Energy

When we wish to observe a change in something we can describe that thing as a 'system'. Everything else relevant to our observations of that system is it's 'surroundings'.

-The internal energy(E) of a system is the sum of the potential and kinetic energies of all its particles.

When energy is transferred between a system and its surroundings it is always transferred in the form of work and/or heat.

-Heat(q) is the energy transferred between a system and its surroundings due to a difference in their temperatures.

-Work(w) is the energy transferred when a force moves an object.

A change in internal energy of a system will be equal to the energy transferred to/from its surroundings in the form of heat and/or work. This statement is described in the equation:

∆E = q+w

where ∆ = 'change in'.

q, w, and ∆E can have positive or negative values depending on whether the system is gaining or losing energy. (loss is -, gain is +)

The 'first law of thermodynamics' or the 'law of conservation of energy' states that the total amount of energy in the universe is constant, i.e. energy cannot be created or destroyed. Any change in the energy of a system is accompanied by an opposite change of equal magnitude in its surroundings, thus there is no net gain or loss of energy.

∆Euniverse = ∆Esystem + ∆Esurroundings = 0