Lesson: Heat

# Question: 1

What is temperature?

## Solution:

The measure of hotness or coldness of an object is called temperature.

# Question: 2

Name the device used to measure temperature?

## Solution:

A thermometer is the device used to measure temperature.

# Question: 3

What is the range of a laboratory thermometer?

## Solution:

The range of a laboratory thermometer is generally from $–$10°C to 110°C.

# Question: 4

Why does the mercury not fall or rise in a clinical thermometer, when taken out of the mouth?

## Solution:

The kink prevents the mercury level from falling on its own.

# Question: 5

How does the heat from the sun reach us?

## Solution:

The heat from the sun reaches us through the process of radiation.

# Question: 6

Name three modes of transmission of heat energy.

## Solution:

Heat can be transferred in three ways:

a.       Conduction, b. Convection, c. Radiation

# Question: 1

Define good conductors and bad conductors of heat.

## Solution:

Good conductors : The substances that allow the heat to flow through them easily are called good conductors of heat. For e.g., copper, gold, aluminium, etc.

Bad conductors : The substances that do not allow the heat to flow through them are called bad conductors of heat. For e.g., wood, plastic, glass, etc.

# Question: 2

Why does the electrician wear rubber gloves when he works?

## Solution:

Rubber is a bad conductor of electricity and so acts as an insulator. An electrician wears rubber gloves while working to prevent himself from electric shock.

# Question: 3

What is heat? How is heat different from temperature?

## Solution:

Temperature is the measure of average energy of each molecule due to its motion. If the speed of the particles increases, temperature also increases.

Heat, on the other hand, is the amount of total energy possessed by all the molecules. So, if the number of molecules is more, the heat energy possessed will also be more.

# Question: 4

Why is mercury used in thermometers?

## Solution:

Mercury is used in thermometers because:

·               It is easily available in the pure state.

·               It is opaque and shiny.

·               Its expansion is uniform on heating.

·               It does not stick to the sides of the glass tube.

# Question: 5

Write the difference between a laboratory and a clinical thermometer.

## Solution:

(a)   A laboratory thermometer is used to measure the temperature of liquids whereas a clinical thermometer is used to measure human body temperature.

(b)   The kink is not present in a laboratory thermometer whereas a clinical thermometer has a kink which controls the flow of mercury.

(c)   The temperature range of a laboratory thermometer is more than that of a clinical thermometer.

# Question: 6

How do woollen clothes keep us warm in winter?

## Solution:

Woollen clothes can keep us warm during winter. Woollen fibres trap air between them, which prevents the flow of heat from our body to colder surroundings. This helps us to keep warm.

# Question: 1

Why do we prefer white coloured clothes in summer and dark coloured clothes in winter?

## Solution:

We prefer to wear white coloured clothes in summer because it has poor heat absorbing capacity. They reflect most of the heat of the sun falling on them and absorb very little. On the other hand, dark coloured clothes are good absorber of heat and hence keep our body warm in winter.

# Question: 2

Write the difference between conduction and convection?

# Question: 3

Explain land and sea breeze.

## Solution:

During the daytime, heat from the sun warms land faster than water. The air above the land gets warm and less dense, due to which it rises upwards, creating a low-pressure area. Cooler air from above the sea moves to occupy the low-pressure area created over the land.

The flow of cool air from the sea towards the land is called a sea breeze.

During night time, the land loses heat faster than the sea does. Since the air above the sea is warmer and less dense, it begins to rise upwards, so a low pressure is created above sea.

The cooler air over the land moves towards the sea to replace the warm rising air. This cool breeze from the land towards the sea is called a land breeze.

# Question: 4

Explain how water is heated by convection?

## Solution:

The water is a poor conductor of heat. So, it does not get heated by conduction. When water is heated, the water became lighter. Hot water rises up. The cold water from the side moves down towards the source of heat, and this water also gets hot and rises and water from the sides moves down again. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

# Question: 5

Give one example of each mode of transfer of heat.

## Solution:

(i)                 Conduction: It is the process of transfer of heat in solids from one object to another by direct contact.

For example, the transfer of heat from one end to another, in iron or copper rods

(ii)              Convection: It is the mode of transfer of heat through a liquid or gas caused by molecular motion from one part to another.

For example, the chimneys are fitted in industries to remove undesirable fumes.

(iii)            Radiation: It is the transfer of heat directly from a hot body to a cold body, without heating the space between them.

For example, the Sun’s heat radiation is reaching the Earth.