Thermodynamics · revision map · A-Level 9702, Topic 16

TOPIC 16 · THERMODYNAMICS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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THERMO-
DYNAMICS16
Internal Energy

Internal energy U is the total of the random kinetic and potential energies of all the molecules of a system, not heat.

U = Σ (KE + PE) of molecules
  • A function of state: ΔU depends only on the end states, not the route.
  • An ideal gas has no molecular PE, so its U is entirely kinetic.
Temperature & Internal Energy

Raising the temperature raises the mean random KE, and so the internal energy.

U ∝ T  (ideal gas, T in K)
  • For an ideal gas U depends only on T, whatever the pressure or volume.
  • Mean KE per molecule = &frac32;kT.
Work Done by a Gas

An expanding gas pushes its boundary. At constant pressure it does work as it moves a piston through a volume change.

  • Work done on the gas: W = −pΔV.
  • Expansion (ΔV > 0): the gas does work on the surroundings, so W on the gas is negative.
W = −pΔV
gas, p F = pA piston moves out
The First Law

Energy conservation for a gas: the change in internal energy equals heat supplied plus work done on the gas.

ΔU = q + W
  • q = heat supplied to the gas; W = work done on the gas. Watch the signs.
  • Constant volume: W = 0, so ΔU = q.
  • Isothermal ideal gas: ΔU = 0, so q = −W.
gas ΔU q in W out heat in raises U or leaves as work
BUILDS ON · Temperature · Ideal gases → THERMODYNAMICS → LEADS TO · Oscillations
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