KVL/KCL with two sources - can a loop have both sources?

In summary: Thanks.In summary, the conversation is about a circuit problem where the goal is to find the current in each resistor and the power dissipated in each resistor. The conversation discusses using equations such as V=IR, P=VI, and KVL and KCL to solve the problem. One person has a question about whether it is valid to have two voltages in an equation and if there is a rule to know when this is not allowed. Another person points out an error in the second equation and suggests including all three currents in the equation. The original poster realizes their mistake and thanks the others for their help.
  • #1
chopnhack
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3
forum question.jpg

Homework Statement


In the circuit shown, find (a) the current in each resistors (b)The power dissipated in each resistor.

Homework Equations


V=IR, P=VI, KVL and KCL

The Attempt at a Solution


See attached.

I was going to post this in the main section as its really a question about whether you can have two voltage sources in your equation, but I read the sticky on the main page for general physics and it directed all coursework type questions here.

In the drawing I have a section underlined. When I set up my equations, I got two voltages in the equation. I wanted to know if that is valid? If I should come across another equation like this, is there a rule for knowing not to do this? The solution to this problem is not the same as what I derived. Please give me some direction with this, thanks!
 
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  • #2
Sure, a given loop can contain zero or more sources. For KVL their potential changes sum just as you've done.
 
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  • #3
chopnhack said:
In the drawing I have a section underlined. When I set up my equations, I got two voltages in the equation. I wanted to know if that is valid? If I should come across another equation like this, is there a rule for knowing not to do this? The solution to this problem is not the same as what I derived. Please give me some direction with this, thanks!
Your second equation is incorrect.
There should be i2 instead of i1.
 
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  • #4
gneill said:
Sure, a given loop can contain zero or more sources. For KVL their potential changes sum just as you've done.
Thanks mate! BTW - if you haven't seen the finale... jumping the shark abit... Still a fan though :-)

cnh1995 said:
Your second equation is incorrect.
There should be i2 instead of i1.
Hi CNH, I think I see the error now. The 5I1 should really be just I1 times the 1 ohm resistor. That makes it all work.
Thanks gents!
 
  • #5
cnh1995 said:
Does it?

I1 flows through the 3 ohm resistance too. Plus, I2 also flows through the 3 ohm resistance. You should have all the three currents in that equation.
OP has 3 individual currents, each one for each branch of the circuit. He then uses a junction equation to relate them.
 
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  • #6
It was almost 12 am when I posted it and I was kinda sleepy:oops:. I guess I have to sleep now before I post anything stupider.
SammyS said:
OP has 3 individual currents, each one for each branch of the circuit. He then uses a junction equation to relate them.
Yes. I wasn't able to delete that because of network problems (I am traveling in a hilly area).
Fixed it now.
 
Last edited:

Related to KVL/KCL with two sources - can a loop have both sources?

1. Can a loop have both a voltage and current source?

Yes, a loop can have both a voltage and current source. This is known as a mixed source circuit.

2. How do you apply KVL/KCL with two sources in a loop?

To apply KVL (Kirchhoff's Voltage Law) in a loop with two sources, you must sum the voltage drops and rises around the loop, taking into account the direction of current flow. For KCL (Kirchhoff's Current Law), you must ensure that the sum of currents entering and exiting the node is equal.

3. Are there any special considerations when using KVL/KCL with two sources in a loop?

Yes, there are a few special considerations when using KVL/KCL with two sources in a loop. One is that the direction of current flow may need to be reversed for certain sources, such as a current source in parallel with a voltage source. Additionally, you may need to use superposition to analyze the circuit, as the presence of multiple sources can make it more complex.

4. Can you simplify a circuit with two sources using KVL/KCL?

Yes, you can simplify a circuit with two sources using KVL/KCL. By applying these laws, you can determine the voltage and current at various points in the circuit and use this information to simplify the circuit or solve for unknown values.

5. What happens if the values of the two sources in a loop are equal?

If the values of the two sources in a loop are equal, then they will cancel each other out and the circuit will behave as if there is only one source present. This is known as a source transformation and can be used to simplify the circuit and make it easier to analyze.

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