# Voltage Sources and Series Resistor

Discussion in 'Homework Help' started by geratheg, Oct 9, 2014.

1. ### geratheg Thread Starter Member

Jul 11, 2014
107
3
Shown in the attachments is a circuit with the ground given, I hope I converted it to the second circuit correctly with the ground in the correct location of the circuit. I would like to know what the voltage across the resistor is?

Since the ground is there I would assume 0V. If ground wasn't labeled, I would choose 5V. Which voltage is correct with ground there?

Thanks!

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2. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
It has to be 5V.

3. ### geratheg Thread Starter Member

Jul 11, 2014
107
3
Wouldn't the ground there make it 0 potential since I'm thinking because the ground is there then it's connected to the negative so there's a short, thus 0V. Am I misunderstanding what ground is?

4. ### wmodavis Well-Known Member

Oct 23, 2010
737
150
Your equivalent circuit, which is correct, shows a 5V source connected directly across the resistor. How can the voltage voltage across that resistor be both 5V & 0V? That makes no sense. Analyse it with KVL.

5. ### geratheg Thread Starter Member

Jul 11, 2014
107
3
Without ground 5V. With ground at that particular location I thought it meant negative is connected there so 0V. I don't think I'm understanding ground in a DC circuit? Where's ground connected to? The negative of the power source?

6. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Where your problem is coming from is you are mixing up the concept of the voltage potential at a point relative to an arbitrary reference point and the concept of the voltage potential between two arbitrary points.

Voltage is ALWAYS a measure of the potential DIFFERENCE between TWO points. (At least for anything remotely like this where we are dealing with conservative electric fields.)

By picking one node and calling it 0V, we are merely saying that when we talk about "the voltage AT a point in the circuit", what we are really talking about is the voltage difference between that point and the node we arbitrarily chose to call 0V.

But when we ask about the voltage ACROSS some component, then we are talking about the voltage difference BETWEEN the two points on either side of that component and it has nothing to do with which node we chose as our common reference point.

Last edited: Oct 10, 2014
7. ### geratheg Thread Starter Member

Jul 11, 2014
107
3
Thanks for all the help!

8. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,402
1,225
Here's your circuit without the ground ... note the meter leads ... and compliance with KVL.

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