The transfer of heat energy is one of the topics studied in GCSE Science. This is one of eight quizzes on that subject and it looks specifically at things which affect the rate of heat energy transfers through convection, conduction, evaporation and condensation.
Thermal energy can be transferred from one place to another by doing work or by heating processes. The rate at which heat transfers depends on several things. The heating processes are conduction, convection, evaporation and condensation and they all depend on how the particles of a particular substance are arranged. Lets have a look at these different heating processes in turn, starting with conduction:
Particles of solids are fixed in place but they can vibrate. They transfer heat energy into kinetic energy, so the more heat that is present, the more they vibrate. Metals are good heat conductors because the particles are closely packed together and the vibrations can be transferred efficiently. They also contain free electrons which drift through the metal - the hotter it is, the quicker they move. So heating one end of a metal bar makes the free electrons move faster at that end and transfer heat quickly throughout the metal bar. In all other substances there are no free electrons to carry the heat through the structure and so conduction is a much slower process. Conduction of heat cannot take place in liquids or gases because the particles are not fixed in place.
Secondly, convection. The particles of a liquid are not fixed in place, so when they are heated they move around faster. They therefore occupy more space and that means the heated part of the liquid is less dense than the cooler parts. In liquids, less dense areas will move upwards so the warmer part of the liquid rises away from the heat source. Cooler liquid moves in to take its place and is heated in its turn, so a convection current is set up that transfers the heat throughout the liquid.
Finally, evaporation and condensation. The fastest moving particles of a liquid are able to escape from the surface, this is called evaporation. The temperature of a liquid is a measure of the average speed of its particles. As these faster particles leave, it means that the average speed of the ones left behind is lower, so evaporation causes cooling. The faster the rate of evaporation, the greater the cooling. Condensation is the opposite of evaporation.