First Law

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


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:

[math]\Delta U = Q + W[/math]


[math]dU = dQ + dW[/math]