BLv是导出公式，原来的公式是E=dΦ/dt=d(BS)/dt如果B和L不变d(BS)/dt=BLds/dt=BLv如果B也在变化，d(BS)/dt不等于BLv本题E=dΦ/dt=d(BS)/dt=d(kt*l*vt)/dt=2klvt
