Thermal Physics and Heat Transfer

Temperature, Heat, and Expansion

Thermal physics is the study of heat, temperature, and their relation to energy and work.

Temperature vs. Heat

Temperature: A measure of the average kinetic energy of the particles in a substance. It is an intensive property (doesn't depend on the amount of substance). Measured in Celsius (°C) or Kelvin (K).

Heat (Q): The transfer of thermal energy between objects due to a temperature difference. Heat always flows from a hotter object to a colder object. It is a measure of energy, in Joules (J).

The Zeroth Law of Thermodynamics

This law defines thermal equilibrium.

The Law: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

Implication: This is the principle that makes thermometers work. When a thermometer reaches thermal equilibrium with an object, they are at the same temperature.

Thermal Expansion

Most substances expand when heated and contract when cooled.

Mechanism: When a substance is heated, its particles gain kinetic energy and move more vigorously, increasing the average distance between them.

Linear Expansion: For a solid rod, the change in length (ΔL) is given by:

ΔL = αL₀ΔT

α is the coefficient of linear expansion (a property of the material).

L₀ is the original length.

ΔT is the change in temperature.

This is why bridges have expansion joints and power lines are hung with a sag.

Calorimetry

Calorimetry is the science of measuring heat transfer. It is based on the principle of conservation of energy. In an isolated system:

Heat lost by hot objects = Heat gained by cold objects

q_lost + q_gained = 0

By convention, q is positive for heat gained and negative for heat lost. Using the formula q = mcΔT, we can solve for unknown quantities.