Thermodynamics

[Jessica]

The Zeroth Law of Thermodynamics

Two systems are said to be in thermal equilibrium if there is no net flow of heat between them when they are brought into thermal contact.

Zeroth Law of Thermodynamics
Two systems individually in thermal equilibrium with a third system are in thermal equilibrium with each other.

The zeroth law establishes temperature as the indicator of thermal equilibrium and implies that all parts of a system must be in thermal equilibrium if the system is to have a definable single temperature. In other words, there can be no flow of heat within a system that is in thermal equilibrium.

[Jessica]

First law of thermodynamics

The internal energy U of a system changes due to the heat Q flowing into it and the work W it does on its environment.


Q > 0: heat gained by system
Q < 0: heat gained by environment

W > 0: work done by system
W < 0: work done by environment

[Jessica]

Thermal processes

Isobaric process

Pressure is constant. If the piston rises by a distance s, the work done by the gas is
W = Fs = (PA)s = PΔV

The internal energy change is
ΔU = Q - W = Q - PΔV

Isochoric process

Volume is constant. If there are no movable parts, the work done by the gas is zero.
ΔU = Q - W = Q

Isothermal process

Temperature is constant. The work is now done by a variable pressure. Taking the area under the curve in the P-V plane yields

Adiabatic process

No heat is exchanged (Q = 0). The work done is the negative change in internal energy, ΔU = -W. So: