Energy of the movement and interaction of molecules of which the body consists is called internal energy. The thermal movement of particles never stops therefore the body always has some internal energy. This energy can be changed (to reduce or increase) commission of work and heat exchange. Distinguish three types of heat exchange: heat conductivity, radiation and convection.
Convection is the heat exchange in liquid gaseous environments which is carried out by streams (or streams) substances. In solid bodies the convection cannot happen because of a strong molecular attraction. Energy in solid bodies is transferred by heat conductivity. It is well-known that liquids and gases heat from below, for example, the teapot with water is put on fire, radiators of heating place under windows about a floor. It is explained by the fact that a part of substance heats up from below, extends, its density becomes less, than at surrounding (colder) Wednesday, and under the influence of the pushing-out force it begins to rise up. And its place is filled below by a cold part of this substance. After a while, having got warm, this layer will rise too up, giving way to the following stream of substance, etc. So there is a convection. Therefore liquids and gases should be heated from below, heated layers cannot fall below cold, heavier. At convection the energy is transferred by streams of gas or liquid. Distinguish two types of convection: natural (free) and forced. Free convection takes place when gas layers or liquids are interchanged the position without the aid of external forces, for example, air heating by the battery in the room happens by natural convection, and here speed of heating of water in a pan, it is possible to provide by hashing of layers of liquid with a spoon, such convection is forced. Most quicker convection happens in gases because of their free favor of particles. They, being at long distances from each other, badly interact with each other and move in almost independent direction therefore gases have bad heat conductivity. Liquids take average place between gases and solid bodies on convection and heat conductivity. That is convection at them takes place more slowly, and heat conductivity - quicker, than in gases. And their heat conductivity is weaker than rather solid bodies.