一个带-Q电量的金属小球与一个不带电的实心球形导体的相互位置如图所示，在静电平衡时，感应电荷在C点产生的场强大小是（ ）
A．零
B．KQ (l+R)2
C．KQ (l+
R)23 2
D．不能确定
金属小球在C点产生的场强E1=k
，所以感应电荷在C点产生的场强E2=E1=kQ (l+
R)23 2
．故C正确，A、B、D错误．Q (l+
R)23 2
故选C．
| When you first drift off into slumber, your eyes will roll about a bit, your temperature will drop slightly, your muscles will relax, and your breathing will become quite regular. Your brain waves slow down a bit, with the alpha rhythm predominating for the first few minutes. This is the first stage of sleep. For the next 30 minutes or so, you will drift down through Stage 2 and Stage 3. The lower your stage of sleep, the slower your brain waves will be. About 40-60 minutes after you lose consciousness, you will reach the last stage. Your brain waves will show the delta rhythm. You may think that you stay at this deep stage all the rest of the night, but that turns out not to be the case. About 80 minutes after you fall into slumber, your activity cycle will increase slightly. The delta rhythm will disappear, to be replaced by the activity pattern of brain waves. Your eyes will begin to dart around under your closed eyelids. This period of Rapid Eye Movements lasts for 8-15 minutes and is called REM sleep. During both light and deep sleep, the muscles in your body are relaxed but capable of movement. As you slip into REM sleep, a very odd thing occurs -- most of the voluntary muscles in your body become paralyzed. Although your brain shows very rapid bursts of neural activity during REM sleep, your body is incapable of moving. REM sleep is accompanied by extensive muscular inhibition. |
The first paragraph of this passage tells us about______.
A．the rhythms of brain waves
B．the daily activities of sleep
C．the stages of sleep in cycles
D．the daily activities in cycles