Can anyone tell me if [itex]Internal\ Energy + Pressure * Volume[/itex] equals heat, total energy, energy in transit? What if all energy is assumed to be in transit (heat all the way down)? Would that mean all energy is heat? I tend to think so for the following reason: In a "complete" system (with no inputs and outputs): [itex]Total\ (E)nergy=Internal\ Energy + Pressure * Volume[/itex] Would imply that [itex]Internal\ Energy + Pressure * Volume[/itex] is constant. It would therefore mean any decrease in internal energy would be matched by a increase in [itex]pressure * volume[/itex]; this implies that entropy must increase if pressure * volume increases. We see this happening with the cores of planetary nebula. However, if pressure * volume decreases (i.e. internal energy increases), this implies an entropy decrease. If volume became Pressure*Volume would amount to "un-internal energy". But am I wrong? I'm trying to figure this out: http://web.mit.edu/16.unified/www/FALL/thermodynamics/chapter_4.htm http://en.wikipedia.org/wiki/Internal_energy#Expressions_for_the_internal_energy Oh wait.... i think this is it: http://184.108.40.206/search?q=cache:tnyYIhREbdMJ:www.physicsforums.com/archive/index.php/t-98358.html+%22total+energy%22%22internal+energy%22+%22pressure+times+volume%22&hl=en&gl=us&ct=clnk&cd=2&client=firefox-a [Broken] Also, if radiation is emitted, at the finest particles and/or at the largest scales, does pressure * volume increase? Is [itex]Internal\ Energy + Binding\ Energy = constant[/itex] a correct statement for closed systems?