If two bodies have different temperatures, heat transfers between them. This phenomenon is known as heat transport, heat transfer or simply heat exchange. Heat transfer always occurs from a body with higher temperature to a body with lower temperature, never in the reverse direction (Fig. 1).
Remember, heat is always being transferred – sometimes more and sometimes less. For example, while reading this article your body is transferring heat to the environment around you. An average human generates and releases 80-100 watts while at rest and over 200 watts while exercising.
The Three Modes of Heat Transfer
There are three modes in which heat is transferred: convection, radiation and conduction. In most cases, multiple modes are occurring simultaneously, although one may be dominant and more noticeable than the other(s). Convection and radiation transfer heat to or from the surface, while conduction transfers heat into or out of the center of the component part.
Convection is heat transfer between two bodies that do not touch, but instead transfer heat via contact with a gas or liquid. Convection can be either natural (e.g., a radiator) or forced (e.g., via a circulating fan in an industrial furnace or oven).
In natural convection the heated media (air, for example) rises due its natural buoyancy, and cold air replaces it from below. It is dependent on gravity and, therefore, is not possible in a weightless environment. Forced convection requires a method of circulation (such as a fan) to move air or gas across a heat source where it gains heat energy then impinges on the material being heated. The material absorbs the heat from the gas.
Radiation is a method of heat transfer that does not rely on any contact between the heat source and the heated object (as is the case with conduction and convection). Heat can be transmitted through empty space by thermal radiation (in which case it is called infrared radiation). No mass is exchanged and no medium is required in the process of radiation. One of the key aspects of radiation heating is that it is line-of-sight heating. If an object is shielded from the radiant source, it cannot heat (the half of the Earth facing away from the sun being the classic example). Other examples of radiation are the heat we receive from the sun or heat released from the filament of a light bulb.
Heat conduction is caused by physical contact between materials, one of which has a higher temperature (contains more heat energy) than the other. Adjacent atoms or molecules transfer (by vibration or excitation) heat energy from particle to particle. The heat transfer is directly proportional to the difference in temperature between the two bodies being considered.
It is important to note that the rate of heat transfer is directly proportional to the difference in temperature between the two bodies being considered. An example is stove-top cooking in a frying pan where the flame heats the metal pan that, in turn, transfers the heat to the contents contained within. Conduction occurs at a rapid rate between the pan and the food. The cause of this heat transfer is simply the difference in temperature between them.
1. Herring, Daniel H., "Heat Transfer," Industrial Heating, December 2014
2. Education for the Information Age (www.edinformatics.com)