Science - Combined (0653) | Thermal physics

Thermal Physics (P2) - Comprehensive Study Notes

Hello future scientists! This chapter, Thermal Physics, is all about heat, temperature, and how energy moves through different materials. Understanding these ideas helps explain everything from why ice melts on a warm day to how a thermos flask keeps your drink hot. Don't worry if some concepts seem a bit abstract—we'll use everyday examples to make them clear!

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P2.1 The Kinetic Particle Model of Matter

The kinetic particle model is the fundamental idea that all matter is made up of tiny particles (atoms or molecules) that are constantly moving. The energy of this motion is directly related to the temperature of the substance.

P2.1.1 States of Matter (Core)

The way particles are arranged determines whether a substance is a solid, a liquid, or a gas.

Quick Review: Distinguishing Properties

• Solids: Have a fixed shape and a fixed volume. Very difficult to compress.
• Liquids: Do not have a fixed shape (they take the shape of the container) but have a fixed volume. Difficult to compress.
• Gases: Do not have a fixed shape or a fixed volume (they fill the container). Easy to compress.

Changes of State (Core)

Energy (usually heat) must be added or removed to change the state of matter:

• Melting: Solid to Liquid (e.g., ice turning into water).
• Freezing: Liquid to Solid (e.g., water turning into ice).
• Boiling (or Vaporization): Liquid to Gas (occurs throughout the liquid at boiling point).
• Condensing: Gas to Liquid (e.g., steam turning into water droplets).
• Evaporating: Liquid to Gas (occurs only at the surface, below boiling point).

P2.1.2 Particle Model (Core & Supplement)

Let's look closely at how the particles behave in each state:

1. Solids:

• Arrangement: Regular, fixed pattern (lattice structure).
• Separation: Very close together.
• Motion: They only vibrate about fixed positions.

2. Liquids:

• Arrangement: Random, no fixed pattern.
• Separation: Still close together, but slightly further apart than solids.
• Motion: They can slide past one another.

3. Gases:

• Arrangement: Completely random.
• Separation: Far apart.
• Motion: Very fast, random motion, constantly colliding with each other and the container walls.

Particle Motion and Temperature (Core)

The key relationship here is:
The higher the temperature of a substance, the faster its particles move (they have higher kinetic energy).

Forces and Properties (Supplement)

The forces and distances between particles determine the physical properties:

• In solids, strong forces lock particles into position, giving them a fixed shape.
• In liquids, forces are strong enough to keep them close but weak enough to let them slide, hence the fixed volume but variable shape.
• In gases, forces are negligible, allowing particles to move far apart, resulting in no fixed volume or shape.

Gas Pressure (Supplement)

Imagine you are trapped inside a massive balloon with hundreds of tiny, bouncy tennis balls flying everywhere. The pressure you feel is caused by these balls hitting you!

In physics, the pressure of a gas is created by the particles colliding with the surfaces of the container. Each collision exerts a small force. Since pressure is defined as force per unit area, these constant, rapid collisions create a measurable pressure: $$P = \frac{F}{A}$$

P2.1.3 Pressure Changes (Supplement)

How does changing temperature or volume affect the pressure of a fixed mass of gas? (Remember, we are thinking about collisions!)

1. Changing Temperature (at Constant Volume):

• If you increase the temperature, the particles gain kinetic energy and move faster.
• Faster particles hit the walls more frequently and with greater force.
• Result: Pressure increases. (Think about a car tire pressure reading increasing after a long drive.)

2. Changing Volume (at Constant Temperature):

• If you decrease the volume (make the container smaller), the particles have less space to move.
• They hit the walls much more frequently.
• Result: Pressure increases. (This is why pressing down on a bicycle pump increases the air pressure inside.)

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P2.2 Thermal Properties and Temperature

P2.2.1 Thermal Expansion (Core & Supplement)

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.

Why does it happen? When a substance is heated, its particles gain kinetic energy and vibrate more vigorously. This increased motion pushes the particles slightly further apart, causing the material to expand in all directions.

Expansion in Solids, Liquids, and Gases (Core)

Gases expand the most, followed by liquids, then solids, because the forces between gas particles are negligible, allowing for the greatest increase in separation.

Everyday Applications and Consequences (Supplement)

• Application: Thermometers use the expansion of a liquid (like mercury or alcohol) to measure temperature.
• Consequence: Railways and bridges need small gaps (expansion joints) to allow for expansion in hot weather. Without these gaps, the materials would buckle.
• Consequence: Hot water pipes sometimes squeak as they expand and rub against fittings.

Common Mistake to Avoid: Expansion doesn't mean the particles themselves get bigger—they just move further apart!

P2.2.2 Evaporation (Core & Supplement)

Evaporation is the process where a liquid turns into a gas (vapour) without reaching its boiling point.

The Particle Explanation (Core):

In a liquid, particles move at different speeds (they have a range of kinetic energies).

1. Only the most energetic particles (the fastest ones) near the surface have enough energy to overcome the forces of attraction holding them in the liquid.
2. These high-energy particles escape into the air as gas.

