问题 问答题

图中左边有一对平行金属板,两板相距为d,电压为U,两板之间有匀强磁场,磁感应强度大小为B0,方向与金属板面平行并垂直于纸面朝里.图中右边有一半径为R、圆心为O的圆形区域,区域内也存在匀强磁场,磁感应强度大小为B,方向垂直于纸面朝里.一质量为m的正离子沿平行于金属板面、垂直于磁场的方向射入平行金属板之间,沿同一方向射出平行金属板之间的区域,并沿直径EF方向射入磁场区域,最后从圆形区域边界上的G点射出.已知弧

FG
所对应的圆心角为θ,不计重力.

求:(1)离子速度v的大小;

(2)离子的电量q等于?

答案

(1)离子在平行金属板之间做匀速直线运动,

由平衡条件得:qvB0=qE0

已知电场强度:E0=

U
d

由①②式解得:v=

U
dB0

(2)离子在圆形磁场区域做匀速圆周运动,轨迹如图所示:

由牛顿第二定律得:qvB=m

v2
r

由几何关系得:r=Rcot

θ
2

解得:q=

mU
BB0Rdcot
θ
2

答:(1)离子速度v的大小为

U
dB0

(2)离子的电量q=

mU
BB0Rdcot
θ
2

阅读理解

阅读理解。

     Millions of Americans run to the bank or visit ATM when they need cash. They use credit cards when

they want to buy clothes, VCRs, or television sets.   

     But there is an underclass-people with low incomes and no credit history -who visit their neighborhood

pawnshops (当铺) when they need cash or a loan.   

     About 20 percent of the US population has no bank account. More than half of this group don't have credit

cards and cannot get bank loans.   

     "These people are borrowing an average of $50," said John P. Caskey of Swarthmore College in

Swarthmore, Pennsylvania."If you add up in terms of how much dollar value pawnshops provide, they don't

look very important. If you add up how much of the population they serve or the number of loans they make,

they are important."   

     Because they make loans, pawnshops are a type of bank, often calling themselves"the bank of the little

people."   

     Caskey and Swarthmore student Brian Zidmund in 1989 looked at the importance of pawnshops in the US

economy-the first serious study of the subject since the 1930s.   

     Their conclusion: pawnshops are the consumer's lender of last resort (最后贷款人).   

     Pawnshop customers typically cannot get credit at banks. They have poor credit records, low and unstable

incomes, or cannot maintain positive bank account balances.   

     Typically, pawnshop customers borrow relatively small amounts that traditional lenders are unwilling or

unable to provide on a secured basis.   

     "If you look at total consumer credit, the amounts provided by pawnshops remain small," Caskey said.

"They are lending mainly to low-income people. In terms of the population they serve, they're really important."   

     In 1988, there were about 6,900 pawnshops in the United States - one for every two commercial banks.

They made about 35 million loans, providing 1 percent of the nation's consumer credit.  

1. The best title for the passage would be ______.  

A. Credit Cards for the Poor       

B. Banks for the Poor   

C. Pawnshops vs. Banks         

D. Commercial Banks  

2. What do the underclass people do when they need cash?   

A. They go to local banks for help.

B. They apply for credit cards.  

C. They ask for a loan from large banks.

D. They apply for a loan in pawnshops.  

3. What can we learn about pawnshops?   

A. Poor people come to pawnshops as their last resort.   

B. Most people prefer pawnshops for their need of cash.   

C. Pawnshops are an important part of the state economy.   

D. Pawnshops are not important because they make up only 1 percent of the nation's consumer credit.  

4. According to John P. Caskey, pawnshops are important because ________.   

A. they provide great dollar value to the poor

B. they make big loans to a lot of people   

C. they are serving the majority of the population

D. they make a large number of loans to the poor

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