CIE IGCSE Physics (0625) Thermal expansion of solids, liquids and gases Study Notes - New Syllabus
CIE IGCSE Physics (0625) Thermal expansion of solids, liquids and gases Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Thermal expansion of solids, liquids and gases
Key Concepts:
- Thermal Expansion of Solids, Liquids, and Gases (Qualitative, Constant Pressure)
- Everyday Applications and Consequences of Thermal Expansion
Thermal Expansion of Solids, Liquids, and Gases
Thermal Expansion
When a substance is heated, the particles gain kinetic energy.This causes the particles to move more energetically – either vibrating faster (in solids), or moving faster and spreading apart (in liquids and gases).
- As a result, the average separation between particles increases, and the substance expands in volume.
- At constant pressure, this expansion happens without any increase in external pressure on the substance.
Thermal Expansion in Solids
- Particles in solids are arranged in a fixed, tightly packed structure.
- They do not move freely, but vibrate about fixed positions.
- Heating a solid increases the vibration amplitude of the particles.
- This causes the particles to push against each other slightly more, resulting in a small increase in volume.
- Thermal expansion in solids is small but measurable
- e.g., metal rods expanding when heated.
Thermal Expansion in Liquids
- Particles in liquids are close together but not fixed – they can slide past each other.
- Heating gives the particles more kinetic energy, so they move faster and push each other further apart.
- Liquids show moderate expansion – greater than solids but less than gases.
- Thermometers use this property: as the liquid (like mercury or alcohol) is heated, it expands and rises in a thin tube.
Thermal Expansion in Gases (at Constant Pressure)
- Gas particles are widely spaced and move randomly at high speeds.
- Heating increases their kinetic energy, so they move even faster.
- To keep pressure constant, the gas must expand – that is, the particles spread out into a larger volume to compensate for the extra energy.
- This results in very large expansion compared to solids and liquids.
- This is why gases are highly sensitive to temperature changes – e.g., hot air balloons rise because heated air expands and becomes less dense.
| State of Matter | Particle Arrangement | Particle Motion | Effect of Heating | Extent of Expansion | Explanation |
|---|---|---|---|---|---|
| Solid | Fixed, closely packed lattice | Vibrate only | Particles vibrate more | Small | Increased vibration slightly increases average spacing |
| Liquid | Close, irregular arrangement | Move and slide past each other | Particles move faster, separate more | Moderate | Weaker forces allow particles to spread apart further |
| Gas | Very far apart, random motion | Move freely at high speed | Particles move much faster and spread far apart | Largest | Minimal forces allow rapid expansion to maintain constant pressure |
Example:
A sealed gas container holds 2.0 m³ of air at 27°C. The air is heated to 127°C at constant pressure. Calculate the final volume of the gas. (Assume ideal gas behavior.)
▶️ Answer/Explanation
Use the volume-temperature relation at constant pressure:
\( \frac{V_1}{T_1} = \frac{V_2}{T_2} \)
Convert °C to Kelvin:
\( T_1 = 27 + 273 = 300\, \text{K} \)
\( T_2 = 127 + 273 = 400\, \text{K} \)
Rearrange for \( V_2 \):
\( V_2 = V_1 \times \frac{T_2}{T_1} = 2.0 \times \frac{400}{300} = 2.0 \times \frac{4}{3} = 2.67\, \text{m}^3 \)
Question:
Which of the following statements best explains why gases expand more than solids or liquids when heated at constant pressure?
- (A) Gas particles are heavier and take up more space.
- (B) Gas particles are in fixed positions and can only vibrate.
- (C) Gas particles gain kinetic energy and move much farther apart due to weak intermolecular forces.
- (D) Gases contain more atoms than solids or liquids.
▶️ Answer/Explanation
Correct Answer: (C)
When gases are heated, their particles gain kinetic energy and move much faster. Because gas particles are far apart and experience very weak forces, they spread out significantly. This causes a large increase in volume when pressure is kept constant.
Everyday Applications and Consequences of Thermal Expansion
Everyday Applications and Consequences of Thermal Expansion
- When materials are heated, their particles gain kinetic energy and move more.
- This increases the average separation between particles, causing the material to expand.
- This expansion can be useful (applications) or problematic (consequences), depending on the situation.
Useful Applications of Thermal Expansion
Bimetallic Strips in Thermostats:
- Two metals with different expansion rates are bonded together.
- When heated, one expands more, causing the strip to bend and switch a circuit on or off.
Thermometers (Liquid-in-glass):
- Liquids like mercury or alcohol expand when heated.
- The rising liquid column indicates the temperature due to expansion.
Riveting in Metal Structures:
- Hot rivets are inserted into holes while expanded.
- As they cool, they contract and pull metal plates tightly together.
Thermal Expansion Joints in Bridges:
- Gaps are left between sections of bridges or railway tracks.
- Prevents damage due to expansion on hot days.
Problematic Consequences of Thermal Expansion
Road Buckling in Summer:
- Asphalt roads expand in high temperatures.
- If there’s no room to expand, roads can crack or buckle.
Overhead Power Lines Sagging:
- Wires expand when heated by the sun and sag downward.
- In winter, they contract and become taut.
Glass Cracking Due to Uneven Heating:
- When one part of glass expands faster than another, internal stress causes cracks.
- This is why glass cookware should not be placed directly on flame or taken from freezer to oven quickly.
Example:
A thermostat contains a bimetallic strip made from brass and iron. When the temperature increases, the strip bends. Explain why this bending happens in terms of the properties of the two metals.
▶️ Answer/Explanation
On heating, particles in both metals gain kinetic energy and vibrate more.
Brass expands more than iron because it has a higher coefficient of thermal expansion.
The side with brass becomes longer, causing the strip to bend toward the iron side.
The difference in expansion rates causes the bending used to open or close electrical circuits.
Example:
Bridges often include small gaps between metal plates called expansion joints. Explain why these are necessary and what would happen without them during summer.
▶️ Answer/Explanation
As the temperature rises, metal particles gain energy and vibrate more.
This causes the metal plates to expand.
Without a gap, the expanding plates would push against each other, causing buckling or cracking.
Expansion joints allow safe expansion and prevent structural damage.
Example:
During summer, power lines are seen to sag more than in winter. Explain this observation using the kinetic particle model.
▶️ Answer/Explanation
In hot weather, metal particles in the wire gain kinetic energy and vibrate more.
This causes the wire to expand in length.
Since the wire is longer, it sags between poles.
The sagging is a result of thermal expansion at constant tension.