Unit 10: Thermal Physics – Long Questions & Answers

Solids: In solids, atoms are arranged in a tight, regular pattern. As the temperature rises, each atom pushes slightly harder against its neighbours due to increased vibration. Since there is no room to move inwards, the structure expands outward instead.
For example, expansion of a metal rod when heated.

Liquids: Unlike solids, which have a fixed shape and expand in a limited way, liquids do not have a definite shape, so they expand freely in all directions when their temperature rises. Liquids expand more freely than solids.
For example, rise of mercury or alcohol in a thermometer.

Gases: Gases exhibit the greatest relative thermal expansion while solids have the least expansion. In a flexible container, the volume of a gas can expand to allow pressure to remain constant.
For example, expansion of air in a hot air balloon.

The sources list several consequences commonly used in daily life:

Thermometers: Thermal expansion in a narrow tube helps in the measurement of temperature.

Gaps in Railway Tracks: Steel tracks of the rails expand on hot days. Small gaps are left between railway tracks to allow them to expand in summer, preventing them from bending.

Bridges: Bridges have small gaps called expansion joints that allow them to expand in hot weather and contract in cold weather without breaking.

Metal Lids on Jars: When a metal lid is stuck on a glass jar, running warm water over it makes the lid expand slightly, making it easier to open.

Gas Containers: Gases expand significantly when heated. Pressure can build up inside and causes explosions; therefore, pressure relief valves are often included.

Pipes and Plumbing: Water pipes can expand and contract, especially with hot water. Flexible joints and loops are used in piping systems to absorb movement.

Hot Air Balloon: The expansion of heated air is intentionally used to make the balloon rise. Heating the air inside causes it to expand, reducing its density so it lifts the balloon.

Latent heat is the heat energy required to change the state of a substance without changing its temperature. This heat is important because it is used to break or form bonds between particles instead of increasing their movement.

• Melting: When a solid melts into a liquid, it absorbs heat without rise in temperature. This heat is used to break the strong bonds between solid particles, allowing them to move more freely as a liquid.
• Vaporization: When a liquid changes into gas, it absorbs heat, but its temperature remains constant until the entire liquid has evaporated. This heat is used to break the bonds between liquid molecules, allowing them to escape as gas.

Expansion occurs because as the temperature rises, the particles within a substance gain more kinetic energy. This causes them to move faster and spread out, leading to expansion of the material.

• Solids Example: A metal rod increases in length when heated because its atoms vibrate with larger amplitude.
• Liquids Example: In a flask, as the liquid heats up, it expands and the level rises from B to C, known as real expansion.
• Gases Example: In car engines, fuel burns and produces hot gases that expand quickly, pushing the pistons and making the vehicle move.

Water in a wide, shallow dish will evaporate quicker than the same amount in a tall, narrow container.

The scientific reason is that a larger surface area allows more liquid to be exposed to the air, which speeds up evaporation. Evaporation occurs at the surface of the liquid, where some of the particles have enough energy to escape into the air as vapour; a wider dish provides more surface for these energetic particles to escape.