Flashcards · 9702 · A Level Topic 16

Thermodynamics (A Level) flashcards

11 cards on A Level Topic 16, subtopics 16.1 to 16.2. Pick a subtopic or study them all.

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Internal energy: The sum of the random distribution of kinetic and potential energies of the molecules in a system.
What does each component of internal energy depend on?: Kinetic: the temperature. Potential: the separation of the molecules, so it changes during a change of state.
Why is the internal energy of an ideal gas purely kinetic?: An ideal gas has no intermolecular forces (except during collisions), so there is no molecular potential energy. Its internal energy depends only on temperature.
What does a rise in the temperature of an object tell you about its internal energy?: Its internal energy has increased: the random kinetic energy of its molecules is greater.
Internal energy is determined by the state of the system. What does that mean?: It depends only on the current state (for example temperature, pressure, volume), not on how that state was reached. Two routes between the same states give the same ΔU.

First law of thermodynamics: ΔU = q + W ΔU: increase in internal energy q: energy supplied to the system by heating W: work done on the system
Work done when a gas changes volume at constant pressure: pΔV. For an expansion this is work done by the gas, so the work done on the gas is W = −pΔV. For a compression, work done on the gas is positive.
A gas absorbs 500 J of heat and does 200 J of work expanding. Find ΔU.: Work done on the gas W = −200 J. ΔU = q + W = 500 − 200 = +300 J.
A gas at a constant 1.0 × 10⁵ Pa is compressed by 2.0 × 10⁻³ m³. How much work is done on it?: W = pΔV = 1.0 × 10⁵ × 2.0 × 10⁻³ = 200 J, positive because work is done on the gas.
Isothermal change of an ideal gas: ΔU and q: Constant temperature, so ΔU = 0 and q = −W.
Adiabatic change: q and ΔU: No heat transfer, so q = 0 and ΔU = W.