FTQ’s – Other

What is the difference between heat transferred by radiation and radiated heat? 

Radiation is the propagation of energy through space – it does not require a medium to carry it. Examples are electromagnetic radiation (a portion of electromagnetic frequencies is light that we can see) and sub-atomic particles such as alpha, beta and gamma radiation.

Radiation has its own energy which is absorbed by materials that they encounter or “hit” and this energy is transferred at a sub-atomic level, raising the energy of the atom, rather than at a molecular level. Some radiation (such as visible sunlight) is not absorbed (or only partially) by a medium through which it is travelling. Sunlight is not absorbed by air, hence it reaches our eyes (where it is then absorbed to create images) rather than being absorbed by the air itself.

Radiated heat requires a medium to carry heat energy, e.g. a coal or electric fire will warm a room up by heating the the air molecules which then flow around the room, plus the air molecules will also contact the walls/windows/ ceiling  / contents of the room and thereby pass the energy from the fire to these material objects.

Radiated heat is the excitation of the air (and other materials) molecules caused by the energy the fire is giving out and it is this excitation which causes the molecules to literally vibrate and “bump” into other molecules with which they are in contact. This is how conventional ovens work – they heat the air in the oven and then the vibrating air molecules bash against the food you are cooking, which in turn causes the molecules comprising the food to vibrate also – this is why food cooks from the outside to the centre.

Microwave cookers use radiation to cook, rather than by heat transfer. Microwave ovens generate electromagnetic radiation at a frequency that causes water molecules to vibrate (heat up). Again this is why microwaves cook from the outside – and also, without extra technology, they do not “brown” the food – heated air molecules are not in contact with the outside of the food.

Food in a microwave would still cook even if there was a vacuum in the microwave, but food in conventional ovens would not. The radiation created by microwaves pass through a vacuum (just as light does in space to reach the Earth) but the heat created by the hot elements of an oven cannot reach the food without intervening air molecules to transfer the heat (technically,some energy would be transferred because a hot element also produces a small amount of radiation in the Infra-red frequency range – but not enough to cook food in any realistic timescale.