Sciences - Co-ordinated (Double) (0654) | Thermal physics

Thermal Physics (Physics Section P2) – Comprehensive Study Notes

Welcome to Thermal Physics! This chapter is all about heat, temperature, and how energy moves through the universe, from your morning cup of tea to the Earth's climate. Understanding these concepts will help you explain why boiling water stays at $100^{\circ}C$ and why painting your house white keeps it cooler. Let's dive in!

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P2.1 The Kinetic Particle Model of Matter

The entire world is made of tiny particles (atoms or molecules) that are constantly moving. The way these particles are arranged and move determines if a substance is a solid, liquid, or gas. This is the Kinetic Particle Model.

P2.1.1 States of Matter (Solids, Liquids, Gases)

Here’s a quick overview of the three states of matter you need to know:

Solid

• Distinguishing Property: Has a fixed shape and a fixed volume.

Liquid

• Distinguishing Property: Does not have a fixed shape (takes the shape of the container) but has a fixed volume.

Gas

• Distinguishing Property: Does not have a fixed shape and does not have a fixed volume (fills the entire container).

Quick Review: Changes of State

• Melting: Solid to liquid
• Boiling: Liquid to gas (throughout the liquid)
• Evaporating: Liquid to gas (only from the surface, below the boiling point)
• Freezing (Solidification): Liquid to solid
• Condensing (Condensation): Gas to liquid

P2.1.2 Explaining the States using the Particle Model (Core & Supplement)

The main difference between the states is the arrangement, separation, and movement of the particles.

1. Solids

• Arrangement: Regular pattern (lattice).
• Separation: Closely packed.
• Motion: They vibrate around fixed positions.
• Forces: Strong forces hold them tightly together.

2. Liquids

• Arrangement: Randomly arranged.
• Separation: Closely packed, but slightly further apart than solids.
• Motion: They move randomly and slide past each other.
• Forces: Weaker forces than solids, allowing movement.

3. Gases

• Arrangement: Randomly arranged.
• Separation: Far apart (large distances between them).
• Motion: Move very rapidly and randomly in all directions.
• Forces: Almost negligible forces between particles, except during collisions.

Analogy: Think of the states like people on a dance floor.
Solid: Everyone is standing in a perfect grid and just wiggling (vibrating).
Liquid: Everyone is touching but moving randomly and swapping places.
Gas: Everyone is running wildly across the huge room, rarely bumping into anyone else.

Particles and Temperature

There is a direct link between the motion of particles and the temperature of the substance:

When you heat a substance, its particles gain kinetic energy, which means they move faster (vibrate or travel more rapidly). Temperature is a measure of the average kinetic energy of the particles.

Evidence: Brownian Motion (Supplement)

Brownian motion is the random movement of tiny particles (like smoke or pollen grains) suspended in a liquid or gas.

• Observation: When viewed under a microscope, the smoke particles appear to move randomly and jerkily.
• Explanation: This erratic movement is caused by the much smaller, fast-moving particles (of the gas or liquid) randomly colliding with the larger smoke particles. This provides strong evidence that the particles of gases and liquids are in constant, random motion.

Gas Pressure (Supplement)

A gas exerts pressure because its particles are constantly moving and hitting the walls of the container.

The pressure is defined in terms of the total force exerted by these particles colliding with the surface per unit area.

P2.1.3 Pressure Changes in Gases (Supplement)

We can use the particle model to explain why gas pressure changes when you change the temperature or volume.

1. Effect of Temperature (at constant volume)

• If you increase the temperature, the gas particles gain kinetic energy and move faster.
• Faster particles hit the container walls more frequently and with greater force.
• Since the volume is constant, this leads to an increase in pressure.

2. Effect of Volume (at constant temperature)

• If you decrease the volume (make the container smaller), the particles have less space to move in.
• The particles will hit the container walls more frequently.
• Since their speed is the same (constant temperature), this increased collision rate leads to an increase in pressure.

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P2.2 Thermal Properties and Temperature

P2.2.1 Thermal Expansion

When most substances are heated, they get bigger. This is called thermal expansion.

The particle explanation: When heated, the particles gain energy and vibrate or move faster. This increased movement forces the particles to push further apart, causing the solid, liquid, or gas to expand.

Everyday Applications and Consequences (Supplement)

• Solids: Gaps must be left in concrete roads, bridges, and railway lines (expansion gaps) to allow for expansion in hot weather. If not, they would buckle.
• Liquids: Used in glass thermometers. When heated, the liquid expands much more than the glass bulb, causing the liquid column to rise.
• Gases: Hot air balloons use the expansion of air. Heating the air makes it less dense, causing the balloon to rise.

P2.2.2 Melting, Boiling, and Evaporation

Melting and Boiling (Energy Input)

When a solid melts or a liquid boils, we are continuously supplying energy, but the temperature often stops rising.

• During melting (solid → liquid) and boiling (liquid → gas), the energy supplied is used to break the bonds or forces holding the particles together, allowing them to move into a new state.
• Since the energy is being used to change the state (potential energy of particles), not increase the average speed of the particles (kinetic energy), the temperature remains constant during the change.

Example: Pure water melts at $0^{\circ}C$ and boils at $100^{\circ}C$ (at standard atmospheric pressure).

