Physics (0625) | Thermal properties and temperature

Welcome to Thermal Properties and Temperature!

Hey there, future physicist! This chapter is all about how matter (solids, liquids, and gases) behaves when you add or remove thermal energy (heat). This knowledge is super useful—it explains why bridges have expansion gaps, why sweating cools you down, and how quickly your cup of tea cools off.

Don't worry if some of the formulas look scary. We will break down every concept into small, easy-to-understand pieces. Let's dive into the fascinating world of heat!

2.2.1 Thermal Expansion of Solids, Liquids, and Gases

What is Thermal Expansion? (Core)

When an object is heated, its particles gain kinetic energy and vibrate more vigorously. This increased motion causes the particles to push further apart from each other. The result? The overall size of the object increases.
This increase in size (length, area, or volume) due to a rise in temperature is called thermal expansion.

Expansion in Solids, Liquids, and Gases

All three states of matter expand when heated, but they do so to different extents.

• Solids: Expand the least. Their particles are held tightly in fixed positions, so they only vibrate slightly further apart.
• Liquids: Expand more than solids. Their particles are less tightly bound and can move around more freely.
• Gases: Expand the most. Their particles are already widely separated and moving randomly, so a small temperature increase leads to a large increase in volume (if pressure is constant).

Supplement Note: Particle Explanation of Expansion Magnitude
The relative order of expansion (Gases > Liquids > Solids) is due to the forces and distances between the particles:

1. In solids, strong forces prevent large movement.
2. In liquids, forces are weaker, allowing slightly larger separation.
3. In gases, there are almost no forces between the widely separated particles, so the increase in kinetic energy results in much greater separation and therefore massive expansion.

Everyday Applications and Consequences (Core)

Thermal expansion isn't just a lab concept; it affects many things we see daily:

• Gaps in bridges and railway tracks: Small gaps are left between sections of road or rail to allow for expansion in hot weather. Without these gaps, the tracks would buckle!
• Fusing two metal pieces (Shrink Fitting): To fit a small metal piece tightly onto a larger one, the larger piece is heated (expanded), the smaller piece is inserted, and then the whole assembly is allowed to cool (shrink).
• Bimetallic strip: This strip is made of two different metals welded together. Since different metals expand at different rates, heating the strip causes it to bend. This is used in thermostats, fire alarms, and circuit breakers.
• Cracked glasses: Pouring very hot water into a thick glass can cause it to crack. The inside layer heats up and expands quickly, while the outer layer remains cool. This unequal expansion causes stress, leading to a break.

2.2.2 Specific Heat Capacity (SHC)

Internal Energy (Core)

When you heat an object, the energy transferred to it is stored as internal energy (also known as thermal energy).

Internal Energy is the total energy contained within a substance, consisting of:

• Kinetic Energy (KE): The energy due to the random movement and vibration of the particles (atoms or molecules).
• Potential Energy (PE): The energy stored in the chemical bonds or forces holding the particles together.

When you heat something and its temperature rises, you are primarily increasing the average kinetic energy of all its particles. (This is the Supplement explanation of temperature rise.)

Defining Specific Heat Capacity (SHC) (Supplement)

Imagine you have two pans, one filled with 1 kg of water and one with 1 kg of iron. If you heat them both with the same flame for five minutes, the iron pan gets much hotter than the water. Why?

Different materials need different amounts of energy to change their temperature. This property is called Specific Heat Capacity (c).

Definition: Specific Heat Capacity (c) is the amount of energy required to raise the temperature of one unit mass (e.g., 1 kg) of a substance by one degree (e.g., 1 °C or 1 K).

The SHC Equation

The amount of thermal energy (\(\Delta E\)) transferred to a substance is related to its mass (\(m\)), its specific heat capacity (\(c\)), and the change in temperature (\(\Delta \theta\)).

$$ \Delta E = mc\Delta \theta $$

We can rearrange this to define SHC:

$$ c = \frac{\Delta E}{m \Delta \theta} $$

• Unit of SHC: Joules per kilogram per degree Celsius or Kelvin: \(\text{J}/(\text{kg}^{\circ}\text{C})\) or \(\text{J}/(\text{kg}\text{K})\).
• Example: Water has a very high SHC (about 4200 J/kg°C). This means it takes 4200 J of energy to heat 1 kg of water by 1 °C.

Measuring Specific Heat Capacity (Supplement)

You can measure the SHC of a substance using the electrical method, based on the principle of conservation of energy (Electrical Energy Input = Thermal Energy Gain).

