temperature
T F = 9 T C + 32 5 5 T C = T F 32 9 T K = T C + 273.15 T C = T K 273.15 Systems are in thermal equilibrium when they have the same temperature. Thermal equilibrium occurs when two bodies are in contact with each other and can freely exchange energy. The zeroth law of thermodynamics states that when two systems, A and B, are in thermal equilibrium with each other, and B is in thermal equilibrium with a third system, C, then A is also in thermal equilibrium with C. 13.2 Thermal Expansion of Solids and Liquids Thermal expansion is the increase, or decrease, of the size length, area, or volume of a body due to a change in temperature. Thermal expansion is large for gases, and relatively small, but not negligible, for liquids and solids. Linear thermal expansion is.