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例1如图1所示,水平放置的两平行金属导轨相距L=0．25m,电池的电动势E =6V,内阻不计,电阻R= 5Ω,匀强磁场的磁感应强度B竖直向下．K合上后,横放在导轨上的导体棒ab(与导轨垂直)在磁场力作用下由静止开始向右运动,导体棒与导轨间的滑动摩擦力f=0．15N,为使棒运动速度最大,B应为多大?此时最大速度v_m为多大?
Example 1 As shown in Figure 1, the two parallel metal rails placed horizontally are separated by L = 0.25m. The electromotive force of the battery is E = 6V. The internal resistance is not counted, and the resistance R = 5Ω. The magnetic induction B of the uniform magnetic field is vertically downward. After K closes, the conductor bar ab (perpendicular to the guide rail) on the guide rail begins to move to the right under the action of the magnetic field force. The sliding friction between the conductor bar and the guide rail is f=0.15N to make the rod move. What is the maximum speed, and how large should B be? What is the maximum speed v_m?