Experiment for a Liquid (e.g., Water)

Apparatus: Immersion heater, beaker of liquid (measured mass \(m\)), thermometer, stopwatch, power source, joulemeter (or ammeter and voltmeter).

Step-by-Step:

1. Measure the mass (\(m\)) of the liquid.
2. Record the initial temperature (\(\theta_1\)).
3. Switch on the immersion heater for a measured time (\(t\)). (If using a joulemeter, record the energy input \(\Delta E\); if not, calculate energy using \(E = VIt\)).
4. Record the final temperature (\(\theta_2\)).
5. Calculate the temperature change \(\Delta \theta = \theta_2 - \theta_1\).
6. Calculate SHC using \(c = \frac{\Delta E}{m \Delta \theta}\).

Common Mistakes to Avoid:

• Heat Loss: A major source of error is heat loss to the surroundings (air, beaker). Use insulation (like a polystyrene cup) to minimise this.
• Stirring: The liquid must be continuously stirred to ensure the temperature is uniform throughout.

2.2.3 Melting, Boiling, and Evaporation

Changes of State and Internal Energy (Core)

When a substance changes state (e.g., solid to liquid, liquid to gas), energy is still being added, but the temperature often stays constant during the process.

Why does the temperature stay constant during melting or boiling?
The energy being supplied is used to increase the potential energy of the particles by breaking the bonds (or loosening the forces) holding them in place, rather than increasing their kinetic energy (which determines temperature). This energy is known as Latent Heat (hidden heat).

Melting and Boiling

• Melting (Solid \(\to\) Liquid): Energy is absorbed to break the rigid bonds, allowing particles to slide past each other. The temperature remains constant at the melting point.
• Boiling (Liquid \(\to\) Gas): Energy is absorbed to completely overcome the inter-particle forces, allowing particles to escape and move freely. The temperature remains constant at the boiling point.

Did you know? The melting and boiling temperatures for water at standard atmospheric pressure are fixed reference points:

• Melting Point: \(0 ^{\circ}\text{C}\)
• Boiling Point: \(100 ^{\circ}\text{C}\)

Condensation and Solidification

These are the reverse processes where energy is released (the substance cools down):

• Solidification (Freezing) (Liquid \(\to\) Solid): Particles lose energy and settle back into fixed positions. Energy (latent heat) is released, but the temperature remains constant at the freezing point.
• Condensation (Gas \(\to\) Liquid): Gas particles lose energy, slow down, and inter-particle forces pull them back into a liquid arrangement. Energy is released.

Evaporation vs. Boiling (Core & Supplement)

Both processes turn liquid into gas, but they are fundamentally different.

Evaporation (Core)

Evaporation is the escape of more-energetic particles from the surface of a liquid.

1. Particles in a liquid have a range of kinetic energies (some fast, some slow).
2. The fastest, most energetic particles near the surface have enough energy to overcome the attractive forces and escape into the air.
3. Since only the most energetic particles leave, the average kinetic energy of the remaining particles in the liquid decreases.
4. A decrease in average kinetic energy means the temperature of the remaining liquid drops. Evaporation causes cooling of the liquid.

Boiling vs. Evaporation (Supplement)

The key differences are crucial for exam understanding:

Feature	Evaporation	Boiling
Location	Only happens at the surface of the liquid.	Happens throughout the entire body of the liquid (forming bubbles).
Temperature	Can occur at any temperature below the boiling point.	Occurs only at the fixed boiling point.
Speed	Slow process.	Rapid process.

Factors Affecting the Rate of Evaporation (Supplement)

The faster a liquid evaporates, the faster it cools. The rate of evaporation depends on three main factors:

1. Temperature: If the liquid is hotter, more particles have enough energy to escape the surface, so the rate increases.
2. Surface Area: Evaporation only happens at the surface. A larger surface area means more particles are near the surface and can escape, increasing the rate (e.g., clothes dry faster when spread out).
3. Air Movement (Wind): Moving air (wind) sweeps away the water vapour particles that have just escaped, reducing the concentration of vapour above the liquid. This allows more liquid particles to escape, increasing the rate.

Cooling Effect of Evaporation Explained (Supplement)

The cooling of an object in contact with an evaporating liquid is directly linked to the energy needed for the liquid to change state.

Example: Wet skin and cooling.

1. When water evaporates from your skin (or from a wet cloth placed on a hot object), the water needs energy to turn into gas.
2. This required energy is taken directly from the liquid itself and the surface it is sitting on (your skin/the object).
3. Because the surface loses thermal energy to the evaporating liquid, its temperature drops, causing a cooling effect.