a narrow ellipse, and then "unsquishing" it back to a circle, I got a noticeable current.

Never mind Mr. Walter Lewin has solved my issue
Professor Lewin is one of the best teachers!

Anyway, if you have learned about Lorentz force law, you should know that when the current loop changes area, the carriers inside experience relative motion and thus Lorentz force. This force would drive them as the emf. In fact, the classical Faraday dynamo is based on this principle.

I was over complicating things little bit I think, and in the process confused myself :)

Thank you for your help though :)