# Dictionary:Thermodynamic functions

(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

*U*=

*T*d

*S*–

*P*d

*V*,

where *U*=**intrinsic energy** and d*U*=∂*Q*–∂*W*, *T*=temperature=(∂*u*/∂**' s'**)

_{v},

*S*=

**entropy**and d

*S*=∂

*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.