# Analyzing Voltage Drops in Complex Circuits

• Defagment
In summary, Kirchhoff's circuit laws state that the sum of currents flowing through a parallel circuit is equal to the sum of the currents flowing through the individual circuit elements. The Attempt at a SolutionThe student is trying to solve a problem involving current and potential. They started by looking at the potential across each resistor in the circuit and came up with the equation I3 = I1 + I2. They are not sure if the equation is correct or how to find the third equation. They also tried a diagram but it didn't look right. Finally, they asked for help and received it.
Defagment

## Homework Statement

Calculate the drop in potential across each resistor:

## Homework Equations

V = IR
Kirchhoff's circuit laws

## The Attempt at a Solution

V1 + V2 has a current of I1 that goes through R1 and R3 which are in a parallel circuit. V3 has a current of I2. The two current combine so they go through R2 with I3 (I1 + I2)

So I came up with:

I3 = I1 + I2

(V1 + V2) - I1*[(1 / R1) + (1 / R2)]^-1 - R2*I3 = 0

I'm not sure if the above is right or how to find the third equation. Any help is appreciated.

A nice drawing always helps. Show the currents with arrows in the circuit diagram. ehild

This does not look right, no.

You should probably think about what the potential over your three top branches are (hint: you can directly read this potential on the diagram).

After that you only have the R2 branch left, which then consist of a simple circuit with two voltage sources (remember to get the sign of the voltage right here) and one resistance.

ehild said:
A nice drawing always helps. Show the currents with arrows in the circuit diagram.

ehild

I've tried that on a sheet of paper (which is how I came up with those 2 equations) but I still don't know what to do. =\

filiplarsen said:
This does not look right, no.

You should probably think about what the potential over your three top branches are (hint: you can directly read this potential on the diagram).

After that you only have the R2 branch left, which then consist of a simple circuit with two voltage sources (remember to get the sign of the voltage right here) and one resistance.

Sorry but I don't really understand this...the potential in the top three branches is V1 + V2? Not sure where to go from there.

Defagment said:
the potential in the top three branches is V1 + V2?

Look at the bottom branch with V3. What does this (ideal) voltage source do to the potential over each of the other three branches?

If you still have trouble answering this it may be that you are confused over the V1+V2 branch. If so, try think about V3 again but now assume that that V1+V2 is zero (ie. that it for a short while has been replaced by a piece of wire). Now you have one voltage source V3 and three resistors in parallel and you should be able to see what the potential over each of the three resistors is. If this still seems puzzling try simplify further by removing two of the resistors so you end up with V3 and one resistor.

filiplarsen said:
Look at the bottom branch with V3. What does this (ideal) voltage source do to the potential over each of the other three branches?

If you still have trouble answering this it may be that you are confused over the V1+V2 branch. If so, try think about V3 again but now assume that that V1+V2 is zero (ie. that it for a short while has been replaced by a piece of wire). Now you have one voltage source V3 and three resistors in parallel and you should be able to see what the potential over each of the three resistors is. If this still seems puzzling try simplify further by removing two of the resistors so you end up with V3 and one resistor.

Ah I think I get it now...so the V3 will counter-act the V1+V2 in the R1 and R3 branch. So the voltage drop in those two branches will be V1+V2-V3. And the voltage drop in the R2 branch would be V1+V2+V3?

Defagment said:
Ah I think I get it now...so the V3 will counter-act the V1+V2 in the R1 and R3 branch. So the voltage drop in those two branches will be V1+V2-V3.

No, that is not correct. The voltage over R1 and R3 (and also V1+V2+R2) is determined solely by V3. Almost by definition (of an ideal voltage source) will V3 force the potential difference over those two branches to be V3. No matter what components each branch contains the voltage over that branch will be fixed by V3. And since two of the branches only contains a single resistance, the voltage over those resistances are given directly as V3.

Defagment said:
And the voltage drop in the R2 branch would be V1+V2+V3?

This, however, is correct.

filiplarsen said:
No, that is not correct. The voltage over R1 and R3 (and also V1+V2+R2) is determined solely by V3. Almost by definition (of an ideal voltage source) will V3 force the potential difference over those two branches to be V3. No matter what components each branch contains the voltage over that branch will be fixed by V3. And since two of the branches only contains a single resistance, the voltage over those resistances are given directly as V3.

This, however, is correct.

Thanks for the help!

If it isn't too much trouble could you clarify why V3 will force the R1 and R3 branches to be V3?
I'm still having trouble understand why V1+V2 won't effect R1 and R3 =\.

Look at Kirchhoffs voltage law which says that the sum of the voltage difference (with sign) over each component in any loop in the circuit is zero. Now apply it to the loop that contains V3 and, say, R1. If we call the voltage from left to right over R1 for U1 (to avoid confusion with V1) and the voltage from left to right over V3 for V3, then Kirchhoffs voltage law for this loop, when we traverse it counter clock-wise on the diagram, says that V3 - U1 = 0 which is the same as saying U1 = V3.

You should note, that this only works so simple as it does in this case because V3 is assumed to be an ideal voltage source which keep the given voltage difference V3 no matter how much current (and therefore power) it has to deliver. This is obviously a theoretical construction as no real voltage source can truly provide limitless power. It is possible to make more realistic models of a voltage source (simple models just include a resistance in series) that as a result will make the voltage drop the more current you draw from it. To employ such a source in your circuit you would replace the V3 source with this more complicated sub-circuit so that you then can "include" into the analysis of the voltage-current behaviour of the complete circuit.

## What is a complex circuit?

A complex circuit is a circuit that contains multiple components, such as resistors, capacitors, and inductors, connected together in a series or parallel arrangement. These circuits can have multiple paths for current flow and can perform various functions, such as amplifying a signal or powering a device.

## Why is it important to evaluate complex circuits?

Evaluating complex circuits is important because it allows us to understand how the circuit works and determine if it is functioning properly. It also helps us identify any potential issues or areas for improvement in the circuit design.

## What methods can be used to evaluate complex circuits?

There are several methods that can be used to evaluate complex circuits, including simulation software, circuit analysis techniques, and physical testing with instruments such as multimeters and oscilloscopes. Each method has its own advantages and limitations, and the best approach may vary depending on the specific circuit being evaluated.

## What factors should be considered when evaluating a complex circuit?

When evaluating a complex circuit, it is important to consider factors such as the circuit's voltage, current, and power requirements, as well as its overall function and intended use. Other factors that may impact the evaluation process include the circuit's design, components used, and any external influences such as temperature or noise.

## What are some common issues that can arise when evaluating complex circuits?

Some common issues that can arise when evaluating complex circuits include incorrect component values, faulty connections or components, circuit instability, and inadequate power supply. It is important to carefully check and troubleshoot these potential issues to ensure accurate evaluation of the circuit.

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