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如图所示，U形导线框内串联着阻值为R的电阻，另有阻值为R的均匀电阻丝MP跨在线框上，接线点N和O三等分长度为L的MP，磁感应强度为B的匀强磁场垂直线框平面，当MP以速率v向右匀速运动时,求：

(1)R上消耗的功率有多大？   (2)M、P间的电势差有多大？

答案

（1）B2L2v2/16R     （2）11BLv/16R

(1)当MP做匀速运动时，产生的感应电动势为E＝BLv，则电流I＝，

　所以R上消耗的功率P＝；

　(2)M，P两点间的电势差也就是R两端的电势差与MN、OP段上产生的感应电动势之和，即

　

　．

解答题

3______46≈3000

______02×3≈1800

6______5×4≈2800．
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单项选择题

One day while a girl was walking in the woods she found two starving songbirds. She took them home and put them in a small (16) She cared for them and felt great love for them.

Once the (17) and per of the two birds flew from the cage. Therefore, (18) he flew close, she grasped him (19) She felt glad, but suddenly she felt the bird go limp. She opened her hand and (20) in horror at the dead bird.

She noticed the other bird moving back and forth on the edge of the cage. She could feel his great need for (21) She lifted him from the cage and (22) him softly into the air. The bird circled once, twice, three times.

The girl watched delightedly at the bird’ s enjoyment. Her heart was no longer (23) with her loss. Suddenly the bird flew closer and (24) softly on her shoulder. It sang the sweetest melody that she had ever heard.

The fastest way to (25) love is to hold on it too tightly, while the best way to keep love is to give it WINGS!

20().

A.stared 

B.glared 

C.glanced 

D.shouted
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