任务型阅读
An Interesting Experiment
Put an ice cube(冰块) from your fridge into a glass of water.____ Thus, you can take out that piece of ice with the help of the string. But you must not touch the ice with your fingers.
You may ask your friends to try to do that when you are having dinner together.____ You must use salt when you carry out this experiment.
____ Then put some salt on the ice. Salt makes ice melt(融化). The ice round the string will begin to melt.____ The cold ice cube will make the salt water freeze again.
After a minute or two you may raise the piece of string and with it you will raise your piece of ice!
This experiment can be very useful to you. If, for example, there is ice near the door of your house, you must use very much salt to melt all the ice.____
A. If you don't put enough salt, the water will freeze again.
B. The string must be at least 10 centimeters long.
C. First you put the string across the piece of ice.
D. We can learn a lot from this physical experiment.
E. But when it melts, it will lose heat.
F. You should have a piece of string 10 centimeters long.
G. There is a salt bottle on the table.
动势大小为E。忽略涡流损耗和边缘效应。关于
、
的大小和铜棒离开磁场前两端的极性，下列判断正确的是