Heating by conduction
Heating by convection
Heating by radiation |
Energy can be transferred by heating in a variety of ways. These are:
- Conduction where energy passes through an object without net movement
of any atoms or molecules in the process.
- Convection where energy is transferred by atoms and molecules actually
moving.
- Radiation, where energy is transferred without the need for atoms or molecules as a medium.
Heating by conduction
When a metal spoon is placed in hot water, energy is transferred or conducted
by atoms of the metal, gaining energy, and vibrating faster. As the atoms
closest to the source of heat vibrate, their energy of vibration is passed
on to atoms around them, so the thermal energy is transferred along the
material. In this manner energy is transferred, without the metal atoms
actually moving out of their place in the crystal structure.
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Conduction |
Conduction is the main energy transfer mechanism in solids. Conduction can also occur in liquids and gases, but is far less effective and falls behind convection in importance. For conduction to be effective, the particles in the substance must be fixed in position with the energy transferred by vibrations between the atoms. Only solids have their particles arranged in a fixed lattice, the particles in liquids and gases are free to move making conduction less effective.
Heating by convection
Convection is transfer of thermal energy in which the particles of the
heated material actually move from one place to another. It is the dominant
method of heating in gases and liquids. A room heater works by convection
of moving air. The heated air expands as the molecules in the air move
around more as a result of the thermal energy being transferred to them.
As the volume increases the density of the air decreases and the warmed
air rises. Cooler air above the warmed air falls due to its higher density
and moves in to take the place of the warmed air. This cooler air also
gets warmed. By this process the air moves in what are called convection
currents. As long as there is a difference in temperature between the
air at the energy source and the surrounding air convection currents will
continue.
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Convection |
Heating by convection occurs in liquids too. Water in a saucepan
on a hotplate is heated by conduction through the pan. Then, because the
water density decreases as it is heated at the bottom of the pan, convection
currents form in the water mixing the heated water with colder water until
the whole lot is at a constant temperature.
Heating by radiation
You can feel the effects of being heated by the Sun, yet you know
that there are very few particles in space. Therefore the heating cannot
be occurring by either conduction or convection. In fact the thermal energy
is being transmitted by radiation, as electromagnetic radiation mostly
in the form of infrared rays. A hot object, such as the Sun, gives off
electromagnetic radiation that travels through space. When these rays
strike another object, like your body for example, the energy is transferred
to the molecules in your skin, their energy increases, they move around
more and you feel warm.
Radiators are efficient heaters because they warm your body directly, not having to warm the air first, then move it by convection. However, they are not effective if you are not in direct 'sight' of the radiator.
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