9th Class Physics: Chapter 9 Transfer of Heat Short Questions Answers

Metals are good conductors of heat because metal contains free electrons which can move from one place to another, by getting energy.

At the hot end, they get energy from the hot flame and move toward cold end, transferring energy to the other atoms and electron by collision on their way. At the same time atoms at the hot end, start vibrating more vigorously and transfer some energy by collision to neighboring atoms, eventually heating the other end.

Conduction is the process of transfer of heat by vibrating atoms and free electrons in solids from hotter to colder parts of the body while convection is the process of transfer of heat by actual movement of molecules from hot place to cold place. So in the gases, molecules do not vibrate at their places, due to weak intermolecular forces, the molecules actually move from their places.

That’s way the conduction of heat does not take place in gases.

Energy in houses can be conserved by using energy servers instead of bulbs.

• Switch off the electric applications when these are not used by humans.
• Geothermal energy can be used to heat up, water into steam for the utilization of cooking food.
• Solar energy is used by solar panel in houses.
• The solar energy is converted into electric energy.
• The bottoms of cooking pots are made black to increase the absorption of heat from fire.
• Hot water tanks are insulated by plastic or foam lagging.
• Wall cavities are filled with plastic foam wool.
• False ceiling is done using insulating materials.
• Double glazed window panes are used. These window panes have air between glass sheets that provides insulation.

Convection is a process of transfer of heat by actual movement of molecules from hot place to a cold place. As we know the intermolecular forces are less in fluids as compared to solid, due to weak intermolecular forces the fluids molecules can move from their actual positions and are responsible for the transfer of heat by convection process.

The path flow or current which is used for the transfer of heat by actual movement of molecules for hot place to a cold place is known as convection current.

An example of convection in daily life is when we use a fire place to heat our home, as the fire heats up the air in front of it, the hot air rises up as it is less dense and than in turn pushes the cool air down so that it is heated and then rises, this motion is called convection currents and is the reason why fire place air effective to heat us.

Heat reaches us from the sun through radiation process.

A Leslie cube is a metal box having faces of different nature.

• A shining silvered surface
• A dull black surface
• A white surface
• A coloured surface

1. Black dull surface is good emitter of heat.
2. A dull black surface is a good absorber of heat as its temperature rises rapidly.

• A polished surface is poor absorber of heat as its temperature rises very slowly.

1. Larger is the area, greater will be the transfer of heat.

In the environment, carbon dioxide and water also behave in a similar way to radiations as glass or polythene. Glass and transparent polythene sheets allow radiations of short wavelengths to pass through easily but not long wavelengths of thermal radiations. Thus, a greenhouse becomes a heat trap. Radiations from the sun pass easily through and warm up the objects in a greenhouse. The objects and plants in greenhouse give out radiation of much longer wavelengths and do not allow them to escape out easily and are reflected back in the greenhouse. Similarly Earth’s atmosphere contains carbon dioxide and water vapours. It causes greenhouse effect and thus maintains the temperature of the earth.

During the recent years, the percentage of carbon dioxide has increased considerably. This has caused an increase in the average temperature of the Earth by trapping more heat due to greenhouse effect. This phenomenon is known as global warming.

This has serious implications for global climate.

Transfer of Heat: When two bodies at different temperature are in thermal contact with each other. Thermal energy from a hot body flows to the cold body in the form of heat.

This is called as transfer of heat.

1. Conduction 2. Convection 3. Radiation

Thermal conductivity: Thermal conductivity of a substance can be defined as;

The rate of flow of heat across the opposite faces of a meter cube of a substance maintained at a temperature difference of one Kelvin.

Metals are good conductors of heat because metal contains free electrons which can move from one place to another, by getting energy.

Rate of flow of heat: The amount of heat flows in unit time is called the rate of flow of heat.

Rate of flow of heat = Q/t

Radiation: Radiation is the mode of transfer of heat from one place to another in the form of waves called electromagnetic waves.

The rate of heat flow in solids depends upon the following factors.

• Cross-sectional area of the solid.
• Length of the solid.
• Temperature difference between ends.

Conduction: The mode of transfer of heat by vibrating atoms and free electrons in solid from hot to cold parts of a body is called conduction of heat.

Uses of conductors:

• Good conductors are used when quick transfer of heat is required through a body.
• Cookers, cooking plats, boiler, radiators and condenser of refrigerators etc. are made of metals such ads aluminum and copper for the better conduction.
• Metal boxes are used for making ice, ice cream, etc.
• Sauce pans are made of metal for quick heat transfer.

Uses of non conductors:

• Insulators are bad conductors are used in home utensils such as handles of sauce pans, hotplates, spoons, etc. They are made up of wood or plastic.
• Air is one of the bad conductors or best insulator.

That is way cavity walls i.e two walls separated by an air space and double glazed windows keep the houses warm in winter and cool in summer.

Effect of length on thermal conductivity:

Larger is the length between the hot and cold ends of the solid, more time it will take to conduct heat to colder end and smaller will be the rate of flow of heat, Thus

Rate of flow of heat ∝ 1/L

Uses of non conductors:

Insulators are bad conductors are used in home utensils such as handles of sauce pans, hotplates, spoons, etc. They are made up of wood or plastic.

Air is one of the bad conductors or best insulator.

That is way cavity walls i.e two walls separated by an air space and double glazed windows keep the houses warm in winter and cool in summer.

Sea Breeze: On a hot day, the temperature of the land increases more quickly than the sea. It is because the specific heat of land is much smaller as compared to water. The air above land gets hot and rise up. The cold air from the sea begins to move towards the land.

Land Breeze: At night, the land cools faster than the sea. Therefore, air above the sea. It is called land breeze.

Uses of convection currents:

Convection currents occur on a large scale in nature. Some uses of convection current as;

• Convection currents set up by electric, gas or coal heats help to warm our homes and offices.
• Central heating systems in buildings work on the same principle by convection.
• The day-to-day temperature changes in the atmosphere result from the circulation of warm or cold air that travels across the region.
• Land and sea breezes are also the examples of convection current.

Land breeze blows from and towards sea because at night, the land cools faster than the sea. Therefore, air above the sea is warmer, rises up and colder air from the land begins to move towards the sea.

Conduction: The mode of transfer of heat by vibrating atoms and free electrons in solid from hot to cold parts of a body is called conduction of heat.

Convection: Transfer of Heat by actual movement of molecules from hot place to a cold place is known as convection.

Land breeze: At night, the land cools faster than the sea. Therefore, air above the sea is warmer, rises up and colder air from the land begins to move towards the sea. It is called land breeze.

It is not advisable to wear dark colours in summer, because dark colours absorb more radiations and heat.

We wear white or light coloured clothes in summer which reflect most of the heat radiation reaching us during the hot day.

• A shining silvered surface
• A dull black surface
• A white surface
• A coloured surface

The factors on which radiations depends.

• Colour and texture of the surface
• Surface temperature
• Surface area

• We wear white or light coloured clothes in summer which reflect most of the heat radiation reaching us during the hot day.
• Interior of the cooking pots are polished for reflecting back most of the heat radiation within them.

Relation of surface area with radiation:

Transfer of heat by radiation is also affected by the surface area of the body emitting or absorbing heat.

Larger is the area, greater will be the transfer of heat. It is due to this reason that large numbers of slots are made in radiators to increase their surface area.

Heat reach us from the sun through radiation process.

What is thermo flask?

Thermo flask: In a thermo flask, most of the heat is prevented to enter or leave the flask. This is done by suitable measures to reduce the transfer of heat due to conduction, convection and radiation. Thus, anything kept in it, maintains its temperature for a long time.