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The transfer of thermal energy from a hot area to a cool area is known as heat flow. This occurs when a certain object or material is at a different temperature than surrounding objects. The principle behind this process involves the fact that thermal equilibration needs to occur between objects, also known as the second law of thermodynamics. In essence, when a difference in temperature exists between materials, the heat flow can only be slowed, not stopped.
Heat flow relies heavily on the concept of conduction. Basically, the transfer of the heat energy relies on free electron diffusion. In order for the heat to move from one object or material to another, adjacent atoms vibrate against each other. This is caused by the electrons moving from one atom to another. In terms of liquids, the molecules are further apart, which is the reason thermal energy is transferred at a slower rate when this state of matter is present.
Convection is also important to heat flow. This occurs when a solid object or surface is located near a gas or liquid. The rate of thermal transference increases as the motion moves faster. Two different types of convection exist within the laws of thermodynamics: natural and forced. Buoyant forces due to density variations of the gas or liquid naturally occur, while forced convection is in response to a man-made method such as a pump or fan.
When heat is transferred from materials through an empty space, the concept is known as radiation. This occurs for all objects above absolute zero, even those located within a perfect vacuum. For example, radiation from the Sun travels through the vacuum of space before impacting the earth and other planets.
The general principle of thermodynamics and heat flow were established by Sir Isaac Newton. The law of cooling states that "the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings, or environment." This can be observed by analyzing food. For example, when a pie is removed from the oven and placed in a cool window, the rate of heat loss is determined by the temperature of the air.
One interesting side effect of heat flow can occur when vapor changes into its liquid phase. Known as condensation, this process relies on the gaseous form of a liquid changing. This is seen in nature in the form of fog or on a cool glass on a hot day.