Distinguish between steady-state and transient heat transfer.

In steady-state heat transfer, temperature at any point in the system remains constant over time (∂T/∂t = 0), meaning heat entering equals heat leaving. Example: a well-insulated pipe carrying hot water at constant flow. In transient (unsteady) heat transfer, temperatures change with time as the system moves toward equilibrium. Example: quenching a hot metal part, heating food in a microwave. Transient analysis requires solving the heat diffusion equation with time-dependent terms and is characterized by the Biot and Fourier numbers.