Heating processes

The kinetic particle model says that all matter is made of tiny particles (atoms or molecules) that are always moving and that attract one another with forces that depend on how far apart they are. In a solid the particles are held close together in fixed positions and can only vibrate. In a liquid they are still close but can slide past one another.

In a gas they are far apart, move rapidly in random directions and interact mainly during collisions. Adding energy makes the particles move faster or pulls them further apart against the attractive forces.

Several quantities that sound alike must be kept separate:

• Temperature is a measure of the average translational kinetic energy of the particles. It does not depend on how much material there is.
• Internal energy (U) is the total of all the kinetic energy and all the potential energy (from the attractive forces) of every particle in a system. A large bath of warm water has far more internal energy than a cup of boiling water, even though its temperature is lower.
• Thermal energy is the part of internal energy associated with the random motion of particles; in everyday use it is the energy that a hotter object can give to a cooler one.
• Heat (Q) is energy in transit from a region of higher temperature to a region of lower temperature. An object does not contain heat; it contains internal energy. Heat is what flows.

Heat moves by three processes. Conduction occurs mainly in solids: energetic particles collide with neighbours and pass on kinetic energy, and in metals free electrons carry energy very quickly, which is why metals are good conductors. Convection occurs in fluids (liquids and gases): the heated fluid expands, becomes less dense and rises, while cooler, denser fluid sinks to take its place, setting up a convection current that carries energy with the moving matter.

Radiation is the emission of electromagnetic waves (mostly infrared at everyday temperatures) by all objects; it needs no medium, which is how energy from the Sun crosses the vacuum of space. Dark, matt surfaces are good emitters and absorbers; shiny, light surfaces are poor ones.

Physics uses the Kelvin scale, an absolute scale whose zero (absolute zero) is the temperature at which particles have the minimum possible kinetic energy. A change of 1 K is the same size as a change of 1 °C, so temperature differences are identical on both scales. To convert, use T(K) = θ(°C) + 273.