问题 选择题

两个小灯泡甲和乙,甲灯标有“6V、3W”字样,乙灯标有“3V、3W”字样,则下列说法错误的是 (  )

A.当甲、乙两个灯泡都正常工作时,两灯的亮度相同

B.甲灯的额定电流小于乙灯的额定电流

C.两灯并联接入一电压可调的电源两端,电路消耗的最大功率为6W

D.两灯串联接入一电压可调的电源两端,电路消耗的最大功率为3.75W

答案

(1)当甲、乙两个灯泡都正常工作时,两灯泡的实际功率和额定功率相等,

∵灯泡的亮暗取决于实际功率的大小,

∴两灯泡额定功率相等,实际功率相等,亮度相同,故A正确,不符合题意;

(2)根据P=UI可得,两灯泡的额定电流:

I=

P
U
=
3W
6V
=0.5A,I=
P
U
=
3W
3V
=1A,

∴甲灯的额定电流小于乙灯的额定电流,故B正确,不符合题意;

根据欧姆定律可得,两灯泡的电阻:

R=

U
I
=
6V
0.5A
=12Ω,R=
U
I
=
3V
1A
=3Ω;

当两灯并联接入一电压可调的电源两端时,

∵并联电路中各支路两端的电压相等,

∴电路中的最大电压U=U=3V,此时P′=P=3W,

甲的实际功率:

P′=

U2
R
=
(3V)2
12Ω
=0.75W,

电路消耗的最大功率:

P=P′+P′=0.75W+3W=3.75W,故C不正确,符合题意;

当两灯串联接入一电压可调的电源两端时,

∵串联电路中各处的电流相等,

∴电路中的最大电流I=I=0.5A,

∵串联电路中总电阻等于各分电阻之和,

∴电路消耗的最大功率:

P=I2(R+R)=(0.5A)2×(12Ω+3Ω)=3.75W,故D正确,不符合题意.

故选C.

选择题
阅读理解

阅读理解。

     Occasional sleepwalking does not tend to present a problem and most children grow out of it. If it

happens every night and causes problems, then sleep disorder specialists suggest that parents spend a

week noting the time when sleepwalking tends to happen, and then gently wake the child about fifteen

minutes before this time. This will often break the cycle.

    So what should you do if you find your relative sleepwalking? For a start, they are in such a deep sleep

that they will probably not notice you, even if you try to wake them. If you do succeed in walking them,

they will become distressed. Everyone has experienced that feeling of confusion when the alarm awakens

you from deep sleep, instead of the lighter sleep we are usually experiencing by the time our alarms

sound. I once had the shock of being woken by a crash and found myself in the kitchen, standing

barefoot and surrounded by broken glass. I'd tried to get a glass of water while sleepwalking, but like

most sleepwalkers I failed to turn on the light, and had smashed the glass against the tap.

    Walking a sleepwalker will not cause them to have a heart attack, but the kindest thing to do is not try

to wake them at all. Lead them gently back to bed so that they do not hurt themselves. They will remain

deeply asleep, and it is likely that will not remember a thing in the morning.

1.What do the specialists advise the parents to do to avoid their children's sleepwalking?

A. Go to bed with their children at the same time.

B. Keep a record of the time when children wake up.

C. Take 15 minutes to wake children from sleepwalking.

D. Try some time to break the cycle of the sleepwalking.

2. When a sleepwalker is suddenly woken, he is most likely to   ___  .

A. die from heart attack    

B. do some cooking

C. feel lost    

D. become relaxed

3.What should we do to deal with a sleepwalking relative?

A. Do nothing but take him back to bed.

B. Try every means to wake him at once.

C. Leave your relative where he stays.

D. Send him to a specialist immediately.

4. What is the best title for the text? 

A. Sleep Disorder  

B. Sleepwalking    

C. Sleep Advice    

D. Sleepwalkers