Heat

Heat (also called kulayl or kuleyl) is energy that flows from one system to another because of a difference in temperature. When two objects at different temperatures touch, energy flows from the warmer to the cooler until both reach the same temperature. This state is called thermal equilibrium.

Heat is not a substance contained in things; it is a way of transferring energy. Therefore one does not say that something "has" heat, but rather that heat was transferred to or from it. Heat is measured in units of energy, particularly joules (J) in the SI system, though calories are still used.

Heat and temperature

Temperature is a measure of how rapidly the tiny particles of a substance are moving. Heat is transferred energy. Two objects at the same temperature but different sizes have different amounts of heat energy. For example, a cup of hot water and a lake of water at the same temperature each contain different amounts of heat, but heat does not flow between them since their temperatures are equal.

Modes of heat transfer

Three modes are common. First: conduction, where heat moves through solid materials as particles bump into each other. Second: convection, where heated liquid or air rises while cooler material sinks, creating circulation. Third: radiation, where energy is transmitted through electromagnetic waves, even across empty space with no air—such as heat from the sun.

Heat storage capacity

Different substances do not warm at the same rate. Specific heat capacity is the energy needed to raise the temperature of one unit of a substance by one degree. Water has high specific heat capacity, which means it stores large amounts of heat without warming quickly. For this reason, oceans affect the climate of coastal regions.

Laws of thermodynamics

The first law of thermodynamics states that energy is conserved: heat entering a system equals the change in internal energy plus the work done. The second law shows that heat naturally flows from hotter to cooler regions and will not reverse on its own without work being done—as in refrigeration machines that require electrical power.

Change of state

When a substance changes from solid to liquid or liquid to gas, it absorbs heat while its temperature remains constant. This energy is called latent heat. For example, ice absorbs heat while melting, but stays near zero degrees Celsius until the melting is complete.