问题 问答题

如图所示,电阻不计的平行金属导轨MN和OP水平放置,MO间接有阻值为R=5Ω的电阻,导轨相距为L=0.2m.其间有竖直向下的匀强磁场,磁感应强度为B=5T.质量为m=1kg的导体棒CD垂直于导轨放置并接触良好,其长度恰好也为L,电阻也为R.用平行于MN的恒力F向右拉动CD,CD棒与导轨间的动摩擦因数为0.2.已知CD棒运动中能达到的最大速度vm=10m/s,重力加速度g取10m/s2.试求:

(1)恒力F的大小;

(2)当CD达到最大速度时,电阻R消耗的电功率.

答案

(1)依题意 CD棒达到最大速度时应处于平衡状态.

    则 F=f+F   

    而f=μmg    

     F=BIL,I=

E
2R
,E=BLv

得到F=μmg+

B2L2v
2R

代入解得  F=3N.

    (2)当CD达到最大速度时,电阻R消耗的电功率

       P=I 2•R=(

BLv
2R
)2R

代入解得    P=5 W.     

答:(1)恒力F的大小为3N;

    (2)当CD达到最大速度时,电阻R消耗的电功率为5W.

阅读理解

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

My daughter Chloe is starting out in college in the fall. With her newfound independence will come the newfound responsibilities of dealing with money.  小题1: Now I have a few lessons I’d like to share with Chloe, and with anyone else starting out in college.

 小题2: OK, this is almost the only lesson you need. The biggest reason you get into money problems is that you spend money you don’t really have. Then you end up in the hole, and it’s hard to get out of the hole. You work too many hours to keep up with your spending. And you end up with a life that’s about nothing but trying to pay for all the spending on something you don’t really need.  小题3:

Make a very simple budget. It’s a terrible thing for people who’ve never done a budget, but it will become very simple and easy if you know how to do it.  小题4: If the bills add up to more than the income, remove some bills.

Pay bills right away. If you have the money, pay the bill as soon as it comes in. You can usually do this online, but if not, it’s just a matter of writing a check, putting it in an envelope, and writing out an address — two minutes.  小题5: If you let the bill-paying get pushed back, it becomes a terrible thing.

That’s all you need to know. If you save when you’re in college, avoid debt, and pay bills on time, you’re golden.

A.Why not enjoy your life in college?

B.Spend less than you earn.

C.So spend less, work less, worry less, be happier.

D.Like many young people, she hates thinking about money.E. First, list your income, and then list your bills.

F. Find ways to get things done you need to get done.

G. Do this two-minute action at once, so you don’t have to worry about it later.

计算题