Section 1 of 8Particle explanation of expansion
Particle explanation of expansion
Energy transfer
- Energy transfer heating gives each particle more kinetic energy
- In a solid the particles vibrate with larger amplitude about fixed positions
- In a liquid or gas the particles move faster
Result
- Result the average separation increases, so the substance takes up more space
Unchanged
- Unchanged the particles stay the same size, their number is unchanged and mass is constant
Cooling
- Cooling the opposite happens and the substance contracts
Section 2 of 8Expansion in solids, liquids and gases
Solids
- Solids expand least because strong forces hold the particles in a lattice
- Larger vibrations push neighbours slightly further apart
- Different materials expand by different amounts, for example aluminium expands more than steel
Liquids
- Liquids expand more than solids because the forces between particles are weaker
Gases
- Gases expand most because the forces between particles are almost zero
Order
- Order gases greater than liquids greater than solids for equal temperature rise
Section 3 of 8Expansion and density
Density
- Density density = mass divided by volume, so it falls when volume rises at constant mass
Fluid movement
- Fluid movement warmer, less dense fluid rises above cooler, denser fluid, which gives convection currents
Water
- Water between 0 °C and 4 °C water contracts on heating, which is why ice floats and lakes freeze from the top
Section 4 of 8Uses of expansion
1Liquid-in-glass thermometer
- Liquid-in-glass thermometer the liquid expands up a narrow tube, so a small volume change gives a visible movement
2Bimetallic strip
- Bimetallic strip two metals expand by different amounts, so the strip bends with the greater-expanding metal on the outside
- Used in thermostats
- Used in simple fire alarms
3Loosening a lid
- Loosening a lid running a metal lid under hot water makes it expand more than the glass jar
4Hot-air balloon
- Hot-air balloon heated air expands, becomes less dense than the cool air outside and rises
Section 5 of 8Problems caused by expansion
Bridges
- Bridges expansion gaps or rollers stop them buckling in summer
Railway tracks
- Railway tracks laid with small gaps or pre-stressed
Power cables
- Power cables hung with slack so they do not snap when they contract in winter
Concrete pavements
- Concrete pavements laid in slabs with joints filled by a soft material
Section 6 of 8More from the specification
1
Thermal energy quantified in joules
- Thermal energy quantified in joules absorbing thermal energy measured in joules increases internal particle kinetic vibrations, driving volumetric expansion.
2
Thermal Expansion and Internal Energy
- When a substance is heated, its particles gain kinetic energy and vibrate or move more vigorously,
- causing them to push further apart and the material to expand;
- internal energy is the sum of the random kinetic and potential energies of the constituent molecules in the system.
3
Specific Heat Capacity and Electrical Calorimetry
- Specific heat capacity c is the thermal energy required per unit mass to increase the temperature of a material by 1 °C without changing state (ΔE = m·c·ΔT); in laboratory investigations, an electrical immersion heater transfers energy to a known mass of water or metal block to determine its specific heat capacity.
- (E = P·t = I·V·t)
4
Evaporation vs Boiling and Phase Changes
- Melting and boiling occur at fixed temperatures where supplied thermal energy breaks intermolecular bonds,
- increasing molecular potential energy without increasing temperature;
- in contrast, evaporation occurs at any temperature from the liquid surface as energetic molecules escape into vapour.
5
Immersion heater
- Electric heater placed in a liquid or block to supply energy
6
Potential energy
- Energy stored by the configuration of a system.
7
Intermolecular force
- An attractive force between particles.
8
Potential difference
- Work done by a unit charge passing through a component.
9
Capacity
- Charge a cell can supply, in A h or C.
10
Evaporation
- Escape of the most energetic molecules from a liquid surface at any temperature
11
Boiling
- Liquid becoming gas throughout the liquid at the boiling point
12
Molecule
- Tiny particle of the fluid, too small to be seen.
13
Melting
- Change from solid to liquid.
14
Heater
- A device that transfers electrical energy to the thermal store.
15
Joule
- Unit of energy, 1 kg·m2/s2.
16
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)
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Thermal expansion | Increase in size of a substance 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 |
| Density | Mass per unit volume; falls when a substance expands |
| Thermostat | Device that uses a bimetallic strip or similar to keep temperature steady |
Section 8 of 8Exam technique
1
Saying the particles themselves expand — the spaces between them increase, not their size.
2
Thinking heating makes more particles or more mass — mass and particle number are unchanged.
3
Saying solids expand more than gases — gases expand most and solids least.
4
Forgetting to link expansion to a fall in density — volume rises while mass is constant.