Here is the question. A coil rotates at 200 revoltuions per second in teh earth's field. If the coil has a cross sectional area of 20cm^2 and has 1000 turns, approximately what emf is induced in the coil as a function of time? I know emf= int of E*dL and that the voltage proudces across a coil is V= NwBA sin(wt). I guess what i don't get is what would i use for my B or do i need to use a different equation.