Re: In energy, what does work equal?

From: Herman Family (celcaps_at_frontiernets.net/without_any_s/)
Date: 08/06/04 

Date: Fri, 06 Aug 2004 02:43:36 GMT

"Hikaru" <none@available.com> wrote in message
news:4112d5b0$0$5262$c3e8da3@news.astraweb.com...
> > > What about the relationship between enthalpy and internal energy?
> >
> > This came up in a previous thread from you no? The bottom line is
(again)
> > this: at constant volume, dE = q; at constant pressure dH = q. This is
> the
> > primary reason why H is defined, as most experiments are carried out at
> > constant pressure.
>
> Hm ok I'm not quite understanding this right now. dE and dH both give you
q.
>
> I guess the original equation is dH = dE + d(PV)
>
> I'll think like the piston thing. At constant pressure, expansion would
> lower temperature so heat would be draw in, so q is positive. At constant
> volume, more heat would mean more pressure...q is positive here, too. I'm
> lost, sorry.
>
>
I think my books use U where you are using E.
In the piston, at constant pressure, lowering the temperature (removing
heat) would decrease the volume, and q is negative.
Now, the heat that you took out of the system should equal the work done on
the system, or PdV. Notice that since V decreased, work was done on the
system, so it recovered a bit of energy from that, allowing it to keep the
pressure up.

Lets then go a little further and add heat to the system. The internal
energy thus starts to rise, but this allows for the piston to expand. q is
now positive and work leaves the system. E is a state variable, dependent
upon temperature and pressure.

H is E without the piston work, sort of. It is the total q to achieve a
given temperature and pressure less the work required to increase the
pressure. To get E from H, just subtract any piston work required.

I've probably screwed a couple things up, but if heat is lost from a system
q is negative. If the system stays at the same temperature and pressure
then q = PdV.

Michael

Relevant Pages

  • Re: pressure and heat
    ... The adiabatic gradient in terms of pressure ... If the temperature is below local melting point, ... But the amount of heat the Earth can hold is constrained by ... All layers of Earth must have either a temperature gradient ...
    (rec.arts.sf.science)
  • travestiazra.com
    ... The adiabatic gradient in terms of pressure ... If the temperature is below local melting point, ... But the amount of heat the Earth can hold is constrained by ... All layers of Earth must have either a temperature gradient ...
    (rec.arts.sf.science)
  • Re: Reply to Eric Rowley -- Ozone
    ... pushing all the heat in a large area into a smaller area. ... temperature is higher, but there is no actual new heat energy in the ... If this is correct, then pressure is not a heat source, as no ... We can move it around the atmosphere. ...
    (talk.origins)
  • Re: Let me simplify it for you...
    ... heat into a Dyson sphere rather than into the 3 K background. ... fourth power of temperature, ... Slight warming leads to slight pressure increase, ...
    (rec.arts.sf.fandom)
  • Re: temperature increases, volume decreases. why?
    ... >> Assume I have a gas in a cylinder that has a piston as a top. ... >> temperature inside the cylinder will increase, ... more collisions per minut means also that pressure got bigger. ...
    (sci.physics)