Heat, temperature, and the particle model
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What happens when heated?
Particles vibrate faster, solid expands slightly.
Liquids:
Particles close together, random arrangement, particles move randomly, can flow, fixed volume but no fixed shape
Gases:
Particles very far apart, move randomly at high speeds, collide with walls → creates pressure, no fixed shape or volume
Absolute zero: -273°C = 0 K
Temperature at which all particle motion stops (theoretically)
Kelvin conversion (IGCSE-essential):
Examples:
0°C = 273 K | 25°C = 298 K | 100°C = 373 K
What is it?
Random, zigzag motion of small particles suspended in a fluid (smoke in air, pollen in water)
Why does it happen?
Tiny particles are bombarded by fast-moving fluid particles. These collisions push the particle randomly in all directions.
Three ways to increase pressure:
1. More particles (pump more gas in)
2. Smaller volume (compress the gas)
3. Higher temperature (particles move faster)
At constant temperature, for a fixed mass of gas:
Pressure and volume are inversely proportional
Real-world example:
Bicycle pump: as you push (decrease V) without changing temperature, pressure increases to push out air.
Question:
Gas at 100 kPa occupies 200 cm³. Compress it to 50 cm³ at constant temperature. What is the new pressure?
Solution:
p₁V₁ = p₂V₂
100 × 200 = p₂ × 50
20,000 = p₂ × 50
Insight: Halve the volume, double the pressure!
Internal energy = Total kinetic + potential energy of all particles
At a higher temperature: Particles have more kinetic energy → higher internal energy
SHC = energy needed to raise temperature of 1 kg of substance by 1°C
where: ΔE = energy (J), m = mass (kg), c = SHC (J/kg°C), Δθ = temperature change (°C)
Example values:
Water: 4200 J/kg°C (very high — hard to heat/cool)
Aluminium: 900 J/kg°C
Lead: 130 J/kg°C
Procedure:
1. Measure mass of liquid (m)
2. Note initial temperature
3. Turn on heater and record power (P) and time (t) until temperature rise Δθ
4. Rearrange ΔE = mcΔθ: c = Pt / (mΔθ)
Question:
A 2 kg block of metal is heated. Energy supplied = 36,000 J. Temperature rises from 20°C to 50°C. Find SHC.
Solution:
ΔE = mcΔθ
Rearrange: c = ΔE / (mΔθ)
Δθ = 50 - 20 = 30°C
c = 36,000 / (2 × 30) = 36,000 / 60
Key point: During melting or boiling, temperature DOES NOT CHANGE even though energy is added
Energy goes into breaking bonds (changing state), not increasing kinetic energy
Water boiling point: 100°C at sea level
Water melting point: 0°C
L = energy needed to change state of 1 kg without temperature change
where: ΔE = energy (J), m = mass (kg), L = latent heat (J/kg)
Example values for water:
Latent heat of fusion (melting): 334,000 J/kg
Latent heat of vaporisation (boiling): 2,260,000 J/kg
| Factor | Evaporation | Boiling |
|---|---|---|
| Temperature | Below b.p. | At b.p. |
| Location | Surface only | Throughout liquid |
| Speed | Slow | Fast |
| Bubbles | None | Yes |
| Energy needed | Only fast particles escape | All particles supplied energy |
Real-world applications:
Railway lines have gaps to allow for thermal expansion
Bimetallic strips (two metals bonded): different expansion rates → used in thermostats
Water's anomaly: water expands when cooled below 4°C (rare!)
Mechanism: Heat travels through a material by vibrating particles bumping into neighbours
Real example: Metal spoon in hot tea — handle gets hot quickly by conduction
Mechanism: Heated fluid (liquid or gas) becomes less dense → rises, cool fluid sinks → circular currents (convection currents)
Real examples:
Boiling water: hot water rises, cool water sinks
Room heating: warm air rises, circulates, cools, falls
Sea breeze: land heats up faster, air rises, cool ocean air rushes in
Mechanism: Heat travels as infrared electromagnetic radiation
Emission rate factors:
Higher temperature → more infrared → faster emission
Darker/rougher surface → better emitter
Larger surface area → more radiation
Light/shiny surfaces: Reflect radiation (poor absorbers, poor emitters)
Dark/rough surfaces: Absorb radiation (good absorbers, good emitters)
Earth's temperature balance:
Sun's energy hits Earth → Earth absorbs it → Earth heats up
Hot Earth emits infrared radiation → atmosphere traps some (greenhouse gases)
At equilibrium: energy in = energy out → constant temperature
Cooking pans: Metal base (conduction), handle away from heat (insulation)
Room heating: Hot radiator + convection current warms entire room
Car radiator: Coolant circulates by convection, releases heat by conduction through metal fins + convection with air
Thermos flask: Vacuum (blocks conduction + convection), reflective surfaces (blocks radiation)
Fire: Heat by radiation (you feel warmth from distance)
✅ Kinetic particle model: solids (fixed), liquids (close, random), gases (far apart, fast)
✅ Absolute zero: -273°C = 0 K. T (K) = θ (°C) + 273
✅ Brownian motion proves particles are moving
✅ Boyle's Law: pV = constant (pressure × volume stays same at constant T)
✅ SHC: ΔE = mcΔθ | Latent Heat: ΔE = mL
✅ Phase changes (melt/boil): temperature constant, energy used for state change
✅ Heat transfer: Conduction (solids), Convection (fluids), Radiation (all, no medium)
✅ Earth's temperature: balance of solar energy in + radiation out
Master thermal physics — ace Topic 2! 🔥