heat

Meaning: Heat (Q) is energy in transit between a system and its surroundings, or between two objects, driven by a temperature difference. Objects have internal energy and temperature; they do not "have heat." Transfer occurs by conduction (contact), convection (bulk fluid motion), and radiation (electromagnetic waves, which needs no medium). Net flow stops at thermal equilibrium.

Equation: Q = mcΔT for a temperature change, and Q = mL during a phase change. Q is heat in joules (J), m is mass in kg, c is specific heat in J/(kg·K), ΔT is the temperature change in K (same size as °C), and L is latent heat in J/kg. The first law is ΔU = Q − W, where ΔU is the change in internal energy (J) and W is work done by the system (J).

On the MCAT: Expect calorimetry (heat lost by the hot object equals heat gained by the cold one), heating curves with flat plateaus at melting and boiling, and comparisons of specific heats (water's is high, so it changes temperature slowly). Passages may also ask which mode of transfer applies, or use ΔU = Q − W on a PV diagram.

Common confusion: Students equate heat with temperature. Temperature reflects average kinetic energy of particles; heat is energy transferred. During boiling, heat flows in but temperature stays constant, because the energy goes into overcoming intermolecular attractions. Work can also raise temperature with no heat flow.