# Find The Voltage (Vab)

Discussion in 'Homework Help' started by joe809, Mar 5, 2018.

1. ### joe809 Thread Starter Member

Mar 4, 2018
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So, The circuit I have to work on has two voltage source. I have to find the voltage across a and b. The problem is that I am not sure how to start with a circuit like this. I have to do this only using KCL, KVl or Ohms law. No nodal or mesh analysis. In the picture I reduced the circuit to find the total resistance which is 1.79 ohms. I'm stuck at this part and don't even know if reducing the circuit is correct.

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2. ### joe809 Thread Starter Member

Mar 4, 2018
32
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I though doing the voltage divider, [4/(6+4)]x10 = 4, would give me the correct answer for Vab, but that is not correct. I have to get 4.545V

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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1. 4 ohm and 6 ohm are not in series, therefore voltage divider is not applicable. which you found out the hard way.
2. kcl and kvl are other names for node-voltage analysis and mesh-current analysis. therefore you statement that you are not allowed to use nodal or mesh analysis is false.

4. ### joe809 Thread Starter Member

Mar 4, 2018
32
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we have not reach the topic of mesh and nodal analysis in class. We were told not to use those methods to solve this. I don't know what other way of doing this and that is why I would like to know if it is possible.

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Is Superposition allowed?

6. ### joe809 Thread Starter Member

Mar 4, 2018
32
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Yeah, that's what I did by eliminating the 10 volts.

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
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The next step is to keep 10 volt and eliminate the 15 volt supply.

8. ### anhnha Well-Known Member

Apr 19, 2012
866
61

Hi joe809,

Is there any current flowing through the middle branch?
If you ignore the middle branch, can you find the voltage across resistor R5?

9. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
There is no current given. Not sure if we have to find the current for R4 or that just equals to 0.
15V/(4+7)= 1.364A
4(1.364) = 5.456 V
10V-5.456V= 4.544V I assume this is Vab?

10. ### anhnha Well-Known Member

Apr 19, 2012
866
61
Hi,

There is no closed path for current flowing through R4 so the current has to be zero.
You correctly found the voltage across R5 resistor.
To find the voltage Vab, you just need to apply KVL for the loop on the left including the voltage source V3, R4 and R5.

You have it wrong sign, just apply KVL to the loop I said above and do it carefully you will get the correct result.

Last edited: Mar 5, 2018
11. ### anhnha Well-Known Member

Apr 19, 2012
866
61
Hi,

You have it wrong sign as I said above.

Apply KVL to the left loop:

Vab = -10 + VR4 + VR5

VR4 = 0 due to no current, VR5 is as you calculated above.
So Vab = -4.544V

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12. ### joe809 Thread Starter Member

Mar 4, 2018
32
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Yes, I am sorry. It is supposed to be a negative sign. Can you please tell me how can I get the current through a and b if there is a short circuit there?

13. ### anhnha Well-Known Member

Apr 19, 2012
866
61
You just need to apply KCL to the top node, let's call it Vx now.

KCL for the Vx node: I3 + I4 + I5 =0.
Now you just need to express I3, I4, I5 in terms of Vx and other parameters and solve for Vx and then all currents.

14. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
That's what I don't understand. I don't have any values for I4, I5, I6 and Vx. I don't know what to do. Isn't I5 the same from before, 1.364A?
I4 is still 0?

15. ### MrAl AAC Fanatic!

Jun 17, 2014
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Hi,

Isnt that then applied to the right loop?

anhnha likes this.
16. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
That's what I don't understand. I can't get Vx if I don't have any values. I thought that Vab doesn't exist if it is in short circuit.

17. ### anhnha Well-Known Member

Apr 19, 2012
866
61
Ah, yes. I made a mistake. It should be right loop.

18. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
I don't understand. What do I need to calculate Vx? I already got Vab = - 4.544V and VR5 = 5.456V.

19. ### anhnha Well-Known Member

Apr 19, 2012
866
61
That answer is correct when ab is not shorted. When you shorten a and b, you want to calculate the current so said how to do it by using Vx.

20. ### joe809 Thread Starter Member

Mar 4, 2018
32
0
That's the issue I have. If I can't get Vx I can't get the current. I have no idea how to get Vx.