What is "first law of thermodynamics"

n. the fundamental principle of physics that the total energy of an isolated system is constant despite internal changes [syn: conservation of energy, law of conservation of energy]

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems. The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. The first law is often formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings. Equivalently, perpetual motion machines of the first kind are impossible.

In physics, the first law of thermodynamics is an expression of the conservation of total energy of a system. The increase of the energy of a system is equal to the sum of work done on the system and the heat added to that system:

dE = dQ + dW

where

• E is the total energy of a system.
• W is the work done on it.
• Q is the heat added to that system.

In fluid mechanics, the first law of thermodynamics takes the following form:

$$\frac{D E_t}{D t}=\frac{D W}{D t} + \frac{D Q}{D t} \to \frac{D E_t}{D t} = \nabla\cdot({\mathbf \sigma\cdot v}) - \nabla\cdot{\mathbf q}$$

where

• $\mathbf\sigma$ is the Cauchy stress tensor.
• $\mathbf v$ is the flow velocity.
• and $\mathbf q$ is the heat flux vector.

Because it expresses conservation of total energy, this is sometimes referred to as the energy balance equation of continuous media. The first law is used to derive the non-conservation form of the Navier–Stokes equations.