In thermodynamics so-called "specific" quantities are quantities per unit mass (kg) and are denoted by lower-case characters. So h, u, and v are the specific enthalpy, specific internal energy, and specific volume (volume per unit mass, or reciprocal density), respectively. In a Joule–Thomson process the specific enthalpy h remains constant. To prove this, the first step is to compute the net work done when a mass m of the gas moves through the plug. This amount of gas has a volume of V 1 = m v 1 in the region at pressure P 1 (region 1) and a volume V 2 = m v 2 when in the region at pressure P 2 (region 2). Then in region 1, the "flow work" done on the amount of gas by the rest of the gas is: W 1 = m P 1 v 1. In region 2, the work done by the amount of gas on the rest of the gas is: W 2 = m P 2 v 2. So, the total work done on the mass m of gas is