Heat: Why Objects Become Hot, Cold, and Change Their State

Heat is something we experience every day.
We feel heat from the Sun, from fire, from an iron, and even from our own body.
But in physics, heat is not just a feeling.
It is a form of energy that explains:

• Why temperature changes
• Why solids melt
• Why liquids boil
• How heat flows from one body to another

Understanding heat helps us understand many natural and daily-life processes.

Effects of Heat

When heat is supplied to a body, it can produce different effects depending on the situation.

Rise in Temperature

One major effect of heat is an increase in temperature.
When heat is supplied:

• Particles gain kinetic energy
• They move faster
• Temperature rises

Example:

When water is heated on a stove, its temperature increases.

Expansion of Matter

Another important effect of heat is expansion.
Most substances expand on heating and contract on cooling.

• Solids expand least
• Liquids expand more
• Gases expand the most

Example:

Metal railway tracks are laid with gaps to allow expansion in summer.

Change of State

Heat can change the state of matter.

• Solid → Liquid
• Liquid → Gas

Example:

Ice changes into water on heating.

Measurement of Temperature and Heat

Heat and temperature are related but not the same.

Temperature

Temperature indicates the degree of hotness or coldness of a body.
It tells:

• How hot a body is
• How cold a body is

Temperature does not depend on the amount of substance.

Heat

Heat is the energy that flows from a body at higher temperature to a body at lower temperature.
Heat depends on:

• Temperature difference
• Mass of the body
• Nature of material

Measuring Temperature

Temperature is measured using thermometers.
Common temperature scales:

• Celsius scale
• Fahrenheit scale
• Kelvin scale

The SI unit of temperature is kelvin (K).

Measuring Heat

Heat is measured in:

• Joule (SI unit)
• Calorie (older unit)

1 calorie = heat required to raise temperature of 1 g water by 1°C.

Change of State and Latent Heat

Sometimes, heat is supplied but temperature does not rise.
This happens during change of state.

Change of State

Change of state occurs when matter changes from:

• Solid to liquid (melting)
• Liquid to gas (boiling)

During this process:

• Temperature remains constant
• Heat is used to change state, not temperature

Latent Heat

Latent heat is the heat required to change the state of a substance without change in temperature.

Latent Heat of Fusion

It is the heat required to change 1 kg of solid into liquid at its melting point.

Example:

Ice at 0°C absorbs heat and changes into water at 0°C.

Latent Heat of Vaporisation

It is the heat required to change 1 kg of liquid into gas at its boiling point.

Example:

Water at 100°C absorbs heat and changes into steam at 100°C.

Modes of Transference of Heat

Heat flows from one body to another in three different ways.

Conduction

Conduction is the transfer of heat through a substance without actual movement of particles.
It occurs mainly in solids.

Example:

When one end of a metal rod is heated, the other end becomes hot after some time.
Metals are good conductors of heat.

Convection

Convection is the transfer of heat through actual movement of particles.
It occurs in liquids and gases.

Example:

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

Radiation

Radiation is the transfer of heat without any medium.
It does not require solid, liquid, or gas.

Example:

Heat from the Sun reaches the Earth through radiation.
All bodies emit heat radiation.

Comparison of Modes of Heat Transfer

• Conduction → solids
• Convection → liquids and gases
• Radiation → no medium required

Understanding these differences helps in answering many MCQs.

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FAQs – HEAT (Exam-Focused)