问题 选择题
如图所示,处在磁感应强度为B的匀强磁场中的单匝矩线圈abcd,以恒定的角速度ω绕ab边转动,磁场方向垂直于纸面向里,线圈所围面积为S,线圈导线的总电阻为R.t=0时刻线圈平面与纸面重合,且cd边正在离开纸面向外运动.则正确的是(  )

A.时刻t线圈中电流的瞬时值icosωt

B.线圈中电流的有效值I

C.线圈中电流的有效值I=

D.线圈消耗的电功率P

答案

答案:C

阅读理解

C

Why are so many coin banks shaped like pigs? Why not dogs or cats or elephants?

Coin banks weren't always made to look like pigs. In fact, the name "piggy bank" may have come from a kind of clay and not from the animal at all.

The history of the piggy bank goes back to the Middle ages. At that time in Western Europe, metals were expensive, so a cheap orange clay known as pygg was used for everyday items such as dishes and jars. In fact something made from pygg clay was often just called a pygg. The first piggy bank may well have been a pygg in which a person put a few extra pennies to buy a treat on market day.

People have always had a hobby of saving pennies, and using a pygg as a coin bank became common practice. Potters (陶工) began making jars specially for saving coins, and eventually these jars took a pig-like shape. Modeling a bank to look like a pig may have been influenced by the name of the clay. The earliest Roman coin bank ever found was decorated with a pig. Because it was considered as a valuable farm animal in many countries, the pig was an appropriate symbol of wealth and prosperity.

These early clay banks were fragile and easily broken. As time passed, mining became less expensive. Iron, copper, and silver gradually replaced pygg clay in the production of household items. Although pygg clay lost its popularity, the habit of saving pennies did not. Today piggy 'banks are made of every material imaginable.

The familiar saying "A penny saved is a penny earned" is as old as the clearly piggy banks. Marry a child received his or her first coin bank accompanied by this wise advice. The modern-day piggy bank may have changed in sine, shape, and spelling, but it saves our pennies just as well as the first pygg did countries ago.

64. In the Middle ages, people used pygg to make dishes because ________.

A. it was precious                             B. it stood for wealth

C. it had a beautiful color                           D. it was cheaper than other materials

65. What do we know about the pygg bank?

A. Its name might come from an animal.                   B. The first pygg bank looked like a pig.

C. It was designed for children at the beginning.         D. The early piggy banks were easily damaged.

66. Why did pygg day lose its popularity?

A. Other cheap and solid materials appeared.    B. It was inconvenient to carry it to the market.

C. It couldn't be made into other shapes.          D. People had run out of this kind of material.

67. In the last paragraph, the author intends to tell us ________.

A. piggy banks nowadays are not practical          B. piggy banks still play a part in our daily lives

C. piggy banks today are mainly made of iron

D. most children don' t have the habit of saving money

单项选择题