How is heat transmitted worldwide

Heat transfer

Introduction and Definitions

warmth and the Transferring heat are an important part of human life. With heating, we generate heat when it is cold outside. We also need heat for cooking. Thousands of years ago, the first humans kindled fires because they wanted to warm themselves.

Definitions and terms

Warmth is one Form of energy and can pass from one body to another. There are three types of Heat transfer: The Conduction, the Heat flow (convection) and the Thermal radiation. In everyday life we ​​use energy converters that generate thermal energy from electrical or chemical energy. The quality of such energy converters is determined by the Efficiency.

$ \ eta = \ frac {Q} {E} $,

the ratio between the heat generated $ Q $ and the energy fed in $ E $. Since there are always losses, $ \ eta <1 $. Another measure of the quality of an energy converter is that thermal performance

$ P = \ frac {Q} {t} $,

with the unit watt (W). It describes the heat generated per time.

Conduction

The Conduction takes place at direct contact two bodies (pot and hob) or within of a body (spoon in tea) instead. Heat excites the smallest particles in a substance Vibrations that are passed on to the next neighboring particles. In this way, the whole body gradually warms up: a metal spoon in hot tea becomes hot even though only the tip is in the tea. It finds Heat transport without matter transport instead of. There are also good and bad Heat conductor. A wooden spoon, for example, hardly gets warm. Try some for yourself Experiments on heat conduction.

Heat flow (convection)

In the Heat flow (convection) the heat transport works over Flows in gases or liquids. If you have ever bathed in a lake, you will have noticed that there are colder and warmer places: In the water there are currents of different temperatures. Even a heater only works through convection, as it heats the air in its vicinity and then distributes it throughout the room.

So convection is one Heat transfer with matter transport. The following applies to both liquids and gases: warm layers rise and cold layers fall down. This ensures a constant exchange of heat.

Thermal radiation

The third and final form of heat transfer is Thermal radiation. In contrast to conduction and convection, radiation requires no transport medium. This is the only way for the heat radiation from the sun to reach the earth through the vacuum of space. A campfire also gives off heat radiation. This is electromagnetic radiation in the infrared range. Every body, including people, trees and houses, gives off infrared thermal radiation that cannot be seen with the naked eye, but can be seen with thermal imaging cameras. Thermal radiation can also be used absorbed become. Dark objects do this particularly well. This is why black always gets so hot in summer.

Thermal insulation

It is not always wanted that heat is transferred. For example, a house should stay warm in winter or the contents of a refrigerator should stay cold. Then you need Thermal insulation. Using the example of a thermos flask, each of the three types of heat transfer Countermeasures clarify. To prevent heat conduction, which is based on direct contact, a airless intermediate layer built-in. So that convection is also suppressed, there may be as little matter as possible that can circulate. Helps against thermal radiation Metal foilthat reflects the radiation. This is why the inside of a thermos has a metallic sheen.