(2).電阻為R,長a寬b之四方迴路,以等速v,向右通過寬2b之均勻磁場B,則感應電流與時
    間之變化:
a.在      0< t < b/v  時:
  感應電動勢ε = Δø
B / Δt = ΔA B / Δt = (a vΔt) B / Δt = avB
  感應電流I = ε / R = avB / R (逆時)

b.在 b/v <,= t < 2b/v 時:
  感應電動勢
ε  = 0
  感應電流I = 0

c.在2b/v <,= t < 3b/v 時:
感應電動勢
ε  = Δø
B / Δt = ΔA B / Δt = (a vΔt) B / Δt = avB
感應電流I = ε / R = avB / R (順時)