Unit 10: Thermal Physics – Short Questions & Answers

Thermal expansion is the change in length, area, or volume of a substance when it is heated. As the temperature rises, particles gain kinetic energy, move faster, and spread out, leading to the expansion of the material.

When a substance is cooled, its particles lose energy and move closer together, causing the material to contract. This phenomenon is known as thermal contraction.

If the length of a solid changes upon heating, it is called linear thermal expansion.

The coefficient of linear expansion \(\alpha\) is defined as the fractional change in length per unit rise in temperature of a substance. Mathematically \(\alpha = \frac{\Delta L}{L_{\circ}\Delta T}\).

The volume of a solid also increases when its temperature rises; this is known as volume thermal expansion.

The coefficient of volume expansion \(\beta\) is defined as the fractional change in volume per unit rise in temperature of a substance. Mathematically \(\beta = \frac{\Delta V}{V_{\circ}\Delta T}\).

Liquids do not have a fixed shape or a tight regular pattern like solids, which allows them to expand freely in all directions when their temperature rises.

When a liquid in a flask is heated, the flask expands first (expansion of flask AB), then the liquid expands. The actual change in liquid volume (BC) is real expansion, while the observed rise from the original level (AC) is apparent expansion.

As the temperature of a gas increases, its particles move faster and collide more frequently and forcefully with the walls of the container. These collisions exert pressure on the walls.

Charles' Law states that the volume of a gas is directly proportional to its temperature at constant pressure.

Sun kinks are sharp, dangerous curves or buckles that appear in railway tracks during periods of extreme heat.

Steel tracks expand on hot days. Small gaps are left between them to allow for this expansion in summer, preventing the tracks from bending or twisting (sun kinks), which could cause derailments.

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

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.

Gases expand significantly when heated. If aerosol cans are exposed to high temperatures, pressure builds up inside, which can cause the container to explode.

Flexible joints and loops are used in piping systems to absorb movement. This is because water pipes can expand and contract, especially with hot water, which may lead to pipe bursts or joint leaks if not properly accounted for.

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

Specific heat capacity \(c\) is the amount of heat energy needed to increase the temperature of \(1kg\) of a substance by \(1^{\circ}C\) or \(1K\). This relationship is expressed using the formula: \(Q = mc\Delta T\). Its SI unit is \(Jkg^{-1}K^{-1}\).

Metal is a conductor, wood is an insulator. The metal takes heat away from our hands quickly, but wood does not, so wood feels warmer than metal.

Water has a high specific heat capacity of \(4,200Jkg^{-1}K^{-1}\), which allows it to absorb and carry away a large amount of heat produced during engine operation with only a small temperature increase.

Large water bodies like oceans and lakes absorb heat during the day and release it slowly at night, preventing drastic temperature changes. Since water has a specific heat capacity of \(4,200Jkg^{-1}K^{-1}\), while dry soil has only \(810Jkg^{-1}K^{-1}\), land temperatures rise and fall more quickly than sea temperatures, making seasonal temperature changes less extreme in coastal areas.

By absorbing or losing energy, the K.E. of the atoms or molecules of a substance changes that can lead to change in the state of a matter. Change in states of matter occurs when matter loses or absorbs energy.

When a substance absorbs energy, the atoms and molecules move more rapidly and hence need more space and separation between atoms or molecules. It may lead to turn a solid to liquid, called melting, and liquid to gas, known as boiling.

By losing energy, the atoms and molecules will slow down and hence there is less space and separation between them. It may lead to turn a gas to liquid, called condensation, and a liquid to solid, known as solidification.

Intermolecular forces are the strongest in solids due to least separation and weakest in the gases, as the molecules are far apart as compared to liquids and solids. This is the reason that the gases exhibit the greatest relative thermal expansion while solids have the least expansion.

When a substance absorbs energy, it may turn from solid to liquid, called melting, and from liquid to gas, known as boiling. When a substance loses energy, it may turn from gas to liquid, called condensation, and from liquid to solid, known as solidification.

Evaporation is a natural process by which a liquid changes into a gas at its surface without boiling. It occurs at all temperatures but is faster at higher temperature.

Rate of evaporation depends upon following factors: 1. Temperature 2. Surface Area 3. Air Movement (Wind Speed) 4. Humidity of the Surrounding Air 5. Nature of the Liquid 6. Pressure.

During evaporation, the molecules that evaporate take away heat from the remaining liquid. This process cools the liquid down because the higher-energy molecules leave, leaving behind cooler molecules. For example, when we sweat, evaporation removes heat from our body and makes us feel cool.

CFCs are harmful to the environment, so modern systems avoid using ozone-depleting CFCs.

Latent heat is the heat energy required to change the state of a substance without changing its temperature.

Latent heat of fusion is the heat energy required to convert \(1kg\) of a solid into a liquid at its melting point while keeping the temperature constant. Latent heat of fusion is expressed as \(Q = mL_{f}\).

Latent heat of vaporization is the heat energy needed to change \(1kg\) of a liquid into gas at its boiling point without changing the temperature. Latent heat of vaporization is expressed as \(Q = mL_{v}\).

Superconductivity is a special property of some materials where they have zero electrical resistance when cooled below a certain temperature, called the critical temperature.

Since there are no collisions, electric current flows without facing any resistance as superconductors have zero electrical resistance.

1. Mercury - Becomes superconducting below \(4.2K\)
2. Lead - Becomes superconducting below \(7.2K\)
3. High-temperature superconductors (ceramics) work at higher temperatures, such as \(135K\).

Superconductors are used in advanced technologies, including: 1. MRI Machines: To generate strong magnetic fields for medical imaging. 2. Maglev Trains: Use of superconductors to float the train above tracks, reducing friction. 3. Particle Accelerators: Help scientists to study fundamental particles.