一匀磁场,磁场方向垂直于xy平面,在xy平面上,磁场分布在以O为中心的一个圆形区

题型:计算题

问题:

一匀磁场,磁场方向垂直于xy平面,在xy平面上,磁场分布在以O为中心的一个圆形区域内。一个质量为m、电荷量为q的带电粒子,由原点O开始运动,初速为v,方向沿x正方向。后来,粒子经过y轴上的P点,此时速度方向与y轴的夹角为30°,P到O的距离为L,如图所示。不计重力的影响。求磁场的磁感强度B的大小和xy平面上磁场区域的半径R。

考点:带电粒子在匀强磁场中的运动
题型:计算题

关于报表数据源设置,以下说法正确的是______。

A.可以是任意对象

B.只能是表对象

C.只能是查询对象

D.只能是表对象或查询对象

题型:计算题

Below is a housing guide for students going to London.

University accommodation (住所)offices

Many university accommodation offices have their own list of registered landlords (房东). Others also provide information on accommodation agencies and other housing organization. The advantage of using your university accommodation office is that you can get some support if you have a problem. The disadvantage is that they are unluckily to have enough registered landlords to houses all their students.

Property papers: Loot and Renting

Loot is an important source (来源) of information about private housing for co-renters. The offers are from private landlords, agencies and individuals looking for other co-renters. They also have a website: www.loot.com. The advantage of using Loot is that there are some excellent bargains. The disadvantage is that there is no quality control over the offers.

Renting is another useful paper. The offers in this paper are mainly from accommodation agencies. Their website is at www. renting.co.uk.

Accommodation agencies

The majority of rented accommodation in London is probably advertised through accommodation agencies. The advantage of using accommodation agencies is that you will have access to a large number of accommodations. A good agent will listen to your requirements and can save you time in looking for the right accommodation. The disadvantage is that they will make a range of charges to potential renters.

Noticeboards

Around the universities you will find a number of noticeboards where offers of accommodation will be posted. These will either be from landlords or from students. Some universities will also have online noticeboards where students can advertise to other students. Advertisements from students can be an excellent way to find accommodation. However, advertisements from landlords can be problematic. 

Word of mouth

Some of the best housing in London is never advertised but is passed on from one group of students to another by word of mouth. It might be that you can find out about good offers from final year students. However, don’t suppose that just because you have found out about housing from a friend it is necessarily going to be better than that found through any other source.

Family

Faced with the very high rents charged in London, some students and their parents will consider buying as an alternative. In some cases this might be a good choice.

小题1: What is the advantage of using Loot?

A.It has more offers from accommodation agencies than Renting.

B.It gives you personal information about other co-renters.

C.Their website is designed mainly for students.

D.There are some good bargains.小题2:The information passed on by word of mouth is important because     .

A.it is better than that found through any other source

B.it helps you find some of the best housing never advertised

C.the landlords have little valuable information

D.the final year students always offer better information小题3:For students going to London for the first time, which of the following provides the most reliable information?

A.University accommodation offices.

B.Loot and Renting.

C.Notice boards.

D.Family.

题型:计算题

巡航表速250千米/小时(含)以下的航空器,在机场区域内目视飞行的最低安全高度距离最高障碍物的真实高度不得小于()米。

A.10

B.50

C.100

D.200

题型:计算题

溺水几分钟后会失去意识,几分钟后身体会遭受不可逆的伤害?

题型:计算题

压井作业中,如果泵速从30冲/分增加到40冲/分,而立压保持30冲/分时的立压,对井底压力有何影响?()

A.增加;

B.减少;

C.不变;

D.先降后升。

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