Condensation and Solidification (Core)

These are the reverse processes of boiling and melting:

• Condensation (Gas to Liquid): Particles lose energy and slow down. The forces between the particles pull them closer together to form a liquid.
• Solidification (Liquid to Solid): Particles continue to lose energy and slow down until they arrange themselves into a fixed pattern (a solid lattice).

Evaporation and Cooling

Evaporation is the process where a liquid turns into a gas only at the surface, occurring at temperatures below the boiling point.

Step-by-step process of Evaporation:

1. Particles in a liquid have a range of speeds (kinetic energies).
2. Some particles near the surface are moving faster than average (more energetic).
3. These high-energy particles overcome the attraction forces from neighbouring particles and escape into the air as gas/vapour.
4. Since the most energetic particles have left the liquid, the average kinetic energy of the remaining particles decreases.
5. A decrease in the average kinetic energy means a decrease in the temperature of the remaining liquid, causing cooling. (This is why sweating cools you down!)

Differences between Evaporation and Boiling (Supplement):

• Evaporation: Occurs at any temperature below boiling point; only occurs at the surface; no bubbles are formed within the liquid.
• Boiling: Occurs at a specific temperature (the boiling point); occurs throughout the liquid; bubbles are formed inside the liquid.

Factors Affecting the Rate of Evaporation (Supplement)

The rate at which a liquid evaporates increases if:

• Temperature is higher: More particles have enough kinetic energy to escape the surface.
• Surface Area is larger: More surface is exposed, so more particles are in a position to escape. (Example: Clothes dry faster when spread out.)
• Air Movement (Wind) is greater: Wind removes the water vapour molecules just above the surface, reducing the concentration of vapour and allowing more liquid particles to escape.

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P2.3 Transfer of Thermal Energy

Thermal energy (heat) can be transferred in three ways: conduction, convection, and radiation. We need to know how they work and their everyday applications.

P2.3.1 Conduction

Conduction is the transfer of thermal energy mainly in solids (especially metals) due to the vibrations of particles and the movement of mobile electrons.

Mechanism in Solids (Supplement)

1. When one end of a solid is heated, the particles at that end gain energy and vibrate more vigorously.
2. These highly vibrating particles collide with their neighbours, transferring energy from particle to particle down the solid. (This is called lattice vibration.)
3. In metals, this process is much faster because they contain delocalised (mobile) electrons. These electrons gain energy, move freely and rapidly through the lattice, and transfer energy quickly by colliding with other particles.

Conductors and Insulators (Core)

• Good Thermal Conductors: Materials that allow thermal energy to pass through them easily (e.g., metals like copper and aluminium).
• Bad Thermal Conductors (Insulators): Materials that do not conduct thermal energy easily (e.g., wood, plastic, air, and trapped gases).

Did you know? Air is an excellent insulator. Materials like wool or foam are good insulators because they trap pockets of air.

P2.3.2 Convection

Convection is the transfer of thermal energy in liquids and gases (fluids) through the movement of the substance itself.

Mechanism of Convection (Supplement)

1. When a fluid (liquid or gas) is heated, the part being heated expands.
2. Expansion means the particles spread out, causing that part of the fluid to become less dense.
3. The less dense (hotter) fluid rises, and the denser (cooler) fluid sinks to take its place.
4. This continuous rising and sinking creates a circular movement called a convection current, transferring energy throughout the fluid.

Example: Boiling water in a pot or how radiators heat a room.

P2.3.3 Radiation

Thermal radiation is the transfer of thermal energy by infrared (IR) electromagnetic waves.

• Key Fact: Unlike conduction and convection, radiation does not require a medium (matter) to travel through. This is how the Sun heats the Earth, travelling across the vacuum of space.

Surface Effects (Core)

The rate at which a surface emits (gives out), absorbs, or reflects radiation depends on its colour and texture.

Surface Property	Good Emitter	Good Absorber	Bad Absorber (Good Reflector)
Colour & Texture	Black, dull surfaces	Black, dull surfaces	White, shiny surfaces
Example	Radiators are often painted black to radiate heat efficiently.	Asphalt roads absorb sunlight quickly and get hot.	Emergency blankets are shiny (metallic) to reflect body heat back onto the person.

Earth's Temperature (Supplement)

The temperature of the Earth is a balance between:

• Radiation absorbed from the Sun (mostly visible light and IR).
• Radiation emitted by the Earth back into space (mostly IR radiation).

P2.3.4 Consequences and Applications (Core)

We apply the principles of heat transfer in many ways:

• Keeping things cold (Conduction/Convection): A freezer is placed at the top of a fridge so that the cold air sinks, setting up convection currents that cool the whole fridge.
• Vacuum Flasks:
  • A stopper prevents convection (stops air escaping).
  • Double walls with a vacuum between them prevent conduction and convection.
  • Shiny silver surfaces facing the vacuum prevent radiation loss/gain.
• Heating Systems (Convection): Air conditioning units are usually placed high up so that cool air (which is dense) sinks and circulates down, cooling the room.

Don't worry if this seems tricky at first—Thermal Physics connects to things you see every day. Try to link each transfer mechanism (conduction, convection, radiation) to a real-life example to solidify your understanding!