# Current through a resistor with two DC voltage sources

#### Kelko

Joined Oct 1, 2020
14
I have no idea where to go with this ive tried multiple different things and I know they’re wrong but I have no examples to go off how’d to do questions like these any help would be appreciated

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#### ronsimpson

Joined Oct 7, 2019
1,020
Idea:
Make R1 and R2 into R12=3.0k Is the circuit unchanged?
Make R3 and R4 into R34=2.0k. Put the one resistor either above or below V2. Is the circuit unchanged?

Same?

What is the current in R34 2K?

#### WBahn

Joined Mar 31, 2012
26,301
You started off just fine, except that because you wouldn't track your units properly, you got an answer that was off by three orders of magnitude.

You have

V = I·R
9 V = I·3
I = 3 A

Your second line is wrong. You have voltage on the left and current on the right. But, like so many people, you don't pay attention to units. Instead, you just throw whatever unit onto the final answer you think you would like the answer to have and call it good.

V = I·R
9 V = I·(3 kΩ)
I = (9 V) / (3 kΩ)
I = 3 mA

#### Kelko

Joined Oct 1, 2020
14
So does the other voltage source not have an impact on the current? That’s mainly what I’m stuck on tbh

#### Audioguru again

Joined Oct 21, 2019
2,067
Look at the voltage across R3 and R4 then calculate the currents in them.

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#### WBahn

Joined Mar 31, 2012
26,301
So does the other voltage source not have an impact on the current? That’s mainly what I’m stuck on tbh
The left hand voltage source fixes the voltage difference between the top and bottom nodes at 9 V and will supply (or absorb) whatever current is required in order to maintain that 9 V difference across those two nodes. The only thing those middle two resistors care about is the voltage difference between the top and bottom nodes. The right hand branch also only sees that fixed 9 V difference between the top and bottom nodes and responds accordingly. It will source or sink current into it, but that current comes or goes into the left hand source -- the middle resistors do not see that interaction at all.

#### Audioguru again

Joined Oct 21, 2019
2,067
If the voltage source on the left side is weak and does not have voltage regulation then if the voltage source on the right side has less voltage then it will cause the voltage on the left side to drop and reduce the current in R1 and R2.

But your circuit shows both sides have exactly the same voltage.

#### WBahn

Joined Mar 31, 2012
26,301
It's a very simple problem early in the TS's curriculum. The sources are ideal. There's not point in needlessly complicating the problem at this stage. Let the TS learn how to analyze idea circuits -- not ideal behavior will be added soon enough.

#### MrChips

Joined Oct 2, 2009
22,099
There is no need to make guesses.
Write out all the KCL and KVL equations that you can find.