Saturday, May 9, 2015

Heat and Temperature Basic science Class 7 solution

Solution of chapter Heat and Temperature Basic science Class 7

A. Answer these question in not more than 20 words
1. Give an example to show that heat is a form of energy?

Answer: When we heat a pan of water, the water boils and at its temperature rises. The energy for this rise in temperature comes from heat. This shows that heat is a form of energy.

2. What is temperature? Name the scale used to measure it?

Answer: Temperature of a body is the measure of the hotness or coldness of the body. Celsius and Fahrenheit are the scales used to measure temperature.

Thermometry is branch of physics which deals with the measurement of temperature.

3. If hotness and coldness of a body is relative, how do we measure its degree of hotness?
Answer: We measure the degree of hotness or coldness of a body by measuring its temperature and comparing with standard temperature scale.

4. Mention three things that heat causes in substance?
Answer: Following are the three changes that heat can cause in any substance:

(a) Heat can change the temperature of a substance.

(b) Heat can expand a substance.

(c) Heat can change the state of a substance.

5. How does heat travel in spoon from hot boiling soup
Answer: Heat travel through the spoon by conduction.

6. How is it that we hold a match while it is burning?
Answer: We can hold a match while it is burning because wood is a bad conductor of heat and does not transfer of heat through conduction hand

7. How does turning on heater in room make the whole room warm?
Answer : Heat from the heater transfers to the whole room through radiation that rises the temperature of the whole room.

8. How does heat from the sun reaches the earth?
Answer: Heat from the sun reaches the earth by the process of radiation

9. Mention one similarity between heat and light?
Answer: Heat and light, both are forms of energy and both travel in a straight line

10. When two bodies are in contact, heat flows from hotter to colder bodies. When does heat stop flowing?

Answer: The flow of heat from the hotter body to the colder body will stop when the temperatures of the two bodies become equal.

B.Answer these question in not more than 40 words
1. Distinguish between the Celsius and Fahrenheit scale

Answer:

Celsius Scale

The lower fixed point is 0°C.

The upper fix point is 100 C.

The fundamental interval is divided in 100 equal parts and each part is called 1°C (one degree Celsius).

It is introduced by Celsius, is usually used in scientific laboratories.

Fahrenheit Scale

The lower fixed point is 32F.

The upper fix point is 212 F.

The fundamental interval is divided into 180 equal divisions. Each division is called one degree Fahrenheit. (1°F).

It is suggested by Gabriel Fahrenheit, is usually used in clinical and meteorological purposes.

2. How does heat travel through water on heating?
Answer: Convection is a process through which heat travels through the water. When water is heated, the water became lighter. Hot water rises up. The cold water from the sides moves down towards the source of heat This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection

3. what happen to the radiant heat when fall on the body? What does the absorption of radiant heat by a body depends on?

Answer: When radiant energy falls on an object, some of its part is absorbed by the object and some is reflected by the object.

Absorption of the radiant energy depends upon the following: (i) The colour of the object. (ii) Nature of the surface.

4. (a) Mention difference between transfer of heat by conduction and that of radiation?
    (b) Mention difference between transfer of heat by conduction and that of convection?

Answer :

(a) Conduction: Flow of heat through a medium occurs without the movement of the material.

Radiation: Flow of heat occurs without the help of any material medium.

(b) Conduction: Flow of heat through a medium occurs without the movement of the medium.

Convection: Flow of heat through a medium occurs with the movement of medium.

C. Answer these question in not more than 100 word
1. Explain how land and see breeze originated?

Answer: Sea takes longer to get heated than the land. So, the land is hotter than the sea during the day. The air in contact with the land becomes hot and rises and the cooler air above the sea moves towards the land to replace the hot air. This movement of air from sea towards the land is called sea breeze.

After sunset, the land cools faster than the sea. The air above the sea is warmer than the air above the land. Air above the sea rises and the cooler air above the land moves towards the sea. This is called land breeze.

2. How would you demonstrate to a friend that gases expand on heating?
Answer: The following experiment explains that a gas expands when it is heated: Take a balloon and a glass bottle. Fit the balloon to the mouth of a glass bottle. Place the bottle in a pan of water. Put the pan over a gas flame and heat it.

It is now seen that the balloon gets inflated as the air inside the bottle becomes warm and expands. Also when the bottle is cooled, the balloon gets deflated as the air inside the bottle gets cooler. This shows that gas expands when it is heated.

3. (a) Why does It is better to wear light shades in summer?

(b) Why do two thin sweaters feel warmer than one thick sweater?

(c) ) Why do handles of utensils are made up of plastic while their handle are made up of plastic?


Answer: (a) It is better to wear light shades in summer because light shades absorb less heat.

(b) Two thin sweaters feel warmer than one thick sweater because the air trapped between the two sweaters acts as an insulator of heat. It does not allow the heat from our body to escape easily.

(c) Handles of utensils are made up of plastic because plastic is a poor conductor of heat. Therefore, heat will not travel through the plastic and the handles will remain cool during cooking. This saves our hand from getting injured from the heat.

Metal on the other is a conductor of heat, which helps in transferring heat from the burner flame to the food and helps in cooking the food.

D. Choose the correct options
1.(a) does not require any medium

2. (b) some metals conduct heat better than others

3. (b) so that the level of mercury does not fall as soon as the thermometer is taken out of the mouth

4. (c) poor absorber and good reflector of heat

5. (b) good absorber and good emitter of heat

6. (d) evaporation

E. Say true and false
Answer :

1. True. Water boils at 212 F.

2. False. Clinical thermometer has kink in the capillary tube.

3. True. Liquid expands more than solid.

4. True. The heat from the sun reaches to the earth through radiation.

5. False. Solid is a good conductor of heat than liquid and gas.

6. True. Heat flows in convection by the movement of the substance.

7. False. Transfer of heat by radiation do not need contact between a hot and a cold body.

8. True. Air is a poor conductor of heat.

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