From: Paul Doherty (pauld@exploratorium.edu)
Date: Tue Feb 03 2004 - 16:31:01 PST
Message-Id: <l03110707bc45ebfef8a3@[192.168.111.197]> Date: Tue, 3 Feb 2004 16:31:01 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole EMF
Hi Algis
Voltage is defined as the electrical potential energy change per unit
charge (coulomb) between two different points. Joules per Coulomb is volts.
If you go around a circuit and return to the original point the potential
energy of the charge must be the same at that point and so the voltage
difference is zero.
However if a charge moves around a metal ring from one point to the same
point and there is a time varying magnetic field inside the ring then the
charge can gain energy and return to the same point with higher energy than
it started. However this energy change is not due to a potential energy
change it is due to work done of the charge by a magnetic field. Joules per
coulomb again but this time it isn't a potential difference it is an
electro motive force.
Whew.
here is what one website said.
The definition of emf (electromotive force):
In a circuit in which a current is present, the total rate at which energy
is drawn from the source of current and dissipated in the circuit per unit
current is defined as the electromotive force, (emf), in the circuit.
E = P/I
The emf is represented by the symbol E. From the above definition,
where P is the power dissipated in the circuit and I is the current flowing
in the circuit.
The potential difference is defined as
"The potential difference between two points on a conductor is the work
done per unit charge by a charge moving from a point of higher potential to
that of lower potential."
Observe that potential difference is defined in terms of work and charge,
whereas emf is defined in terms of power and current.
From the definition, we could define the unit of emf as the watt per
ampere, which is the volt.
Both emf and potential difference have units of volts, as both are
ultimately concerned with energy transformation per unit charge.
Paul D
EMF
>How is electro motive force different than voltage?
>
>I am aware that this is probably a historical term and is indeed not a
>force, but generally the ability to move electrons in a circuit which is
>now called a voltage or electric potential. However, my textbook in some
>sections refers to the voltage of a circuit and in other places refers to
>the EMF. How are they different?
>
>Algis Sodonis
>
>The Urban School of SF
>
>
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