On This Page
Overview
Heat is not a substance stored inside an object. It is energy in transfer. When two systems at different temperatures interact thermally, energy transfers from the hotter system to the cooler system. This transfer continues until thermal equilibrium is approached, unless outside processes maintain the temperature difference.
Energy Transfer
Heat describes energy crossing a system boundary because of temperature difference. Once the energy is inside a system, it is usually described as internal energy or thermal energy, not heat.
Direction of Transfer
Heat flows naturally from higher temperature to lower temperature. This direction is central to the second law of thermodynamics.
Conduction
Conduction is heat transfer through direct particle interaction, especially in solids. Faster-moving particles transfer energy to neighboring particles through collisions and interactions.
Convection
Convection is heat transfer by bulk motion of a fluid. Warm fluid can rise, cool fluid can sink, and circulating currents can carry thermal energy.
Radiation
Radiation transfers energy through electromagnetic waves. It does not require a material medium, which is why sunlight can warm Earth across space.
Heat vs. Temperature
Heat is energy transfer, while temperature is a measure of thermal condition. A small flame can have high temperature but transfer less total energy than a large warm object.
Units of Heat
Because heat is energy transfer, it is measured in joules in SI units. Calories are also used in some contexts, especially food energy and historical heat measurement.
Common Mistakes
A common mistake is saying an object contains heat. In physics, an object contains internal energy, while heat refers to energy being transferred.
Why This Matters in Physics
Heat is essential for thermodynamics, engines, weather, climate, cooking, materials, electronics, energy systems, and biological regulation.