题目

完形填空 Mrs. Brown was tired after her day's shopping in London, so she went into a restaurant for a cup of tea 1 catching the train home. When she had ordered her tea, she suddenly remembered that she had to buy some 2 for her husband, who had a cough. “Is there a chemist's near here?” she asked the waiter. “Yes, madam.” the waiter said. “It's only about three minutes' walk away. Turn right when you go out of the restaurant, then take the second turning to the left. You'll find a big chemist's about a hundred yards up the road on the right-hand side. It closes at five, but if y如图10所示，电源电压保持不变，R1=20Ω，闭合开关，电流表的示数为0.3A。断开开关，把电阻R2并联在R1两端，再闭合开关，电流表的示数变化了0.5A。求： （1）R2的阻值 （2）通电100S电阻R1产生的热量。