6.3 Heat Transfer and Thermal Equilibrium

Heat flows until equilibrium

Heat (q) is energy transferred between objects due to a temperature difference. Heat always flows spontaneously from the hotter object to the cooler object until both reach the same temperature, called thermal equilibrium. At equilibrium, no net heat transfer occurs because both objects have equal average kinetic energy.

Conservation of energy in mixtures

When two substances at different temperatures are combined in an isolated system, energy is conserved: the heat lost by the hotter object equals the heat gained by the cooler object (q_lost = −q_gained). This principle lets us predict final temperatures and is the basis of calorimetry experiments.

Quantifying heat transfer

Heat transfer is calculated with q = mcΔT, where m is mass, c is specific heat capacity, and ΔT is the temperature change. Substances with higher specific heat capacities require more energy to change temperature, so they heat up and cool down more slowly than substances with low specific heat capacities.

Key terms

Heat (q)

Energy transferred between objects due to a temperature difference.

Thermal equilibrium

The state reached when two objects in contact have the same temperature and no net heat flows between them.

Direction of heat flow

Heat always flows spontaneously from a hotter object to a cooler object.

q = mcΔT

Formula for heat transfer, where m = mass, c = specific heat capacity, ΔT = change in temperature.

Specific heat capacity

The amount of energy required to raise the temperature of 1 gram of a substance by 1°C; higher values mean slower temperature change.

Try an AP-style question

A hot metal is placed in cool water in an insulated cup. What happens?

1. Heat flows from the water into the metal
2. The metal and the water both get hotter
3. Heat flows from metal to water until temps match
4. Cold flows from the water into the metal