First Law of Thermodynamics: energy can neither be created nor destroyed (see Refuge, p. 313); or, the energy of the universe is constant.

It is helpful to divide the universe into two parts: the system is that particular part of the universe that is under study, e.g., a planet, a lake, a drop of water, etc; the surroundings include all parts of the universe that are not part the system.

Universe = System + Surroundings

First Law: DE_universe = DE_system + DE_surroundings = 0

DE_system = - DE_surroundings

or, the amount of energy gained by a system is equal in magnitude to the amount of energy lost by its surroundings.

As a result of the First Law, any system that has a constant amount of energy must gain energy from, and lose energy to, its surroundings at equal rates. The amount of energy possessed by the system is its reservoir.

For example, the Lava Lamp^®. After it reaches thermal equilibrium (it is no longer changing temperature after it has been on for awhile), the amount of energy going into the fluid is exactly balanced by the amount of energy leaving.

Energy Input: light from bulb, heat from light bulb

Energy Output: scattered light, emitted light, heat lost from solution

Note that there can be transformations of energy within the system. So for example, the light energy entering the system contributes to both energy outputs. Some of the light is simply scattered and some is absorbed by the materials in the fluid. Recall that whenever light is absorbed, its energy is usually transformed to either heat or emitted light. In this case, both are occurring; most of the abosorbed light is converted to heat, which is lost to the surroundings, and a smaller amount is emitted and gives rise to the characteristic yellowish-green glow observed in the "globs".

What happens when the energy inputs and outputs are not equal? The size of the reservoir must change.

So if,

E_in > E_out; system gains energy and the temperature increases (when the light is "warming up")

E_in < E_out; system loses energy and the temperature falls (when the light is turned off and the stored heat is dissipated to the surroundings)