Induced emf?

Question

a 7.2 diameter wire coil is initially oriented so that its plane is perpendicular to a magnetic field of .63 pointing up
during .15 sec the field is changed to one of .25 T pointing down
Find the average induced emf?
I would really appreciate your help on this one.... its got me all confused.

Answer 1

emf=AdB/dt and
average emf =intergral { (AdB/dt) dt} from 0 to .15
=A [B(.15)-B(0) ]

Answer 2

Use v = − N ∂Φ/∂t. In this case, N = 1, and A (area of coil) is constant; you can also disregard the sign. Thus,

v = A dB/dt.

You don't know B as a function of time, but you can approximate the derivative as ΔB/Δt. Change in B,

ΔB = 0.63 − -0.25 = 0.88 T,

so ΔB/Δt = 0.88/0.15 = 5.86666... T/s. Now, just multiply this by area of coil in m² to get induced voltage. You forgot to state units when giving diameter; if, however, diameter were 7.2 cm, induced voltage would be 23.9 mV