okay, i get that enthaply is a measure of the thermodynamic potential of a system. but what exactly are the 'pressure' and 'volume' components of this? (I'm referring to the definition H = U + pV). i've read that it's "motivated" by the isobaric expansion of a gas inside a cylinder, but how does that describe the thermodynamic potential of any arbitrary system (let's say for example, a slab of metal that has been heated to some temperature)-- is the 'pressure' simply zero? is the pV term only there to describe gases?