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1.B随时间t均匀变化,B=B0+kt. 例1 如图1,金属杆ab横跨在水平光滑且左端闭合的金属导轨上,两导轨间距L1=0.5m,ab距离导轨左端L2=0.8m,由导轨与ab构成的回路电阻R=0.2Ω,竖直向下的匀强磁场以△B/△t=0.2T/s的变化率均匀变化,且当t=0时,B0=1T.经多长时间,质量M=0.04kg的物体恰能被拉离地面?(g=10m/s2) 分析 要使作用在ab上的安培力方向向左,则电流方向应由b向a,由楞次定律知,磁感
1.B changes uniformly with time t, B = B0 + kt. Example 1 As shown in Figure 1, the metal rod ab is straddled on a metal rail with a smooth horizontal surface and a left end. The distance between the two rails is L1 = 0.5m, and the distance between the two rails is L2. = 0.8m, the loop resistance R=0.2Ω formed by the guide rail and ab, the vertical downward uniform magnetic field changes uniformly with the change rate of ΔB/Δt=0.2T/s, and when t=0, B0 =1T. How long does the mass M=0.04kg object just be pulled off the ground? (g=10m/s2) Analysis To make the Ampere force acting on ab go to the left, the current direction should be from b direction. a, Known by Lenz’s law, magnetic induction