Voltage & resistance

Geotech

Joined Feb 6, 2010
4
In a recent forum discussion someone did a copy/paste from your e-book. I'd like to point out a couple of errors:

When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point.

"Potential energy" should actually be "electric potential". As an example, consider a 1uF cap and a 100uF cap both charged to exactly 1 volt... which one has more potential energy? Which one has a higher electric potential?

Also:

Just like voltage, resistance is a quantity relative between two points.

Resistance is an absolute quantity, not a relative one. The voltage at any node is only meaningful if it is given relative to some other node. But the resistance through any branch is absolute, and not dependent on stating it relative to anything else. Yes, we make a measurement across 2 nodes to determine the resistance, but that's an artifact of the way the ohmmeter works.

I admit I haven't thoroughly read the rest of the e-book, but it seems to be well-done.

Regards,
Geotech

Wendy

Joined Mar 24, 2008
21,839
When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point.

"Potential energy" should actually be "electric potential". As an example, consider a 1uF cap and a 100uF cap both charged to exactly 1 volt... which one has more potential energy? Which one has a higher electric potential?
Welcome to AAC, and thanks for the feedback.

If both caps have the same voltage, then they are at the same electric potential measured in volts, but the potential energy in them is very different.

from Wikipedia said:
In classical electromagnetism, the electric potential (denoted by φ, φE or V; also called the electrostatic potential) at a point in space is potential energy divided by charge that is associated with a static (time-invariant) electric field. It is a scalar quantity, typically measured in volts.
Resistance is an absolute quantity, not a relative one. The voltage at any node is only meaningful if it is given relative to some other node. But the resistance through any branch is absolute, and not dependent on stating it relative to anything else. Yes, we make a measurement across 2 nodes to determine the resistance, but that's an artifact of the way the ohmmeter works.
On one side it is assumed we are talking a linear resistance, but there are lots of examples of other kinds, such as negative resistance, or the nonlinear resistance exhibited by a incandescent lamp or a diode. Some diodes also have negative resistance, such as tunnel diodes.

I'm a latecomer to helping out with the book. I am not the arbiter of what is changed, I try to assist the editor, Dennis, where I can.

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Geotech

Joined Feb 6, 2010
4
In classical electromagnetism, the electric potential (denoted by φ, φE or V; also called the electrostatic potential) at a point in space is potential energy divided by charge that is associated with a static (time-invariant) electric field. It is a scalar quantity, typically measured in volts.
Yes, voltage == electric potential. Yes, if you know the charge then you can get the potential energy (PE=q*V), but voltage is not a "measurement of how much potential energy exists" (per the e-book) any more than it is a measurement of how much power exists (P=V*I).

On one side it is assumed we are talking a linear resistance, but there are lots of examples of other kinds, such as negative resistance, or the nonlinear resistance exhibited by a incandescent lamp or a diode. Some diodes also have negative resistance, such as tunnel diodes.
True, but none of this matters in the fundamental definition of what resistance is. Even with non-linear resistance, it is an absolute property, not relative. That is, the interbase resistance of a UJT may vary with voltage, but it is measured (or calculated) as a stand-alone value, not with respect to some other point or component in the circuit. The voltage of a node, on the other hand, is always relative, measured with respect to some other node.

I'm pointing this out because people are using your e-book as a reference from which to quote, and they are getting it wrong.

Thanks!

studiot

Joined Nov 9, 2007
5,003
Thank you for your post Geotech.

On the face of it you are correct, but it would help greatly if you would post a link to the offending page so that I can find it for review.

Wendy

Joined Mar 24, 2008
21,839
The force motivating electrons to "flow" in a circuit is called voltage. Voltage is a specific measure of potential energy that is always relative between two points. When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point. Without reference to two particular points, the term "voltage" has no meaning.

Free electrons tend to move through conductors with some degree of friction, or opposition to motion. This opposition to motion is more properly called resistance. The amount of current in a circuit depends on the amount of voltage available to motivate the electrons, and also the amount of resistance in the circuit to oppose electron flow. Just like voltage, resistance is a quantity relative between two points. For this reason, the quantities of voltage and resistance are often stated as being "between" or "across" two points in a circuit.
OK, I see some of it. I don't think the writer meant to write potential energy, since potential is in italics, but that is how it came out.

I don't really see a problem with the second, since he is talking about networks of resistors between two points, but the word is unnecessary.

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Geotech

Joined Feb 6, 2010
4
I don't really see a problem with the second, since he is talking about networks of resistors between two points, but the word is unnecessary.
"Just like voltage, resistance is a quantity relative between two points."

OK, I think I see the intent here...

"Just like voltage, resistance is a quantity measured or calculated between two points."

The confusion came because voltage truly is a relative concept, whereas resistance is not.