Re: emf, potential, voltage

[Bob Sciamanda <trebor@VELOCITY.NET>]

Yes, we do distinguish among the emf, internal resistance voltage drop,
and terminal voltage of a cell.  The potentiometer (the instrument - not
just the variable resistor) was invented to measure the terminal voltage
of a cell without drawing a current, ie to measure the emf.
Potentiometric instruments of this sort now abound in physics, engineering
and chemistry labs.

Bob Sciamanda
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Michael Edmiston" <edmiston@BLUFFTON.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, March 13, 2001 09:56 AM
Subject: Re: emf, potential, voltage


> Disclaimer... I have not been involved in electrochemical measurements
for
> at least 10 years, so I don't know if the terminology used today is
> beginning to change.  It has not changed in current chemistry textbooks.
>
> In physics it had been common to distinguish between the voltage that
would
> be measured at the terminals of a battery if the measurement could be
made
> electrostatically verses that made at the terminals if current is drawn.
> When I went to college the electrostatic measurement would be the "emf"
and
> the voltage under load would be the "terminal potential difference."
>
> Perhaps many physicists have decided to abandon this wording, but the
last I
> knew, chemists have not abandoned this wording.  I think the words used
by
> electrochemists still involve the idea that a specific chemical reaction
has
> an associated emf.  Measurement of this emf is not necessarily easy and
used
> to involve a null measurement using a potentiometer and sensitive
> galvanometer.  Chemical concentrations i.e. chemical activities are also
> important and sometimes measurements have to be extrapolated to infinite
> dilution.  Today "voltmeters" that draw picoamps or even femtoamps have
> changed the technology chemists use to make these electrochemical
> measurements, but it seems to me that the terminology has not changed.
> Recent physical chemistry texts, including the one I use for my class,
> heavily make use of emf terminology.
>
> This doesn't necessarily mean we can't change, but we should be aware
that
> physical chemists use emf quite a bit.  Physics students take chemistry,
and
> chemistry students take physics, and sometimes people end up being both
> chemists and physicists.  We ought to take a broad picture when we begin
> trying to define what terms mean, and which terms should be discarded,
etc.
> Michael D. Edmiston, Ph.D.              Phone/voice-mail:
419-358-3270
> Professor of Chemistry & Physics        FAX:
419-358-3323
> Chairman, Science Department            E-Mail
edmiston@bluffton.edu
> Bluffton College
> 280 West College Avenue
> Bluffton, OH 45817