Thermal Physics and Heat Transfer

Learning Objectives

Distinguish between temperature and heat.

State the Zeroth Law of Thermodynamics.

Describe thermal expansion in solids.

Perform calorimetry calculations using q=mcΔT and the principle of conservation of energy.

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.

Example: A hot piece of metal is dropped into cool water in an insulated container. The heat lost by the metal (m_metal c_metal ΔT_metal) is equal in magnitude to the heat gained by the water (m_water c_water ΔT_water).

Key Terms

Temperature: A measure of the average kinetic energy of the particles in a substance.
Heat: The transfer of thermal energy from a hotter system to a cooler system.
Thermal Equilibrium: A state in which two physical systems have no net flow of thermal energy between them when they are connected by a path permeable to heat.
Thermal Expansion: The tendency of matter to change its shape, area, and volume in response to a change in temperature.
Calorimetry: The science or act of measuring changes in state variables of a body for the purpose of deriving the heat transfer associated with changes of its state.