问题 选择题

将阻值相等的电阻R1和R2串联后接在一个电压恒定的电源两端.若其中R1的电阻值不随温度的变化而变化.而R2的电阻值随温度升高而增大.随温度的下降而减小.则对R2加热或冷却,R2的电功率的变化情况为:(  )

A.加热升温时,电功率增大.而冷却降温时.电功率减小

B.加热升温时.电功率减小,而冷却降温时.电功率增大

C.无论是加热还是冷却.电功率均增大

D.无论是加热还是冷却,电功率均减小

答案

∵R1、R2串联

∴电路中的电流为:I=

U
R1+R2

R2的电功率为:P=(

U
R1+R2
2R2

当R2=R1时,P=

U2
4R1

当R2=2R1时,P=

2U2
9R1
U2
4R1

当R2=

1
2
R1时,P=
2U2
9R1
U2
4R1

所以无论是加热还是冷却,电功率均减小.

故选D.

完形填空
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Have you ever intended to be fully absorbed in your work but found yourself distracted by e-mail, the Internet and other things throughout the day?
"People often lose their concentration when they are bored, of course, but also when they are engaged in challenging tasks," says Peter Bregman. "They sometimes have a feeling to escape from what's difficult or boring, so they jump out," he says. The part of the brain devoted to attention is connected to the brain's emotional center. Any strong emotion—quarrels with colleagues, problems at home—can interrupt our attention. Studies over the last decade have shown that too much work to do at a time can easily lead to distraction.
Refocusing is hard for many people. Robert Epstein suggests the following, "Stop and listen to music for a few minutes. Go for a short walk or take a deep breath, where you breathe in deeply, count to five slowly, hold it and breathe out very slowly." This can "blow out all the tension and the unwanted in your mind to restore your focus."
Take more control by structuring your time and becoming more aware of your behaviors. "Setting the phone alarm to go off every hour is my way of creating awareness," Mr. Bregman says," You have to inform yourself that you've lost focus in order to do something about it." Starting the day with a to-do list is important, such as when to eat, when to go to the gym or take a walk. But if it's overly ambitious, you will put yourself in a state of anxiety, which makes it hard for the brain to concentrate. "Choosing three or four things as your priority for the day allows your brain to settle down and focus," he says.
Topic
How to become more 小题1: ________ on the work?
小题2:_______ of
getting
distracted
People tend to get away from things that they're小题3:_____ up with.
Strong 小题4: _________ affect the brain' s part for attention.
People get distracted easily if小题5:__________ multitasks at a time.
小题6:______ of
strong emotions
having a 小题7:___________ with a colleague.
meeting with problems at home
Tips on holding
attention
Stop working to restore your focus by 小题8: _______ your breath and try different relaxing things.
小题9:_________ your time effectively and create awareness of your behaviors.
Make a to-do list at the beginning of a working day.
Focus on 3 or 4 things and get them done 小题10:_______ for the day.
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