MadSci Network: Physics |
Dear Laura, The energy of a molecule of water is composed of two parts, a kinetic energy due to its motion, and a potential energy due to the forces of the other water molecules acting on it. The farther away the molecules are from one another, the less each one acts upon others; thus water molecules in a gas have less potential energy than molecules in liquid. When a molecule of water evaporates it must do work to pull away from all the other water molecules. Hence its potential energy increases (because it does work), and this is compensated by a decrease in its kinetic energy. The total energy of the molecule remains constant. You might wonder how water can cool by evaporating if the energy of any molecule stays the same when it evaporates. The reason is this: although any particular molecule keeps its same total energy whether it is in a liquid or a gas, not all molecules have the same energy; some have more total energy, some have less, and the difference is chiefly in the amount of kinetic energy each possesses. The molecules with the most energy (and thus the most kinetic energy) are most likely to evaporate. Hence if the most energetic molecules leave the liquid and the least energetic ones stay behind, energy will flow from the liquid to the vapor. I hope this helps. Larry
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