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Topic: What are space and time?
Replies: 184   Last Post: Nov 19, 2012 9:14 PM

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Darwin123

Posts: 195
Registered: 4/6/08
Re: What are space and time?
Posted: Aug 26, 2010 11:49 AM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

On Aug 24, 3:07 pm, John Stafford <n...@droffats.net> wrote:
> In article
> <d6d4e6d1-7920-4163-b097-8e1445b02...@h19g2000yqb.googlegroups.com>,
>
>  Darwin123 <drosen0...@yahoo.com> wrote:

> >    Most of thermodynamics can be visualized by imagining that entropy
> > is an indestructible fluid. Temperature is a quantity proportional to
> > the pressure this fluid can exert. One can think of entropy as
> > analogous to electric charge, and temperature as analogous to voltage.

>
> Help me understand that.  Given that the greater difference in
> temperature between an object and the environment (think of a hot bar of
> steel in an ambient atmosphere of 0c) then the faster the transfer of
> energy from the bar to the atmosphere.
>
> How does that relate to voltage?


I am relating an analogy, not an equivalence. Similar equations
are used in hydrodynamics, electrodynamics, and thermodynamics even
though they relate to different things. Thus, similar pictures are
used in hydrodynamics, electrodynamics, and thermodynamics. You have
to keep in mind that the quantities are different, especially when
they interact together. There are some differences, that have to be
always kept in mind. However, if you know one field then you can use
some of that knowledge in the other field.
I take it you know a little about electrodynamics, since you are
asking about voltage. So I will present a useful analogy between
thermodynamics and electrodynamics.
On an atomic level, electricity consists negatively charged
particles called "conduction electrons" or positively charged bubbles
called "holes". Of course, electricians don't use that knowledge. On a
macroscopic level, electricity is a fluid as described by Benjamin
Franklyn. The equation for power put out by a DC current crossing a
circuit is:
P=IV
where P is power, I is current, and V is the drop in voltage across
the circuit element.
Of course, this assumes the current is constant. The energy used
by the circuit element in a time, t, is
E=Pt
where Q is energy used up by the circuit element. The electric charge
that passed through that device is
q=It
where I is electric current and t is the time. So, using these two
equations
E=qV
Now, electric charge (q) is analogous to entropy (S). Used
energy (E) is analogous to heat energy (Q), which by definition is the
energy released from entropy. voltage (V) is analogous to temperature
(T). So the result of substituting the electrodynamic quantities for
thermodynamics is:
Q=ST
Often, this is presented as the "definition" of entropy which is:
S=Q/T
So entropy, S, has many properties similar to an electric charge.
Here is a big difference.
The total electric charge can not be destroyed and can not be
created. This is called conservation of electric charge.
The total entropy of a system can not be destroyed but can easily
be created. This is called the Second Law of Thermodynamics.
Calculating the work done by a heat engine turns out to be
similar in many ways to calculating the work done by a circuit element.


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