问题 问答题

如图,在物理实践活动中,王明同学自制了亮度可以调节的小台灯,其电路图及灯泡规格,电源电

压U=220V保持不变,R1=R2=R3=33Ω,假设灯泡电阻不变,请完成下列问题:

(1)调节开关可改变灯泡的亮度,调到哪个触点灯最暗?调到哪个触点灯最亮?

(2)灯泡正常工作时,灯泡的电阻是多少?

(3)灯泡正常工作5分钟所消耗的电能是多少?

(4)当开关打到触点2时,灯泡所消耗的实际功率是多少?

答案

(1)开关在触点0时,电路为灯泡的基本电路,电路中的电阻最小,根据欧姆定律可知通过灯泡的电流最大,灯泡的实际功率最大,灯泡最亮;

同理,当开关在触点3时,电路中的电阻最大,电流最小,灯泡最暗;

(2)灯泡正常工作时的电压为220V,功率为100W,

∵P=

U2
R

∴灯泡的电阻RL=

U2L额
PL额
=
(220V)2
100W
=484Ω;

(3)灯泡正常工作5分钟所消耗的电能:

W=Pt=100W×5×60s=3×104J;

(4)当开关打到触点2时,灯泡与R1、R2串联,

∵串联电路中总电阻等于各分电阻之和,

∴根据欧姆定律可得,电路中的电流:

I=

U
RL+R1+R2
=
220V
484Ω+33Ω+33Ω
=0.4A,

此时灯泡的实际功率:

PL=I2RL=(0.4A)2×484Ω=77.44W.

答:(1)调节开关可改变灯泡的亮度,调到3触点灯最暗,调到0触点灯最亮;

(2)灯泡的电阻是484Ω;

(3)灯泡正常工作5分钟所消耗的电能是3×104J;

(4)当开关打到触点2时,灯泡所消耗的实际功率是77.44W.

单项选择题
阅读理解

Millions of Americans return from long-distance trips by air, but their luggage doesn’t always come home with them. Airline identification tags(标签) can come loose, and the bags go who-knows-where. And passengers leave all kinds of things on planes.

The airlines collect the items and, for 90 days, attempt to find their owners. They don’t keep them, since they’re not in the warehouse business. And by law, they cannot sell the bags, because the airlines might be tempted to deliberately misplace luggage.

So once insurance companies have paid for lost bags and their contents, and they no longer belong to passengers, a unique store in the little town of Scottsboro, Alabama, buys them. The “Unclaimed Baggage Center,” is so popular that the building, which is set up like a department store, is the number-one tourist attraction in all of Alabama. More than one million visitors stop in each year and take one of the store’s shopping carts on a hunt for treasures.

Each day, clerks bring out 7,000 new items, and veteran(老练的)shoppers rush to paw over them. You can find everything from precious jewels to hockey sticks, best-selling novels, leather jackets, tape recorders, surfboards, even half -used tubes of toothpaste.

The store’s own laundry washes or cleans all the clothes found in luggage, then sells them. The Unclaimed Baggage Center has found guns, illegal drugs and even a live rattlesnake.

The store has a little museum where some of its most unusual acquisitions(获得物) have been preserved. They include highland bagpipes, a burial mask from an Egyptian pharaoh's tomb, and a medieval suit of armor.

Statistics indicate that less than one-half of one percent of luggage checked on U.S. carriers is permanently lost and available to the store.

小题1:Paragraph1 shows that many passengers lose their luggage because______.

A.they are forgetful

B.the owners of some luggage can’t be identified

C.they are in a hurry

D.there is no lost and found office in many airports小题2:The reason why the airlines cannot sell the bags is that ______.

A.they have to find the owners

B.they have to keep the bags as long as possible

C.some bags are expensive

D.they are likely to make a profit on the bags on purpose小题3:The Unclaimed Baggage Center is very popular because______.

A.visitors may purchase something undervalued.

B.all thethings there are very cheap.

C.there's a large variety of goods.

D.visitors will enjoy some amusing activities there.小题4:What can we infer from the passage?

A.A little museum will keep all the precious unclaimed baggage.

B.The things in the Unclaimed Baggage Center are articles for daily use.

C.The percentage of passengers who lose their baggage for ever is small.

D.People are not allowed to buy the illegal things in the store.小题5:What is the main purpose of the passage?

A.To introduce an attractive place to tourists.

B.To remind passengers of taking care of their baggage.

C.To advise the airlines to find the owners of the unclaimed baggage.

D.To introduce how the unclaimed baggage in the airports is handled in America.