# FL: The First Law of Thermodynamics

### The First Law of Thermodynamics

##### Law:

The first law of thermodynamics states that the total energy in the universe is constant.

\$\Delta E_{univ} = 0 \$

Stated in this way, the most significant implication of this law is that energy can change forms, but the total amount must remain constant.

Even so, this statement is not of terribly practical utility, so we can restate the first law in a more useful form as:

##### Law:

The first law of thermodynamics can be restated as the total energy change of a system must be equal an opposite to the total energy change of the surroundings.

##### Note:

This means that the energy of a system can change forms, but can only change in total amount through interactions with the surroundings.

##### Example:

A 1 kg stone is held on a 100 m high cliff. What is its gravitational potential energy? It is dropped into the mine shaft. What is its potential energy and kinetic energy when it is passing the ground level. What is its potential and kinetic energy when it is about to hit the bottom of the mine shaft. Ignore air friction. What happens to this energy when the stone hits the bottom and comes to rest?