Touch a hot stove and you feel heat. Heat is the transfer of thermal energy from a hotter object to a cooler one. It is not the same as temperature. Temperature measures the average kinetic energy of particles. Heat is the energy that moves because of a temperature difference. A cup of boiling water has a higher temperature than a bathtub of warm water, but the bathtub contains more heat because it has more water. Heat always flows from hot to cold, never the other way, unless work is done.

Heat moves in three ways. Conduction transfers heat through direct contact, as when a metal spoon heats up in soup. Convection transfers heat through the movement of fluids, as when warm air rises. Radiation transfers heat through electromagnetic waves, as when the sun warms your face. Heat is measured in joules or calories. The specific heat of a substance is the amount of heat needed to raise the temperature of one gram by one degree Celsius. Water has a high specific heat, which is why it takes a long time to boil and why coastal climates are milder than inland ones.

Heat facts

• Definition. Transfer of thermal energy.
• Direction. Always hot to cold.
• Transfer. Conduction, convection, radiation.
• Unit. Joule or calorie.
• Specific heat. Heat needed to raise unit mass by one degree.

Heat is not the same as temperature. Temperature is a measure of average kinetic energy. Heat is the transfer of energy.