What is The First Law of Thermodynamics?

Answer 1

The increase in the internal energy of a thermodynamic system is equal to the amount of heat energy added to the system minus the work done by the system on the surroundings.

Basically, the amount of heat(H) the you input subtracting off the work done (W) gives you internal energy (U).

Answer 2

First Law:
Internal energy can be added to a body either by heating or doing work on it, and conversely, if work is extracted from the body, its internal energy (and hence its temperature) will fall unless an equivalent amount of heat is supplied from outside.
Second Law:
Heat will not pass spontaneously from a cold body to a hotter body.
Third Law:
It is impossible to cool body right down to absolute zero, i.e., below – 273.15oC (see Phenomenon and Terms of Physics

Answer 3

The first law of thermodynamics states that, energy is neither created nor destroyed... it just changes its form from one to another...

Answer 4

The first law is the conservation of energy. The energy in a closed system cannot be created or destroyed.

http://en.wikipedia.org/wiki/Laws_of_thermodynamics#First_law