# Kirchhoff's Law

Discussion in 'Electronics Homework Help' started by D.Pet, Oct 31, 2013.

1. ### D.Pet

4
0
Oct 31, 2013
Hi,
Im keen to learn more about electronic but struggling with this question. For the circuit shown in figure 3, calculate the voltage and current through resistor R2 and R3, using Kirchhoff's Laws. What is the power dissipated by R4.

I have attached a picture of a circuit, if anyone could help and explain that would be great File size:
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184
2. ### D.Pet

4
0
Oct 31, 2013
Hi,

For the circuit shown in figure 3, calculate the voltage and current through resistor R2 and R3, using Kirchhoff's Laws. What is the power dissipated by R4.

I have attached a picture of a circuit, if anyone could help and explain that would be great File size:
13.6 KB
Views:
183
3. ### Laplace

1,252
184
Apr 4, 2010
It is a four-step process: 1. Decide what analysis method to use. 2. Follow the rules for converting the circuit diagram into a set of algebraic equations. 3. Crank out the solution by solving the set of algebraic equations. 4. Use the solution to answer the other questions.

So have you decided whether to use the method of node voltages or the method of loop currents? Node voltages use the current law, whereas loop currents use the voltage law.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,387
2,772
Jan 21, 2010
Firstly, can you state what Kirchoff's laws are?

edit: and do you understand what they mean?

Secondly, can you see where you could apply them (even just one place) in this circuit?

25,387
2,772
Jan 21, 2010

6. ### D.Pet

4
0
Oct 31, 2013
Hi,

I was going to use Kirchhoff's voltage law, so far I've done this but unsure

V1+V2=I1(R1+R2+R3)+I2R2
V2=I1R2+I2(R2+R4+R5)

Then I was going to sub in values? Am I on the right track?

7. ### D.Pet

4
0
Oct 31, 2013
Sorry for posting twice but was unsure of where to post my apologies.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,387
2,772
Jan 21, 2010
OK, if you have learned superposition, this would be an ideal place to use it, but the question suggests otherwise.

You do have two equations, both of which have two unknowns. Both describe the sum of voltages around the loop in terms of the loop currents, resistances and voltage sources.

I find it useful to do it more literally, actually writing the parts of each equation as I go around the loops in a particular direction. You need to be very sure you get the signs of things correct. It is also probably helpful to draw the loops with an arrowed line. This will help you determine where there is a negative sign... Look especially a the direction of the current through R2 & V2

Why not have a go at solving them?

Once you have a solution, you can use the loop currents to determine all the voltages. If they add up correctly then you have checked your results and proven them correct.

9. ### Laplace

1,252
184
Apr 4, 2010
So I took your two equations and plugged them into a computer algebra system for a solution. Have attached a cropped screenshot of the whole thing. Note that other than typing your equations and component values once into the equation editor, all the rest was copy/paste and menu selections. While the solution to your equations yielded I1=1.818 mA and I2=-0.056 mA, how will you know whether the solution to your equations gives the correct answer to the problem?

#### Attached Files:

• ###### KVL_loops.png
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