Unit 10: Thermal Physics — Short Questions

The expansion of a solid depends on its original length (Lo), the temperature change (ΔT), and the thermal expansion coefficient (α).
Formula: ΔL = α Lo ΔT
Factors:

• Original length (Lo): Longer rod expands more
• Temperature change (ΔT): Larger ΔT gives larger expansion
• Thermal expansion coefficient (α): Different materials have different α values (Aluminium has higher α than steel, so expands more)

Latent Heat of Fusion
The heat energy required to convert 1 kg of a solid into a liquid at its melting point while keeping the temperature constant is called Latent heat of fusion. This energy is used to break bonds between solid particles, so the temperature stays constant during melting.
Formula
Q = mLf
Unit: J/kg

Different materials expand by different amount on heating their atoms are bonded with different strengths. Materials with weaker bonding expand more, while those with strong bonding expand less.
Gases expand more than liquids and solids due to their weak bonding.

Evaporation causes cooling because the faster (higher energy) molecules of a liquid escape into the air during evaporation. These molecules take heat energy with them, so the remaining liquid loses heat and becomes cooler
Example
When sweat evaporates from our skin, it absorbs heat from the body to change into vapor. As a result, the body loses heat and we feel cool. This process helps regulate body temperature, especially in hot weather.

The heat energy needed to change 1 kg of a liquid into gas at its boiling point without changing the temperature is called Latent heat of vaporization. When a liquid boils, it absorbs heat without a temperature rise until all the liquid has evaporated.
Formula
Q = mLv
Unit
J/kg

The change in length, area, or volume of a substance when it is heated is called Thermal expansion.
Reason:
As the temperature rises, the particles within a substance gain more kinetic energy. This causes them to move faster and spread out, leading to the expansion of the material. Conversely, cooling causes particles to lose energy and move closer together, resulting in contraction.

The fractional change in length per degree change in temperature for a specific material is called the coefficient of linear thermal expansion.
Formula
ΔL = α Lo ΔT
Here α is coefficient of linear expansion
SI Unit
K⁻¹ (Per Kelvin)

Linear Thermal Expansion and Volume Thermal Expansion
The expansion in length due to heating is called linear thermal expansion. It is given by ΔL = α Lo ΔT Where α is the co-efficient of linear thermal expansion.
Volume thermal expansion is the increase in volume of a solid or liquid when heated. It is given by:
ΔV = β Vo ΔT
Where β is the coefficient of volume expansion.

Real Expansion

• This is the actual increase in the volume of the liquid itself, independent of the container's expansion.
• Real expansion is always greater than apparent expansion.
• It accounts for both the thermal expansion of the liquid and the container.
• Formula: Real expansion = apparent expansion + expansion of vessel

Apparent Expansion

• When a liquid is heated in a container, the container also expands. The observed rise in liquid level, which is less than the actual expansion, is called apparent expansion.
• Apparent expansion is always smaller than real expansion.
• It only accounts for the visible change in liquid level due to unequal expansion rates.
• The difference between real and apparent expansion equals the expansion of the flask itself.

Steel tracks expand on hot days. If no room is left for this expansion, the tracks may bend or twist—a phenomenon known as "sun kinks"—which can cause train derailments. Small gaps are left between tracks to allow for this expansion safely.

When the air inside a balloon is heated, it expands and becomes less dense (lighter) than the cooler air outside. This difference in density creates an upward lift, causing the balloon to rise.

Specific heat capacity is the amount of heat energy needed to increase the temperature of 1kg of a substance by (or 1K).
Formula
Q = m c ΔT
c = specific heat capacity
SI Unit: J kg⁻¹ K⁻¹ (Joules per kilogram per Kelvin)

Water has a high specific heat capacity (4200 J kg⁻¹ K⁻¹), compared to dry soil (810 J kg⁻¹ K⁻¹). This means soil heats up five times faster than the same mass of water under the same heat, causing land temperatures to rise more quickly than sea temperatures.

Water is used because of its high specific heat capacity (4200 J kg⁻¹ K⁻¹), allowing it to absorb a large amount of heat with only a small rise in temperature.

• Water circulates around the engine cylinders, absorbing heat.
• The heated water moves to the radiator, where it is cooled by air drawn in by a fan.
• The cooled water returns to the engine to repeat the cycle.

