题干

如图所示是对光电效应中产生的光电子进行比荷测定的原理图,两块平行金属板相距很近,板间距为d,放在真空中,其中N为锌板,受紫外线照射后将激发出沿不同方向的光电子,光电子打在M板上形成电流,引起微安表指针偏转,若调节变阻器R,逐渐增大两板间电压,可以使光电流逐渐减小到零,当电压表读数为U时,电流恰好为零。断开开关,在MN之间加一垂直纸面的磁场,逐渐增大磁感应强度,也能使光电流逐渐减小到零,此时的磁感应强度为B,那么光电子的比荷为____

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答案(点此获取答案解析)

8
U
B
2
d
2

同类题2

    The world has many beautiful sounds. We can hear the songs of birds and the laugh of people. We can listen to the cool music and news reports. And it's all because of our ears! They bring us a world of sounds. But some people can't hear any sound at all. Some are born deaf. Others can hear well in their childhood, but too many loud sounds hurt their ears later on. Many people in China can't hear ordinary sounds clearly. A large number of them are children. Most of them are born with good hearing. But six children of every 1,000 have hearing problems when they are born. And if a child has hearing problems, he or she usually will also have some learning problems at school. A lot of things can cause hearing problems. For example, too much earwax is not good for our hearing. And noise louder than 85 decibels (分贝) can hurt our ears and make us lose our hearing for a short time or even for ever. Hearing is important for all of us. So we should do our best to give our ears special care. Don't forget to do the following:

● Try to stay away from places with too much noise. If you have to go, wear earplugs. A pair of good earplugs can cut the noise by 30 decibels. ● When swimming, remember to put earplugs into your ears to stop water from getting in.

● Never put anything sharp into your ears. If you think you have too much earwax, ask your mum or dad to help you clean it out. Keep these things in mind! Then you won't be saying “WHAT? WHAT?" when you are older.