2.2.1Thermal Physics
Thermal Expansion OF Solids, Liquids AND Gases — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 8 parts Lesson 13 of 45 1 quiz
Lesson summary
2.2.1 Thermal Expansion of Solids, Liquids and Gases — IGCSE PHYSICS notes: Explain thermal expansion in solids, liquids and gases using the particle model and give its uses and consequences.Heated material — when heated at constant pressure its temperature rises, its volume incr…
- Particle cause
- Sensitivity
- Response time
- Bimetallic strip
- Thermostat or fire alarm
- Prevention
Part 1 of 6What thermal expansion is
1Heated material
- Heated material when heated at constant pressure its temperature rises, its volume increases and its density decreases
- Mass stays the same while the volume rises
- On cooling the material contracts
2Particle cause
- Particle cause the particles gain kinetic energy and vibrate or move faster
- The average separation of the particles increases
- The particles themselves do not get bigger and their number does not change
3Sequence
- Sequence heat transferred in, temperature rises, kinetic energy of particles rises, particles move or vibrate faster, particles move further apart, substance gets bigger
Part 2 of 6Expansion in the three states
| State | Expansion | Reason |
|---|---|---|
| Solid | Smallest | Strong forces hold particles in fixed positions, so they only vibrate slightly further apart |
| Liquid | Larger than solids | Weaker forces let particles move further apart than in a solid |
| Gas | Largest | Almost no forces between particles, so they move much further apart |
- Order
- for the same temperature rise gases expand most, liquids less and solids least
Part 3 of 6Liquid-in-glass thermometer
Liquid-in-glass thermometer
Principle
- Principle the liquid expands up a thin glass capillary tube when the bulb is heated
Parts
- Parts a bulb holding a large volume of liquid, a narrow capillary tube and a scale along the side
Sensitivity
- Sensitivity a narrower capillary makes the thermometer more sensitive because the same expansion moves further along the tube
Response time
- Response time thinner glass around the bulb makes the thermometer respond more quickly
Part 4 of 6Bimetallic strip and temperature-activated switches
Bimetallic strip
- Bimetallic strip two different metals joined together that expand by different amounts when heated
- The strip bends by a predictable amount at a given temperature
- The metal that expands more is on the outside of the curve
Thermostat or fire alarm
- Thermostat or fire alarm the bending strip makes or breaks a circuit to switch a heater or alarm
Thermostat sequence
- Heater is on and current flows through the strip
- Temperature rises and both metals expand
- One metal expands more, so the strip bends away from the contact
- The circuit breaks and the heater switches off
- On cooling the strip straightens and the heater switches on again
Part 5 of 6Consequences of expansion
1Buckling
- Buckling solids that get too hot can buckle
- Railway tracks
- Road surfaces
- Bridges
2Prevention
- Prevention gaps and expansion joints are built in to leave room for expansion
3Power cables
- Power cables hung with slack so they do not snap when they contract in winter
4Shrink-fitting
- Shrink-fitting a metal part is heated, fitted, then cooled so it contracts tightly
Part 6 of 6More from the specification
More from the specification
Heat input measured in joules
- Heat input measured in joules absorbing thermal energy measured in joules accelerates vibrational kinetic motion, driving dimensional expansion in heated metal rods.
Thermal expansion
- Solids, liquids and gases expand when heated because molecules vibrate or move with greater energy and take up more space;
- gases expand most and solids least.
Specific heat capacity
- The energy needed to raise the temperature of 1 kg of a substance by 1 degree Celsius, using E = mc(delta T),
- as measured with an immersion heater in a block or water.
Evaporation and boiling
- Evaporation occurs at the surface at any temperature as faster molecules escape,
- while boiling occurs throughout the liquid at its boiling point.
Melting and potential energy
- During melting and boiling, temperature stays constant because energy increases the potential energy of molecules as bonds are overcome, not their kinetic energy.
System applications
- Gaps in bridges, bimetallic strips and liquid thermometers all use expansion with temperature.
Immersion heater
- Electric heater placed in a liquid or block to supply energy
Potential energy
- Energy stored by the configuration of a system.
Intermolecular force
- An attractive force between particles.
Potential difference
- Work done by a unit charge passing through a component.
Capacity
- Charge a cell can supply, in A h or C.
Evaporation
- Escape of the most energetic molecules from a liquid surface at any temperature
Boiling
- Liquid becoming gas throughout the liquid at the boiling point
Molecule
- Tiny particle of the fluid, too small to be seen.
Melting
- Change from solid to liquid.
Joule
- Unit of energy, 1 kg·m2/s2.
ΔE
ΔE mcΔθ [1 mark]
What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
Key terms
| Term | Meaning |
|---|---|
| Thermal expansion | Increase in size of a material when its temperature rises |
| Contraction | Decrease in size when the temperature falls |
| Bimetallic strip | Two metals joined together that bend when heated because they expand by different amounts |
| Capillary tube | Very narrow tube in a thermometer along which the liquid expands |
| Expansion gap | Space left in structures so expansion causes no damage |
Avoid these mistakes
1
Saying the particles themselves expand — only the average separation between particles increases.
2
Thinking heating adds more particles or mass — mass and particle number stay the same, so density falls.
3
Saying solids expand most — gases expand most and solids least for the same temperature rise.
4
Putting the more-expanding metal on the inside of the bend — it is on the outside of the curve.
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