law of thermodynamics

Meaning: Zeroth law: if A and B are each in thermal equilibrium with C, they are in equilibrium with each other, so temperature is well defined. First law: energy is conserved; a system's internal energy changes only by heat added to it or work done by it. Second law: the entropy of system plus surroundings increases in a spontaneous (irreversible) process and stays constant in an ideal reversible one. So heat does not flow spontaneously from cold to hot, and no heat engine converts heat fully to work. Third law: the entropy of a perfect crystal approaches zero as T approaches 0 K.

Equation: ΔU = Q − W, where ΔU is the change in internal energy (J), Q is heat added to the system (J), and W is work done by the system (J).

On the MCAT: Passages use PV diagrams and ideal gases. Work is the area under the curve, W = PΔV at constant pressure. Isothermal ideal gas: ΔU = 0, so Q = W. Adiabatic: Q = 0. Isovolumetric: W = 0. The second law appears in spontaneity (ΔG = ΔH − TΔS) and heat-engine efficiency.

Common confusion: The second law does not forbid a decrease in entropy within a system. Water freezing or a cell building a polymer lowers local entropy, but the surroundings gain more, so the total still rises. Also check the work convention: some texts write ΔU = Q + W, with W as work done on the system.