The process by which a liquid change into a gas at its surface without boiling is called Evaporation.
Cooling Effect:
It occurs when more energetic particles escape from the surface of the liquid as vapors. As these high-energy particles leave, the remaining liquid has lower average kinetic energy, resulting in a drop in-temperature (cooling).

Temperature Higher temperature increases the rate of evaporation.
Surface Area: Larger surface area speeds up evaporation.
Air Movement (Wind): Wind carries away vapor's molecules, preventing saturation and speeding up the process.
Humidity: High humidity slows down evaporation; dry air speeds it up.
Nature of Liquid: Liquids with weaker molecular forces (like alcohol) evaporate faster than water.

Environmentally friendly refrigerants are used to evaporate at low temperatures inside cooling coils. As the refrigerant evaporates, it absorbs a large amount of heat from inside the refrigerator. The vapour is then compressed and condensed back into a liquid outside the compartment, releasing the heat.

The heat energy required to convert 1 kg of a solid into a liquid at its melting point while keeping the temperature constant is called Latent heat of fusion.
Formula
Q = m Hf
Hf = latent heat of fusion
Value for Ice: The latent heat of fusion of ice is 3.36 × 10⁵ J kg⁻¹ (or 336 J g⁻¹)

The heat absorbed during melting is used to break the strong bonds between solid particles rather than increasing their kinetic energy. Therefore, the temperature remains constant until the entire solid has melted.

Latent heat of vaporization is the heat energy needed to change 1 kg of a liquid into gas at its boiling point without changing the temperature.
Formula
Q = m Hv
Hv = latent heat of vaporization
Value for Water: The latent heat of vaporization of water is 2.26 × 10⁶ J kg⁻¹ (or 2260 J g⁻¹)

Superconductivity is a state in which materials exhibit zero electrical resistance. This occurs when the material is cooled below a specific temperature.
Examples
Mercury (becomes superconductor below 4.2 K) and Lead (below 7.2 K).

The temperature below which a material exhibits zero electrical resistance is called critical temperature for e.g. the critical temperature Hg is 4.2 K.

MRI Machines Generating strong magnetic fields for medical imaging.
Maglev Trains: Floating trains above tracks to reduce friction.
Particle Accelerators: They help scientists study fundamental particles. Superconducting magnets accelerate particles to nearly the speed of light.

Bridges are long metal structures, so they expand significantly when heated. Small gaps (expansion joints) are placed in bridges to allow this expansion and contraction without damage. A glass jar with a metal lid is much shorter, so heating only causes a very small increase in length. In fact, warming the metal lid (for example by running it under warm water) makes it expand just enough to loosen the lid.

As heat capacity is given by Q=mc∆T for same heat and mass 'm' ∆T α c⁻¹, it means liquid which heats up faster has smaller specific heat capacity.

Evaporation depends on humidity. Low humidity increases evaporation, while high humidity reduces it.
Explanation
A liquid evaporates faster when the air is dry because dry air can hold more water vapor. When humidity is low, water molecules easily leave the liquid and mix with the air. Since the air is not saturated, evaporation continues quickly. If the air is humid, it already contains a lot of water vapor. In this case, the air cannot absorb much more vapor, so evaporation slows down.

When ice melts, the heat supplied is used to break the bonds between water molecules. This energy is called latent heat of fusion. It helps change ice into water instead of raising the temperature. Therefore, the temperature remains at 0 °C until all the ice has melted. This is important in phase change because it allows a substance to change its state at a constant temperature.

Electric transmission wires are made of metal and follow the formula
ΔL = α Lo ΔT

Explanation
Electric transmission wires are made of metal. Metals expand when temperature increases and contract when temperature decreases. In summer, the high temperature makes the wires expand, so they become longer and sag. In winter, the low temperature causes the wires to contract, so they become shorter and tight.

Thermal expansion differs because the distance between particles and the strength of forces between them are different.
Solids: Particles are very close and strongly bonded, so expansion is very small.
Example: Railway tracks expand slightly in summer.
Liquids: Particles are less tightly packed, so expansion is more than solids.
Example: Mercury rises in a thermometer when heated.
Gases: Particles are far apart with very weak forces, so expansion is very large.
Example: A balloon expands when filled with hot air.
Conclusion
Gases expand most, liquids expand more than solids, and solids expand the least due to differences in particle spacing and bonding.

Shallow dish has larger surface area compared to tall, narrow container, so more water molecules will evaporate in dish.