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Re: Circuit Question



Tina, this is truly the source of much of the students' confusion. Many
types of devices are possible. In particular there are "Constant Voltage"
sources and "Constant Current" sources (and other possibilities). When
these devices appear in problems, their specifications should be clear.

In all introductory courses the EMF which appears in textbook problems is
intended to be a constant voltage source, though this is often unspoken.
It has the "magical" property that it will maintain a fixed, specified
potential difference between its terminals. A real cell is more closely
modeled (even at this intro level) as this "pure EMF" in series with an
internal resistance.

It follows that when such a real cell carries a current, its terminal
voltage will vary from the specified EMF. The difference will be the IR
potential difference across its internal resistance. Think about this for
both the case when the cell is sourcing a current and for the case when a
current is being forced backward through it (perhaps for charging
purposes) by a "stronger" emf.

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