问题 计算题

如图1所示。一对平行光滑轨道放置在水面上,两轨道间距l=0.20m,电阻R=1.0 Ω;有一导体杆静止地放在轨道上,与两轨道垂直,杆及轨道的电阻皆可忽略不计,整个装置处于磁感强度B=0.50T的匀强磁场中,磁场方向垂直轨道面向下。现有一外力F沿轨道方向拉杆,使之做匀加速运动,测得力F与相同t的关系如图2所示。求杆的质量m和加速度a。

答案

解:导体杆在轨道上做匀加速直线运动,用v表示其速度,t表示时间,则有v=at

杆切割磁力线,将产生感应电动势,ε=Blv

在杆、轨道和电阻的闭合回路中产生电流I=

杆受到的安培力为f=IBL

根据牛顿第二定律,有F-f=ma

联立以上各式,得

由图线上取两点代入式,可解得a=10m/s2,m=0.1kg

阅读理解

阅读理解。

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1. What do you have to take to the American embassy if you request a visa for the first time?

A. A government form.

B. Banking and tax records.

C. Pictures and fingerprints.

D. Both A and B.

2. When should you apply for a visa?

A. The moment you have been accepted to a school in the US.

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A. When to get a visa.

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C. How to enter the US.

D. Ways to be accepted by American universities.

判断题