SCIENCE CHAP 4. Heat and Its Transfer

 

4. Heat and Its Transfer

Choose the correct option.

a) In solids, heat is mainly transferred by the process of .

i) conduction

iii) radiation

ii)

iv)

convection none of these

b) Temperature is measured in .

i) calories

iii) joules

ii)

iv)

degree celsius

all of these

c) The temperature at which a substance changes from a liquid to a gas is called its

.

i) melting point ii) boiling point

iii) freezing point iv) none of the above

2. Assertion and Reason type questions:

In the following questions, two statements are given- one labelled Assertion (A)

and the other labelled Reason (R). Read the statements carefully and choose the

correct alternative (A), (B), (C) and (D) as given below.

(A) Both A and R are true and R is correct explanation of the assertion.

(B) Both A and R are true but R is not the correct explanation of the assertion.

(C) A is true but R is false.

(D) A is false but R is true.

a) Assertion: The exteriors of houses are painted in light colours in hot climate.

Reason: Light colour absorbs large amount of radiant heat.

 Assertion is true but the Reason is false.

b) Assertion: When an iron rod at 50°C is brought in contact with water at 30°C, heat

flows from the iron rod to water.

Reason: Heat always flows from a body at a higher temperature to a body at a lower

temperature.

Both Assertion and Reason are true and the Reason is a correct explanation of

the Assertion

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b) Heat emitted from a light bulb

3. Identify the type of heat transfer depicted in the following examples.

a) Touching

a hot spoon

Conduction

 Radiation

c) Cooling a room using an air conditioner

 Convection

4. Answer the following questions in brief.

a) What are the advantages of an alcohol thermometer

over a mercury thermometer?

 Alcohol is useful in measuring the lower temperatures as its freezing point is low.

 It has another advantage of being less hazardous in comparison to mercury.

b) On a cold winter day, the temperature was 23 degree fahrenheit. What

would this temperature be in degree celsius?

 9 5

F 32 C = −

 ( 32) 9

5 C = F −

 (23 32) 9

5 = −

 = −5°C

c) Differentiate between conductors and insulators.

Conductors Insulators

The materials that allow heat to pass

through them easily are called

conductors of heat

The materials that do not allow the heat

to pass through them easily are called

insulators.

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5. Give reasons for the following statements.

a) Rooms are provided with exhaust fans at the top near the ceiling.

The warm air from the floor rises up. Placing the exhaust fans near the top will

keep

the room cool by allowing the escape of the hot air in the top layer.

b) Cooking utensils are provided with wooden handles.

Cooking utensils are provided with wooden handles because wood is

a poor conductor of heat. Hence, the heat from the hot cooking

appliances does not flow into our hands. As a result, we can hold the

appliances by their handles while they are hot.

6. The diagram depicts a beaker containing water kept on a burner.

a) Which mode of heat transfer does the arrow indicate?

 Convection

b) Explain how the water in the beaker gets heated.

Water molecules closest to the source of heat gain heat and expand. As a

result, they become lighter and move upwards. Their place is taken by water

molecules from the top, which are comparatively less warm, thereby heating

the entire liquid. This movement of the liquid in a loop pattern gives rise to a

convection current through which heat energy flows and the entire liquid gets

heated.