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9th Class Physics Chapter 9 Notes

Heat flows from a body at higher temperature to a body at lower, temperature.

There are three ways of heat transfer. These are conduction, convection and radiation.

The mode of transfer of heat by vibrating atoms and free electrons in solids from hotter to colder part of a body is called conduction of heat.

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

The rate at which heat flows through solids depends on the cross sectional area of the solid, length between hot and cold ends temperature difference between hot and cold ends and nature of the material.

The rate of flow of heat across the opposite faces of a metre cube maintained at a difference of 1 K is called the thermal conductivity of the material of the cube.

Good conductors are used for quick transfer of heat. Thus cookers, cooking plate, boiler, radiators and condensers of refrigerators etc. are made of metals.

Water is a poor conductor of heat.

Materials which trap air are also bad conductors such as wool, felt, fur, feathers, polystyrenes and fibre glass.

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

Land and sea breezes are also the examples of convection.

Gliders use upward movement of hot air currents due to convection of heat. Air currents help them to stay in air for a long period.

Birds are able to fly for hours without flapping their wings due to the upward movement of air currents.

The term radiation means the continual emission of energy from the surface of a body in the form of electromagnetic waves.

Radiations are emitted by all bodies. The rate at which radiations are emitted depends on various factors such as colour and texture of the surface, temperature and surface area.

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.

Radiations from the Sun pass easily through glass/polythene and warm up the materials inside a greenhouse. The radiations given out by them are of much longer wavelengths. Glass/polythene does not allow them to escape out and thus maintains the inside temperature of the greenhouse.

Earth’s atmosphere contains carbon dioxide and water vapours. It causes greenhouse effect and thus retains the temperature of the Earth.

The bottoms of cooking pots are made black to increase the absorption of heat from fire.

White surfaces reflect more heat than coloured or black surfaces. Similarly, polished surfaces are good reflectors than rough surfaces and reflection of heat radiations is greater from polished surfaces. Therefore, We wear white or light coloured clothes in summer

We polish the interior of the cooking pots for reflecting back most of the heat radiation inside the hot pots.

A thermos flask consists of a double-walled glass vessel. It reduces the transfer of heat by conduction, convection and radiation.

Class 9th Physics Chapter 8th Notes

The temperature of a body is the degree of hotness or coldness of the body.

Thermometers are made to measure the temperature of a body or places.

The lower fixed point is the mark that gives the position of mercury in the thermometer when it is placed in ice.

The upper fixed point is the mark that shows the position of mercury in the thermometer when it is placed in steam from boiling water at standard pressure.

Inter-conversion between scales:

From Celsius To Kelvin Scale:

T(K)=273 + C

From Kelvin to Celsius Scale:

C =T(K)-273

From Celsius to Fahrenheit Scale:

F =1.8C+32

Heat is a form of energy and this energy is called heat as long as it is in the process of transfer from one body to another body. When a body is heated, the kinetic energy of its molecules increases, the average distances between the molecules increase.

The specific heat of a substance is defined as the amount of heat required to raise the temperature of a unit mass of that substance through one degree centigrade (1°C) or one kelvin (1K).

The heat required by unit mass of a substance at its melting point to change it from solid state to liquid state is called the latent heat of fusion.

The quantity of heat required by the unit mass of a liquid at a certain constant temperature to change its state completely from liquid into gas is called the latent heat of vaporization.

It has been observed that solids expand on heating and their expansion is nearly uniform over a wide range of temperature. Mathematically, L =L0(1+arT)

The thermal coefficient of linear expansion of a substance is defined as the fractional increase in its length per kelvin rise in temperature.

The volume of a solid changes with the change in temperature and is called as volume or cubical expansion. V =V0(1+ b rT)

The thermal coefficient of volume expansion b is defined as the fractional change in its volume per kelvin change in temperature.

There are two types of thermal volume expansion for liquids as well as for gases. Apparent volume expansion and real volume expansion.

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