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A-LEVEL 9702 · A2 · TOPIC 14
Temperature .The chain of the topic: thermal equilibrium fixes what temperature means, the thermodynamic Kelvin scale makes it independent of any substance, and energy then lets us count what a temperature change actually costs through specific heat capacity and specific latent heat. Around the hexagon are the three ideas; above is what it builds on, below is where it leads.
TOPIC 14: TEMPERATURE
CAMBRIDGE A-LEVEL PHYSICS 9702 · PATHWAYS
TheLucidSTEM · thelucidstem.com
BUILDS ON
T1 Units: the kelvin base unit T5 Energy: heat as energy transfer T6 Matter: solids, liquids, gases 14.1
14.2
14.3
TOPIC 14
TEMPERATURE
1 · THERMAL EQUILIBRIUM
Temperature is what decides the flow of heat.
Thermal energy flows from a hot region to a cold one.
When no net flow occurs the bodies are in thermal
equilibrium and share one temperature.
Temperature measures the direction of energy flow,
not the quantity of energy a body holds.
T(hot) > T(cold) → net flow hot to cold
hot
cold
heat Q
flow stops when both reach equal T
2 · TEMPERATURE SCALES
A scale that depends on no substance at all.
An empirical scale uses a property that varies with T:
a column length, a resistance, an e.m.f., a volume.
The thermodynamic (Kelvin) scale is independent of
any substance; absolute zero is its true fixed point.
T / K = θ / °C + 273.15
absolute zero: 0 K = −273.15 °C
373 K
273 K
100 °C
0 °C
same interval: 1 K = 1 °C
3 · SPECIFIC HEAT & LATENT HEAT
Count the energy a temperature change costs.
Specific heat capacity c: energy to raise 1 kg by 1 K.
On a slope the temperature climbs as energy is added.
Specific latent heat L: energy to change the state of
1 kg at constant temperature (a flat plateau).
Q = m c Δθ
on the sloping parts
Q = m L
on the flat plateaus
energy in
T
melt: mL(f)
boil: mL(v)
slope mcΔθ
USING THE IDEAS · METHOD
Pick the term for each leg of the change.
Temperature changing, one state: Q = m c Δθ
State changing, constant temperature: Q = m L
A full change (ice to steam) adds each leg in turn:
Q = mcΔθ + mL(f) + mcΔθ + mL(v) + ...
Power heats at a rate: Q = P t when losses are zero.
Use Δθ as an interval, so 1 K equals 1 °C exactly.
P
heater
Q = P t heats the mass; measure Δθ
LEADS TO
T15 Ideal gases: Kelvin T drives pV = nRT and mean KE = (3/2)kT T16 Thermodynamics: internal energy and the first law, U = Q + W T17 Oscillations: damping dissipates energy as a temperature rise Ideal gases reuse the Kelvin scale; thermodynamics treats c and L as energy flowing in or out of a body.
A higher temperature means more random molecular energy, the bridge from this topic into the kinetic theory of gases.
heat in, on a slope, raises T
heat in, on a plateau, changes state
← Builds on IGCSE: Thermal properties