问题 不定项选择

如图,空间存在水平向左的匀强电场和垂直纸面向里的匀强磁场,电场和磁场相互垂直。在电磁场区域中,有一个竖直放置的光滑绝缘圆环,环上套有一个带正电的小球。O点为圆环的圆心,a、b、c为圆环上的三个点,a点为最高点,c点为最低点,Ob沿水平方向。已知小球所受电场力与重力大小相等。现将小球从环的顶端a点由静止释放。下列判断正确的是

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A.当小球运动到b点时,洛仑兹力最大

B.当小球运动到c点时,洛仑兹力最大

C.小球从a点到b点,重力势能减小,电势能增大

D.小球从b点运动到c点,电势能增大,动能先增大后减小

答案

答案:D

单项选择题
阅读理解

信息匹配。

     Read the following text and choose the most suitable heading from A-F for each paragraph. There is

one extra heading which you do not need.

     A. Goods for auction (拍卖) sales

     B. Definition of bidding

     C. Ways to sell more goods by auction

     D. Auction sales in history

     E. Brief introduction to auctions

     F. Making a larger profit as an auctioneer

1. _____

     Auctions are public sales of goods, conducted by an officially approved auctioneer. He asks the crowd

assembled in the auction- room to make offers, or "bids", for the various items on sale. He encourages buyers

to bid higher figures, and finally names the highest bidder as the buyer of the goods. This is called "knocking

down" the goods, for the bidding ends when the auctioneer strikes a small hammer on a table at which he

stands.

2. _____

     The ancient Roman probably invented sales by auction, and the English word comes from the Latin auction,

meaning "increasing". The Romans usually sold in this way the goods taken in war. In England in the eighteenth

century, goods were often sold "by the candle": a short candle was lit by the auctioneer, and bids could be made

while it stayed alight.

3. _____

     Practically all goods whose qualities vary are sold by auction. Among these are coffee, skins, wool, tea, furs,

silk and wines. Auction sales are also usual for land and property, furniture, pictures, rare books, old china and

similar works of art.

4. _____

     An auction is usually advertised beforehand with full particulars of the articles to be sold and where and when

they can be viewed by potential buyers. If the advertisement cannot give full details, catalogues are printed, and

each group of goods to be sold together, called a "lot", is usually given a number. The auctioneer need not begin

with Lot 1 and continue in the order of numbers; he may wait until he notices the fact that certain buyers are in

the room and then produce the lots they are likely to be interested in.

5. _____

     The auctioneer's services are paid for in the form of a percentage of the price the goods are sold for. The

auctioneer therefore has a direct interest in pushing up the bidding as high as possible. He will not waste time

by starting the bidding too low. He will also play on the opponents among his buyers and succeed in getting a

high price by encouraging two business competitors to bid against each other.