This chapter has been kept exam-focused for RAS Prelims + Mains. It includes heat and temperature, temperature scales, heat transfer, specific heat, latent heat, expansion, change of state, gas laws, thermodynamics, and applications in daily life.

1 What is Heat? by RAS A2Z

Heat is that energy which is transferred from one object to another due to a difference in temperature.

The flow of heat generally occurs from:

SI unit of heat:

Other commonly used units:

• Calorie (cal)
• Kilocalorie (kcal)

2 What is Temperature? by RAS A2Z

Temperature is a measure of the thermal state of an object, and it indicates how hot or cold the object is.

Temperature is related to the average kinetic energy of the particles of a substance.

3 Temperature Scales by RAS A2Z

Three major temperature scales:

Celsius (°C)

Water's:

• Freezing point = 0°C
• Boiling point = 100°C

At normal atmospheric pressure.

Fahrenheit (°F)

• Freezing point = 32°F
• Boiling point = 212°F

Kelvin (K)

• 0°C = 273.15 K
• 100°C = 373.15 K

In practical calculations, often:

is taken.

4 Temperature Conversion by RAS A2Z

And

Therefore:

5 Thermal Equilibrium by RAS A2Z

When the temperature difference between two objects disappears and there is no net flow of heat between them, they are in Thermal Equilibrium.

This concept is the basis of the Zeroth Law of Thermodynamics.

6 Transfer of Heat by RAS A2Z

Heat is transferred in three major ways:

1. Conduction
2. Convection
3. Radiation

7 Conduction by RAS A2Z

The transfer of heat from one part of a substance to another without the actual movement of its particles is called conduction.

It is mainly important in solids.

Example

Heating one end of an iron rod makes the other end hot as well.

Good Heat Conductors

• Silver
• Copper
• Aluminium
• Iron

Poor Conductors/Heat Insulators

• Wood
• Plastic
• Rubber
• Glass
• Air

8 Convection by RAS A2Z

The transfer of heat by the actual movement of the particles of a liquid or gas is called convection.

Example

When water is heated, hot water rises and cold water sinks, creating convection currents.

Daily Life

• Sea breeze
• Land breeze
• Hot air rising in a room
• Hot gases rising in a chimney

9 Sea Breeze by RAS A2Z

During the day, land heats up faster than water.

• The air above the land becomes hot and rises.
• The relatively cooler air from the sea moves towards the land.

This is called sea breeze.

10 Land Breeze by RAS A2Z

At night, land cools faster than water.

• The relatively warm air above the sea rises.
• The cool air from the land blows towards the sea.

This is called land breeze.

11 Radiation by RAS A2Z

The transfer of heat in the form of electromagnetic waves is called radiation. It does not require any physical medium.

12 Difference between Conduction, Convection and Radiation by RAS A2Z

13 Thermal Expansion by RAS A2Z

On increasing the temperature, the dimensions of most substances increase. This is called thermal expansion.

Its three types:

1. Linear Expansion
2. Areal Expansion
3. Volume Expansion

14 Linear Expansion by RAS A2Z

The change in length is called linear expansion.

Where:

• α = Coefficient of linear expansion
• L₀ = Initial length
• ΔT = Change in temperature

Daily Life

Small gaps are left between railway tracks so that the tracks do not bend due to expansion in summer.

15 Expansion Joints in Bridges by RAS A2Z

Expansion Joints are provided in long bridges and metal structures.

Reason:

Temperature change → expansion/contraction of metal → stress in the structure.

Expansion joints help reduce this stress.

16 Bimetallic Strip by RAS A2Z

A strip made by joining two strips of different metals is called a bimetallic strip.

Since the coefficients of expansion of the two metals are different, the strip bends when heated.

Uses

• Thermostat
• Electric switch
• Temperature control devices

17 Anomalous Expansion of Water by RAS A2Z

The behaviour of water is different from most substances.

When water is heated from 0°C to 4°C, its volume decreases and density increases.

At about 4°C, water has maximum density.

Above 4°C, water expands like normal substances as temperature increases.

Importance

In cold climates, the upper surface of lakes may freeze, but the water below can remain at about 4°C. This provides protection to aquatic life.

18 Specific Heat Capacity by RAS A2Z

The heat required to raise the temperature of a unit mass of a substance by 1°C or 1 K is called its specific heat capacity.

Where:

• Q = Heat
• m = Mass
• c = Specific heat capacity
• ΔT = Change in temperature

SI Unit:

19 Specific Heat of Water by RAS A2Z

The specific heat capacity of water is relatively high.

Because of this, water:

• Heats up slowly
• Cools down slowly
• Helps keep temperature stable.

Daily Importance

Seas and large water bodies play an important role in moderating the temperature of coastal areas.

20 Latent Heat by RAS A2Z

The heat absorbed or released by a substance during a change of state is called latent heat.

Where:

• m = Mass
• L = Specific latent heat

21 Latent Heat of Fusion by RAS A2Z

The heat required to convert a solid into a liquid at its melting point is called the latent heat of fusion.

While ice melts, the temperature can remain steady at about 0°C, while the heat is used for the change of state.

22 Latent Heat of Vaporization by RAS A2Z

The heat required to convert a liquid into a gas at its boiling point is called the latent heat of vaporization.

Example

A considerable amount of heat is absorbed when water turns into steam.

For this reason, steam can cause more severe burns than hot water at the same temperature.

23 Evaporation by RAS A2Z

The slow conversion of a liquid into a gas from its surface at any temperature is called evaporation.

It:

• Is a surface phenomenon.
• Can occur at any temperature.
• Produces cooling.

Example

Sweat evaporates and cools the body.

24 Factors Affecting Evaporation by RAS A2Z

The rate of evaporation increases when:

• Temperature increases
• Surface area increases
• Wind speed increases
• Humidity decreases

Example

Wet clothes dry faster in strong wind.

25 Boiling by RAS A2Z

The rapid conversion of a liquid into a gas, with vapour bubbles forming throughout its volume, is called boiling.

At normal atmospheric pressure, the boiling point of water is:

But the boiling point changes with pressure.

26 Why Does Water Boil Faster at Higher Altitudes? by RAS A2Z

As altitude increases, atmospheric pressure decreases.

At lower pressure, the boiling point of water decreases.

Therefore, in high-altitude regions, water can boil even at a temperature below 100°C.

27 Pressure Cooker by RAS A2Z

In a pressure cooker, the boiling point of water increases as pressure increases.

Therefore, food can be cooked at a temperature above the normal 100°C and cooks relatively faster.

28 First Law of Thermodynamics by RAS A2Z

The application of the principle of conservation of energy to thermal processes is the first law of thermodynamics.

General form:

Where:

• ΔQ = Heat given to the system
• ΔU = Change in internal energy
• W = Work done by the system

That is, the heat supplied can be spent in increasing internal energy and doing work.

29 Second Law of Thermodynamics by RAS A2Z

Heat does not spontaneously flow from a cold body to a hot body.

Spontaneous heat flow generally occurs:

In a refrigerator, with the help of external work, heat is transferred from a cold part to the relatively warmer surroundings.

30 Applications of Heat in Daily Life by RAS A2Z

• Cooking food
• Refrigerator
• Air conditioner
• Pressure cooker
• Thermos flask
• Solar cooker
• Engine
• Heater
• Thermostat
• Weather and climate
• Sea/Land breeze

SF RAS Prelims — Super Facts by RAS A2Z