Matric Notes Physics 9th Ch 8 Thermal Properties of Matter Short Questions

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Q.1) Ordinary electric fan increases the kinetic energy of the air molecules caused by the fan blades pushing them means the air temperature increases slightly rather than cool the air? why use it?

Answer: Ordinary fans work on the principle of convection. The moving air increases the rate of evaporation or perspiration from our bodies, so we get a cooling sensation. The flowing air takes away the heat from the body and is replaced with the colder air.

Q.2) Why are small gaps left behind the girders mounted in walls?

Answer: Usually, small gaps are left behind the girders mounted in walls to give room for expansion. One end of the iron is fixed while the other end is allowed to expand into the left out gaps in summer.

Q.3) Why you should not put a closed glass jar into a campfire. What could happen if you tossed an empty glass jar, with lid on tight, into fire?

Answer: A closed jar in a campfire tends to increase the kinetic energy of the air molecules inside the jar. As a result, the air expands, putting pressure on the walls and the lid of the jar. Consequently, the jar could break or it could even explode.

Q.4) Explain why it is advisable to add water to an overheated automobile engine only slowly, and only with the engine running.

Answer: If we add water quickly to an overheated engine, water comes into contact with the hot metal part of the engine. Metals tend to contract on cooling and expand on heating. Some area of the metal part will cool down very rapidly, while other parts will not. This could cause a crack in the engine or could even cease the pistons.

So water should be added slowly with the running engine. This will create a gradual and uniform rate of decrease in the temperature of the engine. 

Q.5) Explain why burns caused by steam at 100 °C on the skin are often more severe than burns caused by water at 100 ° C?

Answer: The steam produces more severe burns than boiling water at 100°C because steam has more latent heat. As water absorbs extra heat to change from the liquid state into its gas state (steam) that is up to 100°C, the heat raises the temperature of the water. But at 100 °C, as the water changes to steam, more heat is added without changing the temperature.

That extra heat is called the heat of vaporization, it is stored in the steam as a latent heat that will be released when the steam liquefies. And that extra heat is passed on into your body as the steam condenses on your skin. That way, the steam produces more severe burn effects than boiling water.

Q.6) Explain why cities like Karachi situated by the ocean tend to have less extreme temperatures than inland cities at the same altitude.

Answer: Karachi is situated near a sea shore. The specific heat of water is five times the specific heat of the land. It means that the heat require to increase 1oC or 1 K temperature of land is less than that of water. Hence, the land gets heated much more easily than water, it also cools down easily. Hence a large temperature difference arise that give rise to land breeze and sea breeze, which keeps the temperature of the coastal areas moderate.

Q.7) An iron rim that is fixed around a wooden wheel is heated before its fixture. Explain why?

Answer: In order to make a tight fit, the diameter of the iron rim is made slightly lesser than the diameter of the wooden wheel. When the iron rim is heated, it expands on heating and can be placed around the wooden wheel. When the iron ring cools down it contracts in this way, it produces a tight fit

Q.8) Why is ice at 0oC a better coolant of soft drinks than water at 0oC?

Answer: The ice is a better coolant of soft drinks than water at 0°C because the latent heat of fusion of ice is large. Which is the amount of heat absorbed by a solid body to convert 1 kg of a substance from solid to liquid. So the heat extracted from the soft drink by ice is more than the heat extracted by water.

Q.9) Why we feel cool after perspiration ?

Answer: We feel cool after perspiration because during evaporation more energetic molecules escape from the liquid surface. While molecules with less K.E remain in the liquid, that’s why the cooling effect is produced due to evaporation.