问题 计算题

如图所示,正方形线圈abcd在磁感应强度B=1T的匀强磁场中绕轴OO′以角速度ω=10π rad/s匀速转动,线圈共10匝,线圈的电阻值为5 Ω,ab=0.6 m,bc=0.6 m,负载电阻R=45 Ω.求

(1)电阻R在0.05 s内发出的热量;(结果保留两位小数)

(2)电阻R在0.05 s内流过的电荷量(设线圈从垂直中性面开始转动).

答案

(1) Q=5.76 J. (2) q=0.072 C.

题目分析: (1)矩形线圈abcd在匀强磁场中旋转产生正弦交变电流,电动势的峰值为

Em=nBSω=10×1×0.6×0.6×10π V=113.04 V

电流的有效值

所以0.05 s内R上发出的热量Q=I2Rt=5.76 J.

(2)矩形线圈旋转过程中产生的感应电动势的平均值

电流的平均值

0.05 s内电路流过的电荷量q=It=0.072 C.

点评:本题要注意交变电流的最大值和平均值的区别,在计算通过的电荷量的时候,只能用电压的平均值来计算,不能用电压的有效值.

阅读理解

A kind of little cars may be seen in the streets in the future. People will like this kind of small cars better than the big ones. The car is as small as a bike. But it can carry two people in it. Everybody can drive it easily, just like riding a bike. Even children and old people can drive them to schools or parks.

If everyone drives such cars in the future, there will be less pollution in the air. There will be more space for all the cars in cities, and there will also be more space for people to walk in the streets.

The little cars of the future will cost less money to buy and to drive. These little cars can go only 65 kilometers an hour, so driving will be safer. The cars of the future will be fine for going around the city, but they will not be useful for a long trip.

This kind of cars can save a lot of gas. They will go 450 kilometers, then they have to stop for more gas.

They are nice cars, aren’t they?

小题1:If you drive this kind of little cars for four hours, you can probably go      at most.  

A.260 kilometers

B.65 kilometers

C.450 kilometers

D.130 kilometers小题2:Why do these little cars have to stop after going 450 kilometers?

A.For more water.

B.For more gas.

C.To have a rest.

D.To charge (充电)小题3: Which of the following statements is NOT true?

A.Driving big cars can make the air dirtier.

B.The little cars can make more space for other cars and people.

C.These little cars will be useful for a long trip.

D.This kind of cars can save much gas.

单项选择题