Temperature is a measure of the degree of hotness or coldness of a body and determines the direction of heat flow.

Heat is a form of energy transferred from one body to another due to a temperature difference.

What is thermal equilibrium?

Thermal equilibrium is the state in which bodies in contact attain the same temperature and no net heat transfer occurs.

State the Zeroth Law of Thermodynamics.

If two systems are each in thermal equilibrium with a third system

What is the importance of the Zeroth Law?

It forms the basis of temperature measurement and the construction of thermometers.

Define thermal expansion.

Thermal expansion is the increase in dimensions of a substance when its temperature rises.

Define coefficient of linear expansion.

It is the fractional change in length per unit original length per unit rise in temperature.

Write the formula for linear expansion.

Change in length is given by \( \Delta L = L_0 \alpha \Delta T \).

What is areal expansion?

Areal expansion is the increase in surface area of a solid due to rise in temperature.

What is volumetric expansion?

Volumetric expansion is the increase in volume of a substance with temperature.

Why are gaps provided between railway tracks?

To allow thermal expansion and prevent buckling during high temperatures.

What is anomalous expansion of water?

Water contracts on heating from \(0^\circ\text{C}\) to \(4^\circ\text{C}\) and expands beyond \(4^\circ\text{C}\).

Free NCERT Solutions, Notes & MCQs for Class 9–12

Learning Objectives

By the end of this chapter, you should be able to link microscopic motion of molecules to macroscopic quantities like temperature, heat, expansion and heat transfer.

Conceptual

Distinguish clearly between heat, internal energy and temperature, and interpret temperature scales (Celsius, Fahrenheit and Kelvin) using conversion relations.

Expansion

Explain and calculate linear, areal and volume expansion of solids and liquids, and relate coefficients of expansion to real-life situations (rails, bridges, glassware).

Calorimetry

Solve calorimetry problems using energy conservation, specific heat capacity, latent heat and the principle of mixtures to find final temperature or mass.

Heat Transfer

Analyse conduction, convection and radiation, use the conduction formula for slabs and rods, and understand basic ideas of thermal conductivity and thermal resistance.

Chapter at a Glance

Temperature scales & thermometers

Thermal expansion of solids, liquids & gases

Heat capacity & specific heat capacity

Calorimetry & principle of mixtures

Change of state & latent heat

Conduction, convection & radiation

Core Concepts Explained

Heat, temperature & internal energy
Temperature is a measure of the average kinetic energy of molecules, heat is energy in transit due to temperature difference, and internal energy is total microscopic energy of a system.

Thermal expansion
Most materials expand on heating because molecular separation increases as average kinetic energy rises; different materials have different coefficients of expansion, which must be considered in engineering design.

Specific heat capacity & calorimetry
Specific heat capacity tells you how much heat is needed to raise unit mass through 1 K, and calorimetry uses this along with the principle that heat lost by hot bodies equals heat gained by cold bodies in an isolated system.

Heat transfer modes
Conduction occurs through molecular collisions in solids, convection through bulk motion of fluids, and radiation via electromagnetic waves even through vacuum, with good absorbers being good emitters.

Everyday Applications

Why gaps in railway tracks?
Rails expand in hot weather; small expansion per metre becomes large over hundreds of metres, so expansion gaps prevent buckling of tracks and accidents.

Cooking utensils & specific heat
Utensils are often made of metals with low specific heat and high thermal conductivity so that they heat up quickly, while handles use materials with high specific heat and low conductivity for safety.

Thermos flask design
A thermos combines poor conduction (insulating walls), reduced convection (narrow neck) and low radiation (shiny surfaces) to keep liquids hot or cold for long durations.

Weather and coastal breeze
Land heats and cools faster than water because of different specific heats, leading to sea breeze in the day and land breeze at night due to convection currents in air.