# Consequences of energy transfer

The radiator

A radiator does radiate some: heat, and if you stand near a hot radiator your hands can feel the infrared radiation being emitted by the front surface of the radiator. However, this is only around one quarter of the heat being released by the radiator. Three quarters of the heat is taken away by the hot air that rises from the radiator. Colder air from the room flows in to replace this hot air, and convection current is formed.

You will note that the convection current is far more efficient at heating the top of the room than it is at heating the person standing in front of the radiator.

The vacuum flask

The vacuum flask will keep a drink hot or cold for hours by almost completely preventing the flow of heat out or in.

Conduction is almost eliminated by making sure that any heat flowing out must travel along the glass of the neck of the flask. The path is a long one, the glass is thin, and glass is a very poor conductor of heat. The bung in the top of the flask must also be a very poor conductor of heat: cork or expanded polystyrene is good.

Convection is eliminated because the space between the inner wall and the outer wall of the flask is made a vacuum so that there is no air to form convection currents.

If the contents are hot, radiation is almost eliminated because the inner walls of the flask are coated with pure aluminium. Because the aluminium is in a vacuum, it stays extremely shiny forever, and so the wall in contact with the hot liquid emits very little infrared radiation.

The vacuum flask works j ust as well t he other way, at keeping the contents cool. And in fact it was invented in the 1890s by a Scottish physicist so that he could keep liquefied gases, such as liquid hydrogen that boils at $-250^{0}C$. Scientists usually call it a Dewar flask in his honour.