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Re: emf, potential, voltage



Some useful points I like to emphasize in DC circuits:
1) Consider any two terminal device with a current of I amperes flowing
through it down a potential hill of V volts. That means there is an
electrostatic field in the device which is giving energy to the charge
carriers as they pass through the device, down the potential hill. If this
were the whole story, the carriers would emerge from the device with
increased kinetic energy, and the current out of the device would exceed the
current into the device. Since the measured current is the same (in and
out), the device is a sink of energy - a device which takes energy from the
electrostatic field and equivalently increases energy of some other form
(heat for a resistor - kinetic energy for a motor, etc) . In all such cases
the rate of energy transformation is P = VI watts. (Analogies of stones
falling through molasses in the earth's graviational field, and water
turning a waterwheel as it falls in the earth's gravitational field may be
helpful - but note the differences.)

2) By the same reasoning a two terminal device which has an unchanged
current I following through it UP a potential hill of V volts must be giving
to the carriers the energy which they lost in climbing that potential hill,
else the current would be lessened. This device is a source of energy (to
the carriers) and is able to use some other form of energy to do this work
at the rate P=VI.

3) The "normal" ideal emf will (attempt to) move the charge carriers in and
on its terminals, and any conductors connected to them, until a (static or
dynamic) equilibrium is reached wherein the potential difference (of the
resulting electrostatic field) across its terminals is numerically equal to
the emf (Volts=Joules/Coulomb).

This is a convenient idealized model of (a part of ) certain existing
devices (eg. batteries), but "circuit energy sources" of differing, more
complicated behavior are certainly conceivable.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor