问题 问答题

电暖气是常见的家用电器之一,某电暖气的电路示意图如图甲所示.L1和L2是两个指示灯,R1和R2为电热丝,通过调节旋转开关可以改变电暖气的档位.为避免引起火灾,使电暖气被碰倒时能够自动断开电路,可以在电路中接入一个“防倾倒开关”.“防倾倒开关”及其原理如图乙所示,在电暖气的底部有一个伸出外壳的塑料杆,当电暖气放置在水平地面时,塑料杆受到地面竖直向上的力而被顶起,接通顶部的触点.如果电暖气发生倾倒,塑料杆底部不受力,在弹簧的作用下顶部触点断开,电暖气便不再工作,该装置起到了“自动开关”的作用.在某次使用中,电暖气的旋钮开关接到档位“1”时,2min消耗的电能是48000J.已知R1:R2=4:7,指示灯L1和L2的电阻很小可忽略不计.

(1)“防倾倒开关”最好应安装在图甲的电路示意图中A、B、C、D、E哪一点处;

(2)求电热丝R2的阻值;

(3)当旋钮开关分别置于档位1和档位2时,求此电暖气的最大和最小功率的比值.

答案

(1)为了防止短路现象,在电热器被碰倒时,切断火线,“倾倒开关”要接在C点;

(2)旋钮开关接触点“1”时的等效电路如图1所示:

∵指示灯L2的电阻很小可忽略,

∴旋钮开关接静触点“1”时,2min消耗的电能:

W=

U2
R1+R2
t,

R1+R2=

U2
W
t=
(220V)2
48000J
×120s=121Ω,

∵R1:R2=4:7,

∴R1=44Ω,R2=77Ω;

(3)旋钮开关接触点“2”时的等效电路如图2所示

∵指示灯L1的电阻很小可忽略,

∴图2中,电热器消耗功率最大,P1=

U2
R1

图1中,电热器消耗功率最小,P2=

U2
R1+R2

P1
P2
=
U2
R1
U2
R1+R2
=
R1+R2
R1
=
121Ω
44Ω
=
11
4

答:(1)“防倾倒开关”最好应安装在图甲的C处;

(2)电热丝R2的阻值为77Ω;

(3)当旋钮开关分别置于档位1和档位2时,电暖气的最大和最小功率的比值为11:4.

填空题
阅读理解

We've all heard the old saying,“Sticks and stones can break my bones,but words can never hurt me.” Don't you believe it?Words are powerful,and they can hurt—a lot. In fact,if you're not careful,you can even hurt yourself,by using the wrong word in your writing.

Recently a friend sent me a copy of an e-mail she received from the customer service department of her credit card company,after she contacted them with a question about her account. She was happy with their positive response. But as I read the e-mail,one line jumped out at me:“In lieu of good customer service...”

Did they really mean they were offering her a few extra reward points instead of good customer service?I doubt it. More likely,what the customer service department meant to say was,“Because good customer service is important to us...” followed by the details of what they were doing to make sure she remained a happy customer.

So why didn't the writer say that?I can only guess that perhaps the writer liked the phrase “in lieu of”,thought it sounded more interesting than saying “because”,and so used it—without bothering to check the true meaning.

Have you ever done that?Try to impress others with a big word,only to find out you've used the word incorrectly?It's just the opposite effect,isn't it?You may impress people all right,but it's not the impression you were going for.

When you write,remember to choose your words carefully. People may still disagree with what you say,but they won't be able to dismiss you because you made careless mistakes in how you said it.

小题1:What's the writer's purpose in giving the saying?

A.To amuse the reader.

B.To persuade the reader.

C.To support a conclusion.

D.To lead to the topic.小题2:What does the phrase “in lieu of” most probably mean?

A.Because of

B.In honour of

C.Instead of

D.In favour of小题3:According to the writer,the customer service department ______.

A.didn't prefer to use familiar words to attract customers

B.didn't express what they really meant

C.didn't need to offer their customers extra reward points

D.succeeded in impressing their customers