(thur’ mō dī nam, ik) See Figure T-2. The first law of thermodynamics simply recognizes that heat is a form of energy and thus a factor in the conservation of energy. The second law states that a cyclic heat engine requires a difference of temperature over which to operate. These laws may be expressed as
where U=intrinsic energy and dU=∂Q–∂W, T=temperature=(∂u/∂'s')v, S=entropy and dS=∂Q/T, P=pressure=–(∂u/∂v)s, V=volume, Q=heat, W=work, and a subscript indicates a variable which remains constant during a differentiation; u=intrinsic energy/unit mass, s=entropy/unit mass, v=volume/unit mass=specific volume. Only two of the variables u, s, T, v, P are independent and describe the state of a substance; an equation of state is a relation among any three.