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## Homework Statement

An aluminum ring of radius r1 and resistance R is placed around the top of a long air-core solenoid with n turns per meter and smaller radius r2 as shown in Figure P31.7. Assume that the axial component of the field produced by the solenoid over the area of the end of the solenoid is half as strong as at the center of the solenoid. Assume that the solenoid produces negligible field outside its cross-sectional area. The current in the solenoid is increasing at a rate of ΔI / Δt.

## Homework Equations

## The Attempt at a Solution

A=pi(r2)

^{2}

ε=d[itex]\Phi[/itex]/dt = (1/2)μ

_{0}n[pi(r2)

^{2}](current in solenoid at increasing rate)

I = ε/R

I solved this problem using numbers using the same equations I have. I dont know what symbols to use to represent "current in solenoid at increasing rate". I tried ΔI / Δt - didnt work. I tried dI/dt - didnt work. I am entering the formula into a website and it has to take it strictly which is really ticking me off because I know how to do these types of problems but dont know "what they are looking for" ... grrr, someone please help me or give me suggestions :)