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问题 多选题

如图所示，正方形容器处在匀强磁场中．一束电子从a孔垂直进入磁场射入容器中，其中一部分从c空射孔射出，一部分从d孔射出，则下列说法正确的是（　　）

A．从两孔射出的电子速率之比为vc：vd．=2：1

B．从两孔射出的电子在容器中运动所用时间之比为tc：td=1：2．

C．从两孔时出的电子在容器中运动时的加速度大小之比ac：ad=1：2

D．从两孔射出的电子在容器中运动时的加速度大小之比ac：ad=2：1

答案

电子从c点射出，d为圆心，Rc=L，圆心角θc=

π
2

由R=

mv
qB
，得vc=
eBL
m

运动时间tc=

T
4
=
πm
2Be

电子从d点射出，ad中点为圆心，Rd=

1
2
L，圆心角θd=π

所以vd=

eBL
2π
，td=
T
2
=
πm
Be

故vc：vd=2：1，tc：td=1：2

电子做匀速圆周运动f洛=F向=ma a=

f
m
=
Bev
m
，

ac
ad
=
vc
vd
=
2
1
；

故选：AD．

实验题

某同学做“探究蒸发快慢与哪些因素有关”的实验，如图所示，在两块相同的玻璃片上分别滴上一滴水，观察图中情景可知该同学主要探究蒸发快慢是否与                             有关。

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问答题

A department store made an announcement in the local newspaper to launch a bonus sale promotion in July 2008. Under this promotion plan any consumer who bought commodities for RMB 200 yuan would be granted one bonus coupon with a particular number for a lucky-draw. The lucky-draw would be made publicly on 31 July 2008. During July Mr Zhou received five coupons for buying commodities totalling more than RMB 1,000 yuan in the department store.On 31 July 2008 the department store made the lucky-draw in public and announced all the numbers of coupons in winning the bonus.On 2 August 2008 the department store put a written notice on the notice board in the front of its premises, stating that the winners should come to cash the bonus before 10 August 2008, otherwise they should be regarded as automatically giving up the bonus.On 12 August 2008 Mr Zhou got to know the lucky number on one of his coupons was the same as the lucky number for the first class bonus of RMB 5,000 yuan. He went to the department store to cash the bonus but was refused by the department store on the grounds that he was too late to take the bonus.Required:Answer the following questions in accordance with the relevant provisions of the Contract Law of China and give your reasons for your answers:

(a) State the legal nature of the announcement for the bonus sale promotion by the department store. (2 marks)
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