Which solution is correct for finding Vth and Rth?

In summary, the conversation discusses two solutions for finding the values of Vth and Rth in a circuit. The first solution involves calculating Vth using 10A=I1+I2 and 4V+2xI1=2xI2, resulting in Vth=12V. The second solution involves transforming the voltage source into a current source and applying KCL, resulting in Vth=8V. The correct solution is the second one, although it is noted that the first solution is incorrect due to missing definitions and incorrect polarities. The conversation also mentions the possibility of applying a theorem three times for a more practical circuit.
  • #1
sciman
24
1
hello!
can you tell me please which solution is correct:

upload_2015-4-6_18-43-31.png


We need to find the Vth and Rth. Rth is easily 4Ohms, but what about Vth?

1st solution:
Calculate Vth, by doing 10A=I1+I2 and 4V+2xI1=2xI2, where you find I1=6A and I2=4A, so Vth=12V

2nd solution:
Calcullate Vth, by transforming the voltage source into current source and then apply KCL, so Vth=8V

which is the correct?thanks!
 
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  • #2
Perhaps, in this case, it's good to apply the theorem thrice, in 3 easy steps... - that would help to choose the values of the resistors if you're doing a real practical circuit.
 
  • #3
Hi sciman.

In future be sure to use the homework template when you post a problem here in the homework sections! It's in the rules.

I can see by the result that your first method is incorrect, but it is also incorrect because you haven't defined I1 and I2 so your equation doesn't mean anything!

Your second approach is much better, but you need to be careful about the polarities of things. What direction will the net current be flowing through the combined resistance?
 

Related to Which solution is correct for finding Vth and Rth?

1. How do I determine the correct solution for Vth and Rth?

The correct solution for Vth and Rth can be found by using Kirchhoff's voltage and current laws to analyze the circuit and solving for the voltage and resistance values at the open terminals.

2. Can I use any method to find Vth and Rth?

Yes, there are different methods that can be used to find Vth and Rth, such as the open-circuit voltage method, the short-circuit current method, and the superposition method. However, the most commonly used method is the open-circuit voltage method.

3. What does Vth and Rth represent?

Vth stands for Thevenin voltage, which is the voltage at the open terminals of a circuit. Rth stands for Thevenin resistance, which is the equivalent resistance between the open terminals of the circuit.

4. Do Vth and Rth change with different circuit configurations?

Yes, Vth and Rth can change depending on the circuit configuration. For example, if a resistor is added or removed from the circuit, the values of Vth and Rth will change accordingly.

5. Are Vth and Rth always necessary to find in a circuit?

No, Vth and Rth are only necessary if the circuit needs to be simplified for further analysis. In some cases, the circuit can be solved without finding Vth and Rth.

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