# First Law

Note: This section needs a very basic knowledge of calculus. To see a simplified version (without equations) go HERE

## Observations

The laws of thermodynamics are based on observations of the natural world. The first law is based on two observations concerning energy:

1. Energy can be transferred between a system and its surroundings by only two ways: work and heat
2. The total energy of a system and its surroundings is always constant (The conservation of energy)

## First Law

These two observations can be combined into the First Law of Thermodynamics:

The internal energy of a system is constant unless changed by doing work or by heating

## Mathematical Statement

Mathematically, the change in internal energy is the sum of the work and heat entering or leaving the system:

$\Delta U = Q + W$

or

$dU = dQ + dW$