问题 选择题

如图所示,PQ、MN是两条平行金属轨道,轨道平面与水平面的夹角为θ,轨道之间连接电阻R。在空间存在方向垂直于轨道平面的匀强磁场。金属杆ab以一定的初速度沿轨道从底端上滑的过程中,金属棒克服重力做功W1,动能的减小量为ΔE,回路中的电流产生的热量为Q1,金属杆与轨道间摩擦产生的热量为Q2。则下列关系式中正确的是(         )

A.  W1+Q1=ΔE+Q2    B.W1+ΔE=Q1+Q2   C.W1-ΔE=Q1+Q2   D.W1+Q1=ΔE-Q2

答案

D

题目分析:重力、安培力、摩擦力都对物体做负功,跟据动能定理可得

W1+Q1=ΔE-Q2,应该选D

单项选择题 A1/A2型题
填空题

You might think that hamburgers were invented in the United States, but that is not totally true. These (1) , or patties, actually came from Germany in the middle of the nineteenth century. They were brought to the United States by (2) who came from the city of Hamburg. That is why (3) was "hamburger steak".

However, people in other places (4) that they invented the hamburger. Perhaps we’ll never have a (5) . But there’s no question that the hamburger was a (6) . Why Perhaps because at that time, industry was growing and a kind of (7) food was needed for workers.

The hamburger became even more popular (8) when the first chain of fast food restaurants was started. This chain was called "White Castle". It served tiny hamburgers that were sold for only (9) . Then, in the 1930s there came the (10) where customers were served in their cars by waiters in uniform. And the hamburger was one of the most (11) .

By now, The hamburger was ready to (12) the world. And this happened with McDonald’s, which was actually a (13) at first. But by the early 1950s the hot dog was replaced by the hamburger. McDonald’s and other fast food restaurants (14) around the world throughout the rest of the twentieth century. McDonald’s alone has (15) for everyone person in the world.

The importance of the hamburger to (16) remains significant. About (17) of all sandwiches that are eaten are hamburgers. According to some sources, (18) of current workers in the United States had their first job at McDonald’s. But the face of the hamburger is changing (19) . Nowadays it is possible to buy a chicken burger, a turkey burger, (20) , or veggie burger.

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