Thermodynamics – Energy Changes and Heat Capacity

24 vocabulary flashcards summarizing core thermodynamic concepts: the three laws, internal energy formulas, ideal-gas processes, heat capacity, and limitations of heat engines.

First Law of Thermodynamics

The internal energy of a system changes only through heat transfer (Q) or mechanical work (W): ΔEₙ = Q + W.

Perpetual-motion Machine of the First Kind

A hypothetical device that would produce work indefinitely without energy input; forbidden by the First Law.

Internal Energy of an Ideal Gas

Sum of the translational (and, if present, rotational/vibrational) kinetic energies of its particles: Eₙ = (f/2) nRT = (f/2) pV.

Degrees of Freedom (f)

Independent ways a molecule can store energy; mono-atomic gases f = 3, di-atomic f = 5, poly-atomic f = 6.

Average Molecular Kinetic Energy

ε = (f/2) kT, proportional to absolute temperature.

Change in Internal Energy

ΔEₙ = (f/2) nR ΔT for an ideal gas.

Isochoric Process

Volume constant (V = const); W = 0, so ΔEₙ = Q.

Adiabatic Process

No heat exchange (Q = 0); ΔEₙ = W (work done on/by gas changes its internal energy).

Isothermal Process

Temperature constant (T = const); ΔEₙ = 0, so Q = –W (all supplied heat becomes expansion work).

Isobaric Process

Pressure constant (p = const); both heat and work enter: ΔEₙ = Q + W with W = –p ΔV.

Cyclic Process

Series of thermodynamic steps returning a system to its initial state, modeling heat engines.

Thermal Efficiency (η)

Ratio of net work output to heat input in a cycle: η = ΣW / ΣQ_in.

p‒V Diagram Area

The area enclosed by a cyclic path equals the net work performed during the cycle.

Heat Capacity (C)

Quantity of heat required to raise the temperature of a body by 1 K; C = Q / ΔT (unit: J K⁻¹).

Specific Heat Capacity (c)

Heat required to raise 1 kg of a substance by 1 K; c = Q /(m ΔT) (unit: J kg⁻¹ K⁻¹).

Mass–Heat-Capacity Relation

Total heat capacity equals specific heat times mass: C = c m.

Water’s Specific Heat

c_water ≈ 4200 J kg⁻¹ K⁻¹; 1 kg of water needs 4200 J to warm by 1 K.

Specific Heat at Constant Pressure (c_p)

Heat capacity per unit mass at constant p; always greater than at constant volume: cp = cV + R/M.

Second Law of Thermodynamics

Natural thermal processes are irreversible and involve entropy increase; no engine can convert all heat into work.

Irreversibility

Characteristic of real processes that cannot be perfectly reversed without external changes or energy losses.

Heat Engine Limitation

No heat engine can transform heat entirely into mechanical work (η < 100 %).

Heat Flow Direction

During contact, heat spontaneously flows from hotter to colder bodies until equilibrium.

Third Law of Thermodynamics

Absolute zero (0 K) is unattainable; as T → 0 K, processes would cease because particle motion approaches zero.

Absolute Zero

0 K (–273 °C), the theoretical temperature at which molecular motion would stop; cannot be reached in practice.