# Kirchoffs Laws and Thevinen Equivalent

• coldturkey
In summary, Kirchoffs Law is correct, but Thevinens Theorem is also correct. Thevinen's theorem is simpler to do, but Kirchoffs Law is more accurate.
coldturkey
I have a question here which asks me to find the Voltage and Current of RL in two different ways: using Kirchoffs Laws and then Thevinens Theorem.

I have tried it a couple times and get different answers using each method.

Perhaps somebody could tell me where I have gone wrong or if somebody that has access to multisim or some other circuit emulator could enter in the circuit and tell me what the answer is that would be a huge help.

Attached is my workings and a picture of the circuit.
P.s. sorry for the handwriting (its my quick working writing).

Many thanks

Your Vth is correct in the thevenin example, it is indeed 76 Volts. I'm guessing you messed up a sign somewhere in the Kirchoff example--- being the nice person I am I'll try to find it for you :D

Also I'll check your Thevenin Rth by using a short circuit current method.

Whoops nm i didnt know that they gave you the resistance of the load! With that in mind, with your Thevenin voltage you can easily find the current through the load.

For your Kirchoff example, I found your mistake--- although your method is on the right track, you would do well to make use of Mesh Currents... that is, instead of labeling i1, i2, i3 etc... you should label each in terms of mesh currents... that is--- i1 would equal the current ia thru first mesh minus ib current through second etc...

Do that, use matrices to solve your system of equations, and you should get it.

And might I add, don't use a supermesh in this problem!

Thanks heaps.
I knew one of them would be right, and the thevinen is the simpler one to do, but i couldn't see where I went wrong in the kirchhoffs one. I will have another crack. Cheers

Oh, did you put the circuit in multisim to make sure it is correct? Thanks

I don't have Pspice or any other relevant software installed on my laptop, and I'm not at home unfortunately!

I don't doubt that it's correct.

Check your EE dept's IEEE student branch or local EE student organizations to see if they hold Pspice, Hspice, Multisim, Matlab, etc tutorials, you'd definitely benefit from them when you take the harder courses in ckt theory and digital design!

I still can't find the error in my kirchhoffs one. Could you point me to the right part?

## 1. What are Kirchoff's Laws?

Kirchoff's Laws are fundamental principles in circuit analysis that describe the behavior of electric currents and voltages in electrical circuits. There are two main laws: Kirchoff's Current Law (KCL) and Kirchoff's Voltage Law (KVL).

## 2. What is Kirchoff's Current Law (KCL)?

Kirchoff's Current Law states that the sum of all currents entering a node in a circuit must equal the sum of all currents leaving that node. In other words, the total current flowing into a junction or node must equal the total current flowing out of that junction or node.

## 3. What is Kirchoff's Voltage Law (KVL)?

Kirchoff's Voltage Law states that the sum of all voltages around a closed loop in a circuit must equal zero. This means that the sum of the voltage drops (or rises) across each element in a closed loop must equal the sum of the voltage sources in that loop.

## 4. What is Thevenin's Theorem?

Thevenin's Theorem is a method for simplifying complex circuits into an equivalent circuit with only a single voltage source and a single resistor. It states that any linear, bilateral circuit can be replaced by a single voltage source in series with a single resistor, called Thevenin's equivalent circuit.

## 5. How do you calculate Thevenin's equivalent voltage (Vth) and resistance (Rth)?

To calculate Thevenin's equivalent voltage (Vth), you need to remove all sources from the original circuit and find the open-circuit voltage at the load terminals. To calculate Thevenin's equivalent resistance (Rth), you need to remove all sources from the original circuit and find the equivalent resistance at the load terminals. This can be done using different methods such as short-circuiting the load and calculating the total resistance, or using the equivalent resistance formula for parallel and series resistors.

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