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## Chapter: 9th Science : Physics : Heat

Science : Physics : Heat : Book Back Important Questions, Answers, Solutions : I. Choose the correct answer, II. Fill in the blanks, III. State whether true or false. If false, correct the statement, IV. Match the following, V. Assertion and reason type questions

HEAT

### TEXT BOOK EXERCISES

I. Choose the correct answer :

I. Calorie is the unit of

(a) heat

(b) work

(c) temperature

(d) food

2. SI unit of temperature is

(a) fahrenheit

(b) joule

(c) celsius

(d) kelvin

3. Two cylindrical rods of same length have the area of cross section in the ratio 2:1. If both the rods are made up of same material, which of them conduct heat faster?

(a) Both rods

(b) Rod-2

(c) Rod-1

(d) None of them

4. In which mode of transfer of heat, molecules pass on heat energy to neighbouring molecules without actually moving from their positions?

(b) Conduction

(c) Convection

(d) Both B and C

5. A device in which the loss of heat due to conduction, convection and radiation is minimized is

(a) solar cell

(b) solar cooker

(c) thermometer

II. Fill in the blanks :

1. The fastest mode of heat transfer is radiation .

2. During day time, air blows from sea to land .

3. Liquids and gases are generally poor conductors of heat.

4. The fixed temperature at which matter changes state from solid to liquid is called melting point .

III. Assertion and reason type questions :

Mark the correct choice as:

(a) If both assertion and reason are true and reason is the correct explanation of assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.

1. Assertion : Food can be cooked faster in vessels with copper bottom.

Reason : Copper is the best conductor of heat.

(a) If both assertion and reason are true and reason is the correct explanation of assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.

[Answer: (a) Both assertion and reason are true and reason is the correct explanation of assertion]

2. Assertion : Maximum sunlight reaches earth’s surface during the noon time.

Reason : Heat from the sun reaches earth’s surface by radiation.

(a) If both assertion and reason are true and reason is the correct explanation of assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.

[Answer: (b) Both assertion and reason are true but reason is not the correct explanation of assertion]

Reason : When the sun is at its highest point, the earth's surface absorbs more heat and retains. This heat is slowly radiated out causing increase in temperature.

3. Assertion : When water is heated up to 100° C, there is no raise in temperature until all water gets converted into water vapour.

Reason : Boiling point of water is 10° C.

(a) If both assertion and reason are true and reason is the correct explanation of assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.

[Answer: (c) Assertion is true but reason is false]

Reason : When a substance changes from one state to another, a considerable amount of heat energy is absorbed or liberated. This energy is called latent heat.

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