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Find the potential difference between 2 junctions (nodes)

by CogitoEAS
Tags: circuits, electricity, loop rule
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CogitoEAS
#1
Jul2-14, 09:15 PM
P: 9
Hello,

I'm wondering how to determine the value of the potential difference between two points (junctions specifically) in a circuit with 2 emfs. The junctions are points x and y in the circuit below.

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I thought I could use the junction rule to determine the current at each junction, but I don't see how that can help me find the potential at those points as I don't see how I can find the resistance there. Then I tried using the loop rule on loop abcda to find the difference of the potentials (-(2A*5ohms)+28)-(-6A)(3ohms)=36V) but I'm not sure if that's correct, and this is a written assignment so I can't check if the answer is right or wrong.

Thanks
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CogitoEAS
#2
Jul2-14, 09:39 PM
P: 9
Also, I tried to edit the title to read "Find" but I guess it won't update..............don't judge me
DaleSpam
#3
Jul2-14, 11:00 PM
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Quote Quote by CogitoEAS View Post
I'm wondering how to determine the value of the potential difference between two points (junctions specifically) in a circuit with 2 emfs.
...
I thought I could use the junction rule to determine the current at each junction
You don't need to determine the currents. All of the currents are already given. Just use Ohms law with the known currents and known resistances to determine any unknown voltages.

PS, I fixed the title.

CogitoEAS
#4
Jul2-14, 11:10 PM
P: 9
Find the potential difference between 2 junctions (nodes)

Thanks for the reply! So if I want to calculate the potential at x I would add the potentials at both batteries and resistors 1 & 2? Or are you telling me to use ohms law at that point with the given current? In which case I don't know what the value for resistance would be at a junction
DaleSpam
#5
Jul2-14, 11:23 PM
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P: 16,947
Unfortunately, your ascii art got seriously degraded, so I cannot tell you about specific points and so forth. From what I can tell, if you set your ground on the left then the only unknown voltage is on the right. So just use Ohm's law across any of the resistors to obtain it.
CogitoEAS
#6
Jul2-14, 11:55 PM
P: 9
Sorry about that - this is actually my first post and I didn't realize you could add an attachment. I've attached an image of the circuit to this reply and the original post. Just so we're clear, you're suggesting that the voltage across the 3 ohm resistor is equal to the potential at point y?

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sophiecentaur
#7
Jul3-14, 06:11 AM
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P: 11,900
Quote Quote by DaleSpam View Post
You don't need to determine the currents. All of the currents are already given. Just use Ohms law with the known currents and known resistances to determine any unknown voltages.

PS, I fixed the title.
Absolutely.
It is often very tempting to feel you have to solve the whole circuit when they give you more than minimal information. You can save yourself loads of time by reading questions carefully before plunging in and doing the whole thing. It's smart not bad.


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