Temperature and Heat

About this topic

Temperature measures the average kinetic energy of a substance's particles, while heat is the energy transferred between objects because of a temperature difference, always flowing from hot to cold. Heat is measured in joules. A bath of warm water holds more thermal energy than a cup of boiling water, even though its temperature is lower, because there is far more of it.

Internal energy is the total of all particle energies, so it depends on temperature, amount of material and the type of substance. Heat is the energy in transit, not something an object contains. Two objects in contact reach thermal equilibrium when they have the same temperature, and heat flow stops. This zeroth law idea is what makes thermometers work.

Worked example: when a hot coffee cools, its particles slow down as energy flows into the cooler air and cup. Eventually everything reaches the room temperature. A metal spoon feels colder than a wooden one at the same temperature because metal conducts heat away from your hand faster, not because it is colder.

Thermometers rely on properties that change with temperature, such as liquid expansion or electrical resistance. A common mistake is to say a body has heat, or that cold flows in; it is heat that flows out. Remember too that temperature is not energy, and the two are linked through the amount and type of material.