Evaporation Causes Cooling (Core):

When the highest energy particles escape, the average kinetic energy of the remaining liquid particles decreases. Since temperature is related to average kinetic energy, the temperature of the remaining liquid drops. This is why you feel cool after swimming, or why sweating cools your body.

Factors Affecting the Rate of Evaporation (Supplement):

The rate at which a liquid evaporates depends on:

• Temperature: Higher temperature means more particles have enough energy to escape, so evaporation is faster.
• Surface Area: A larger surface area allows more particles to be near the surface and escape, so evaporation is faster (e.g., drying clothes spread out).
• Air Movement (Wind): Moving air removes the water vapour particles just above the surface, reducing the concentration of vapour and allowing more liquid particles to escape. This speeds up evaporation.

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P2.3 Transfer of Thermal Energy

Thermal energy (heat) always flows from a region of higher temperature to a region of lower temperature. There are three ways this energy transfer can happen: conduction, convection, and radiation.

P2.3.1 Conduction (Core & Supplement)

Conduction is the transfer of thermal energy through a material without the material itself moving from one place to another. This is the main method of heat transfer in solids.

Conductors and Insulators (Core)

• Good Thermal Conductors: Materials that allow heat to pass through them easily (e.g., all metals).
• Bad Thermal Conductors (Insulators): Materials that do not allow heat to pass through them easily (e.g., wood, plastic, air).

The Mechanism of Conduction (Supplement)

In solids, conduction occurs via two primary methods:

1. Atomic or Molecular Lattice Vibrations:

• When one end of a solid is heated, the particles gain energy and vibrate more intensely.
• These vibrating particles bump into their neighbours, passing the kinetic energy along the structure (or lattice).

2. Movement of Delocalised Electrons (In Metals Only):

• Metals have free (delocalised) electrons that can move freely throughout the structure.
• When heated, these electrons gain kinetic energy and move rapidly to the colder parts of the metal, transferring energy quickly through collisions.
• This is why metals are much better conductors than non-metals, which lack these mobile electrons.

P2.3.2 Convection (Core & Supplement)

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

How Convection Works (Core & Supplement)

Convection relies on changes in density due to heating:

1. When a fluid is heated (e.g., water at the bottom of a pot), the particles gain energy and move faster, spreading further apart.
2. The heated fluid becomes less dense than the surrounding cooler fluid.
3. The less dense (hot) fluid rises, and the denser (cooler) fluid sinks to take its place near the heat source.
4. This continuous rising and sinking creates a convection current, transferring heat throughout the fluid.

Analogy: Think of a hot air balloon! Heating the air makes it less dense, causing it to rise.

P2.3.3 Thermal Radiation (Core & Supplement)

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

Key Fact (Core): Thermal radiation does not require a medium (like solid, liquid, or gas) to travel. It is the only way heat can travel through a vacuum, such as from the Sun to the Earth.

Surface Colour and Texture (Core)

The colour and texture of a surface heavily influence how much thermal radiation it emits, absorbs, and reflects:

• Black / Dull Surfaces: Are good emitters and good absorbers of thermal radiation. (They get hot quickly and cool down quickly).
• White / Shiny Surfaces: Are poor emitters and good reflectors (or poor absorbers) of thermal radiation. (They stay cooler in sunlight and keep heat inside).

Did you know? Car radiators are often painted black to maximise the emission (transfer) of heat away from the engine.

Thermal Radiation and Earth's Temperature (Supplement)

The temperature of the Earth is a balance between the radiation it absorbs and the radiation it emits:

• The Earth absorbs thermal radiation (mainly visible light and IR) coming from the Sun.
• The Earth emits thermal radiation (mainly IR) back into space.
• If absorption equals emission, the temperature remains constant. If absorption exceeds emission, the Earth heats up.

Testing Absorbers and Emitters (Supplement):

Experiments often use a Leslie cube (a box with sides of different colours/textures, filled with hot water) and a thermometer or detector to show:

• The black, dull surface emits the most radiation (it heats the detector fastest).
• A black surface placed in sunlight heats up faster than a shiny silver one (it absorbs radiation better).

P2.3.4 Everyday Applications (Core)

We use our knowledge of C, C, and R constantly to design appliances:

• Conduction Applications: Metal cooking pots use good conductors (metal) for the base but bad conductors (plastic/wood handles) to protect the user.
• Convection Applications: Heaters placed low down in a room use convection to warm the air, creating a circulation pattern.
• Radiation Applications: Emergency blankets are shiny silver to reflect thermal radiation back onto the person, minimising heat loss by radiation.

The Vacuum Flask (Thermos): A perfect example!

A vacuum flask is designed to minimize all three types of heat transfer:

1. Convection: The sealed cap prevents heat escaping via convection (air movement).
2. Conduction: The stopper (cork/plastic) and the thin glass walls are poor conductors. The vacuum (empty space) between the walls prevents conduction entirely.
3. Radiation: The inner glass walls are silvered (shiny) to reflect thermal radiation back into the liquid, preventing heat loss.

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You've successfully covered the core concepts of thermal physics! Remember to use the particle model explanations when describing expansion, pressure, or heat transfer mechanisms.