voltage drop

Thread Starter

mOOse

Joined Aug 22, 2007
20
I don't understand the following from Volume 1 Chapter 5 (page 146).
It's about a simple circuit with a battery and three resistors in series.
The middle resistor R2 fails open, changing to infinite resistance.
(Quotes in blue.)

With zero circuit current, there is no electron flow to produce
voltage drops across R1 or R3. R2, on the other hand, will
manifest the full supply voltage across its terminals.

You say R1 and R3 have no voltage drop because they have no current. But R2 also has no current, so how can it have a voltage drop? Is electron flow necessary to produce voltage drops? Page 20 shows a break in a wire that otherwise shorts a battery, and there is a voltage drop across the break but "no flow!"

PS, a few small things:

Volume 1 Chapter 1 (page 16)
:registers appears as a mysterious paragraph of its own.

Volume 1 Chapter 5 (page 149)
this new kind of analysis without precise numerical figures
something I like to call qualitative analysis.
Missing the word "is"

Volume 1 Chapter 6 (page 187)
i The current through R1 is still exactly twice that of R3,
despite the fact that the source voltage has changed.
There's an extra "i" at the beginning of this sentence.
 

Dave

Joined Nov 17, 2003
6,970
I don't understand the following from Volume 1 Chapter 5 (page 146).
It's about a simple circuit with a battery and three resistors in series.
The middle resistor R2 fails open, changing to infinite resistance.
(Quotes in blue.)

With zero circuit current, there is no electron flow to produce
voltage drops across R1 or R3. R2, on the other hand, will
manifest the full supply voltage across its terminals.

You say R1 and R3 have no voltage drop because they have no current. But R2 also has no current, so how can it have a voltage drop? Is electron flow necessary to produce voltage drops? Page 20 shows a break in a wire that otherwise shorts a battery, and there is a voltage drop across the break but "no flow!"
The e-book is correct. To clarify, the potential across R2 is the supply voltage since the two terminals coming into and out of R2 are connected to to positive and negative terminals of the supply, via R1 and R3. In the absence of current flow, no voltage is dropped across R1 or R3 hence this is the reason as to why it is the 'full supply voltage' that is dropped across R2.

Does that make it clearer?

I will have a look at the other issues later and get back to you.

Dave
 

Thread Starter

mOOse

Joined Aug 22, 2007
20
The main problem I had was with:

With zero circuit current, there is no electron flow to produce
voltage drops across R1 or R3.
I understand how there are no voltage drops across R1 and R3 because
they are not connected across the battery.
But is the statement "there is no electron flow to produce voltage drops"
the proper explanation?
As I (a beginner) currently understand it, "electron flow"
is not necessary to produce "voltage drops". Perhaps it's a matter
of definition, but to give another example, I would say there is a
"voltage drop" across the terminals of a 9V battery even if there was
no "electron flow" between them.
 

Dave

Joined Nov 17, 2003
6,970
I can understand how this can be confusing. The statement "there is no electron flow to produce voltage drops" (or more specifically current flow) is correct if you consider Ohm's Law:

V = IR

When I = 0, V must also be 0 irrespective of the value of R. The problem with the case where R2 is open circuit and hence R2 = \(\infty\) is that Ohm's Law falls down because of the infinity variable. This condition arises in other circuit scenarios, you have already mentioned a voltage source, but also the steady-state (i.e. circuit currents equal zero) conditions for a series RC-circuit (ref. http://www.allaboutcircuits.com/vol_2/chpt_4/3.html).

I will alert the project coordinator to provide his comments on the suitability of the statement and the context in which it is written. I will also make comments on your other question later.

Thanks for your contributions, if you wish to clarify any further points feel free to ask.

Dave
 

recca02

Joined Apr 2, 2007
1,214
the confusion might also arise from the way we perceive things,
this is again a cause-effect based confusion. voltage is seen as cause and current as effect.
when we say electron flow is required for voltage drop we get confused as we understand voltage as the force required to move electrons so electron flow exist wherever a potential drop exist ,consequently for a resistance offered voltage is dropped across it while forcing electrons.
it is advisable to not really get confused with the terminologies used but to have a clear idea of the happenings.
 

Thread Starter

mOOse

Joined Aug 22, 2007
20
Thanks for your responses.
My problem seems to have been a confusion between the terms "voltage" and "voltage drop".
I was thinking that they were basically the same, but now I consider
"voltage" to be a mearurement of emf between two points whereas a
"voltage drop" is the amount of emf used up (dissipated as power?) in a resistance when current flows.
I'm sure that's not quite right, but at least I'm not considering the two
terms to be equivalent anymore.

Re-PS: Here are the three typos again.
I've re-referenced them to AAC's online format of the ebook.

http://www.allaboutcircuits.com/vol_1/chpt_1/4.html
":registers" appears as a paragraph.

http://www.allaboutcircuits.com/vol_1/chpt_5/7.html
"this new kind of analysis without precise numerical figures
something I like to call qualitative analysis"
Missing the word "is".

http://www.allaboutcircuits.com/vol_1/chpt_6/3.html
"i The current" sounds kind of cool, but the "i" is extraneous.
 

recca02

Joined Apr 2, 2007
1,214
i actually knew what your confusion was but if u analyse it u wont find much difference in the two the potential drop across a two can be compared to application of same voltage across it.
 

Dave

Joined Nov 17, 2003
6,970
Thanks for your responses.
My problem seems to have been a confusion between the terms "voltage" and "voltage drop".
I was thinking that they were basically the same, but now I consider
"voltage" to be a mearurement of emf between two points whereas a
"voltage drop" is the amount of emf used up (dissipated as power?) in a resistance when current flows.
I'm sure that's not quite right, but at least I'm not considering the two
terms to be equivalent anymore.
Glad to see your understanding of the subject has improved. I did think of an example that would demonstrate the e-book concept using Ohm's Law and voltage divider equations, but this may be unnecessary. If you have any further questions about this feel free to ask.

Re-PS: Here are the three typos again.
I've re-referenced them to AAC's online format of the ebook.

http://www.allaboutcircuits.com/vol_1/chpt_1/4.html
":registers" appears as a paragraph.

http://www.allaboutcircuits.com/vol_1/chpt_5/7.html
"this new kind of analysis without precise numerical figures
something I like to call qualitative analysis"
Missing the word "is".

http://www.allaboutcircuits.com/vol_1/chpt_6/3.html
"i The current" sounds kind of cool, but the "i" is extraneous.
Thanks for that, you have indeed found 3 typos in the e-book text. I will arrange for them to be corrected.

Dave
 
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