# The Principle of Conservation of Energy In its most general form, the ```METR 201: Physical Processes Dr. Dave Dempsey The Principle of Conservation of Energy In its most general form, the Principle of Conservation of Energy says that energy is “conserved”—that is, it can’t be created or destroyed, so that the total amount of energy in any closed system (that is, a system that can’t exchange energy with anything outside the system) doesn’t change. However, energy exists in many forms, and it can change from one form to another. For example, when an object emits radiative energy (which most objects do, all the time), that energy must come at the expense of some other form of energy, and it in fact comes at the expense of an equivalent amount of heat energy in the emitting substance. (Heat is a form of internal energy associated with the random molecular motions of a substance). Similarly, when an object absorbs radiative energy, that energy is converted into an equivalent amount of heat in the object. As another example, when a substance changes phase (from gas, liquid, or solid to one of the other two phases), energy transforms from heat in the substance to an equivalent amount of latent heat (a form of chemical potential energy) in the substance, or vice versa, depending on the direction of the phase change. We can apply the principle of conservation of energy to write a version specific to the heat content of an object, creating a heat budget equation for the object. In it’s most general form, we can write the heat budget equation as: Rate of change of heat content of the object = Sum of rates at which object gains heat −
by various mechanisms (sum of “sources” of heat) Sum of rates at which object loses heat by various mechanisms (sum of “sinks” of heat) If we apply this general form of the heat budget equation to patch of the earth’s surface (and ignore very small sources and sinks), we get: Rate of change of heat content of the object = Rate of absorption of solar radiative energy + &plusmn; Rate of transformation &plusmn; of latent heat to/from heat due to phase changes of water Rate of conduction of heat from/to air in contact with it &plusmn;