Energy - Transfer of Energy

As well as conduction, convection and radiation, evaporation and condensation transfer heat energy from place to place. Evaporation takes place from the surface of a liquid. The particles in liquids are in constant motion, changing places and colliding with one-another. The kinetic theory predicts that the energy of majority of particles will lie within certain limits but there will always be some with higher than average and lower than average energies (higher tier candidates may have seen a diagram showing this).

When they are near the surface of a liquid, particles with higher than average energies can overcome the forces holding the particles together as a liquid and pass into the air. As this happens, the number of particles in the liquid becomes smaller so the liquid volume gradually decreases. The total energy contained within the liquid is decreased and therefore the temperature of the liquid drops. Some liquids naturally evaporate faster than others, for example, ethanol will evaporate faster than water. Evaporation is a heat transfer process that transfers the thermal energy into the surroundings from a liquid. The faster that a liquid evaporates, the cooler it will become. There are several factors that affect the rate of evaporation of a liquid other than the type of liquid - you need to revise them for the exam.

The opposite of evaporation is condensation. This is the process where vapor turns back into a liquid. A vapor is made from billions of particles, if they are slowed down, they will whizz around less, become closer together and the forces they exert on each other will increase. In other words, the vapor becomes a liquid during condensation. To slow down the speed of movement of particles, it is necessary to remove heat energy, this occurs where a vapor meets a cold surface or is mixed with cooler vapor (for example when you breathe out on a cold day the water vapor in your breath condenses as it loses its energy to the cold air around you). Condensation therefore transfers thermal energy from a vapor into the surroundings. In senior high school, you need to know the factors that affect the amount of condensation that occurs.