F3 PHY (HEAT) EASY!

KIKI YEUNG

2010-05-22 5:18 am

下有D關於S3 PHY ( HEAT )既問題
有D有答案但5明點解希望有人幫到我!

1. 2 identical thermometers are put in the sun for 5 minutes. Their bulbs are covered with different materials. Explain which thermometer will have the highest reading.
Thermometer A: covered with blackened foil Thermometer B: covered with tin foil
（ＡＮＳ：Thermometer A）

2. Today s a freezing sunny day. Adason decides to wear both the following coats: a transparent plastic coat and a black coat. Which one should he wear on the outside for keeping warm? Explain your answer briefly.
（ＡＮＳ：a transparent plastic coat ）

3. Can carbon dioxide block the infra-red? Give examples to support your answers.

4. A metal blockis first immersed in boiling water for some time. The block is then transferred to a cup of cold water. After a while, the temperature of the water is measured.The specific heat capacity of the metal obtained in the experiment is found to be higher than its true value.Whcih of the following is a probable reason?

A Some hot water is still adhered to themetal block when the metal block is transferred to the cold water.

B Some energy is loss to the surroundings when the metal block is transferred to the cold water.

C Some energy is absorbed by the cup

(ANS: A)
(我5明點解B/C 5得可5可以簡單咁解釋下？)

我想問以下2題點様用conducion &convection D theory 去解釋?

5. How does a covered container keep food hot?

6. Air trapped in cotton and feathers is good for preventing energy loss. But why does a hot pan surrounded by air loss heat so quickly?

D ANS 5使長篇大論
答案要肯定
5肯定既話就飛左佢LA
ＴＨＸ　：）

回答 (1)

天同

2010-05-22 5:51 am

✔ 最佳答案

1. Black objects are good absorber of heat. Hence the blackened foil absorbs more heat than the tin foil, giving a higher temperature to the thermometer.

2. The transparent plastic coat permits light, as well as infrared to pass through. The infrared (i.e. heat radiation) will then be absorbed by the black coat. Also, the layer of air between the two coats prevents heat lost by conduction because air itself is a poor conductor of heat.
If the black coat is worn outside, the absorbed heat cannot be easily transfer to the inner coat and to the body because of the presence of air layer between the two coats.

3. Yes, carbon dioxide is a greenhouse gas. It allows infrared from the sun (which is of short wavelength) to pass through, but prevents infrared of long wavelength (far infared) to escape from earth. This results in global warming because of the accumulation of CO2 from industrialization in the atmosphere.

4. Option B: The heat balance equation now becomes,
M.C.T = m.c.T' + Q
where M and m are the masses of the metal block and cold water
C and c are their respective specific heat capacities
T and T' are the corresponding change in temperature
Q is the heat lost to the surroundings
hence, C(true value) = (m.c.T' + Q)/M.T
Clearly, the true value of C is higher, but NOT LOWER than if Q is ignored in the experiment (i.e. experimental value).

Option C: the heat balance quation is,
M.C.T = m.c.T' + Q'
where Q' is the heat absorbed by the cup
The same explanation as given in option B applies.

5. The cover keeps the air in the container, so that convection current cannot be set up. Thus preventing heat loss.

6. Air trapped in cotton and feathers cannot move, thus convection cannot be set up. The poor conduction property of air keeps the heat in.
A hot pan is surrounded by air that is free to move. Thus, convection can easily set up and carries the heat away rapidly.

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