Find the time δt it takes the magnetic field to drop to zero. express your answer in terms of some or all of the quantities a, b0, i, n, and r.

Δt= 

This because ε = dΦ/dt, and emf is a voltage so it can also be written as dΦ/dt = IR

Because ΔΦ= ABcosθ for each of the N loops and cosθ is just 1 here only A needs to be replaced, and it can be replaced with A=πr^2, which makes that part of the equation ΔΦ_m=N*B0*πa^2