# Lenz's Law problem

I'm reviewing for a midterm and I'm stuggling with this question because I'm not sure what equation would be appropirate. I believe that it is Lenz's Law, but I need some suggestions as to what equation to use.

Question:
To monitor the breathing of a hospital patient, a thin belt is girded around the patient's chest. The belt is a 213 turn coil. When the patient inhales, the area encircled by the coil increases by 39.0 cm2. The magnitude of the Earth's magnetic field is 50.0 µT and makes an angle of 28.0° with the plane of the coil. Assuming that a patient takes 1.80 s to inhale, find the average induced emf in the coil during this time.

any help would be appreciated!

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Doc Al
Mentor
Look up Faraday's law. That's the one you need.

GmL
Yeah, Doc Al is correct.

You know that emf= - (d/dt) (magnetic flux)

and since n=213, B=50x10^-6, and theta=62 degrees (remember, you're finding the angle between the area vector A, which is perpendicular to the loop, and B), we can solve.

the n, cos(theta), and B all come out of the derivative, leaving dA/dt, which is trivial to solve. And thus, you have the answer.