题目

 D Jupiter is the largest planet in our solar system. It is also one of the most interesting. It was named after Jupiter, the most important of the Roman gods, and nicknamed the “King of Planets.” It is the fifth planet from the sun and the fourth brightest object in the sky. Before the invention of the telescope, Jupiter was thought to be a bright star. In 1610, the famous astronomer Galileo realised that Jupiter was actually a planet. He also noticed that it had four large moons. These are known as the Galilean moons. The biggest is Ganymede, and it is larger than the planets Pluto and （2011?天门二模）如图所示为两根间距不等的光滑金属导轨MN、PQ，它们水平放置在竖直向下的匀强磁场中．导轨的一端接入电阻R1=10Ω和电流表，另一端接入电阻R2=5Ω．质量为m=0.1kg的金属棒横放在导轨上．当它以初速度υ0=4m/s从ab处滑到a′b′处，同时t=0.08s．导轨间距Lab=0.4m，La'b'=0.8m．若金属滑动时电流表读数始终不变．不计电流表棒与导轨的电阻和摩擦．试求：（1）电流表的读数；（2）磁感应强度．