1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to find the cell voltage of this circuit?

  1. Jan 13, 2017 #1
    1. The problem statement, all variables and given/known data
    ws3VciU.png

    Find the cell voltage, the current at the 15 OHM resistor is 2A.

    2. Relevant equations
    How would I solve to find the cell voltage?

    3. The attempt at a solution
    I tried to find the voltage of the top part of the circuit but I just cannot remember how you would do that or if it would even result in the solution.
     
  2. jcsd
  3. Jan 13, 2017 #2

    cnh1995

    User Avatar
    Homework Helper

    Do you know Ohm's law? Are you familiar with series and parallel connections? Using the given information, what is the voltage across the 15 ohm resistor?
     
  4. Jan 13, 2017 #3
    Yes I am familiar with it, the voltage is 30V.
     
  5. Jan 13, 2017 #4
    Just take it step by step....
    If you know the current in the 15 ohm, this will give you the current in the 10 ohm ... from this the current in the 14 ohm ... hence the voltage across all three ... which is the voltage across the 3,4 and 4 ohm ....and so on ...
     
  6. Jan 13, 2017 #5

    cnh1995

    User Avatar
    Homework Helper

    Right. So, what is the current through the 10 ohm resistor?
    Using that, you can find the voltage across the the network containing 14 ohm, 10 ohm and 15 ohm resistors.
    Also, you need to simplify the network of 3 ohm and 4 ohm resistors in order to find the current through it.
     
  7. Jan 13, 2017 #6
    The current at 10 ohms would be 3A, I had figured this part out. What I can't figure out is how I would simplify and find the current through the 3 and 4 resistors. The current at 14 OHMS should be 5A?
     
  8. Jan 13, 2017 #7

    cnh1995

    User Avatar
    Homework Helper

    Yes. So, what is the voltage across the 14, 10 and 15 ohm network?
    What is the voltage across the 3, 4 and 4 ohm network?
    Current through the network would be equal to voltage across the network/equivalent resistance of the network.
    What is the equivalent resistance of the 3,4 and 4 ohm network? Can you simplify it using series-parallel reduction technique?
     
  9. Jan 13, 2017 #8
    I don't know how to find the voltage across the 3, 4 and 4 ohm network.
    The equivalent resistance of 3,4 and 4 ohms should be 5 ohms?
     
  10. Jan 13, 2017 #9

    cnh1995

    User Avatar
    Homework Helper

    Yes.
     
  11. Jan 13, 2017 #10
    Okay so the voltage at the 14 OHMS should be 70V (V=IR) 70=(14)(5). From what I know, 3 OHMS and 14 OHMS are parallel so they'll have the same voltage? Therefore 70/3 should be 23.3A, that's the total current across the 3,4 and 4 ohm network? So 23.3A + 5A = 28.3A. Next to find the total cell voltage I do V=(28.3)(9) which equals to 255V. Now something in my work there is wrong as the correct answer in my textbook is 225V. I've tried to figure out what I did incorrect but I just can't seem to put a finger on it. It's now 4am and i've been on this single question for over an hour.
     
  12. Jan 13, 2017 #11

    cnh1995

    User Avatar
    Homework Helper

    They are not.
    Right. So what is the voltage across the "entire network of 14, 10 and 15 ohm resistors"?
    That will also be the voltage across the "entire network" of 3, 3 and 4 ohm resistors since these two "networks" are in parallel and not individual resistors.
    I am getting the same answer.
     
  13. Jan 13, 2017 #12
    Shouldn't the voltage across that entire network be equal to 70V as well?
     
  14. Jan 13, 2017 #13

    cnh1995

    User Avatar
    Homework Helper

    What about the voltage across the 15 ohm resistor you calculated earlier?
     
  15. Jan 13, 2017 #14
    70+30=100 Is it 100V?
     
  16. Jan 13, 2017 #15

    cnh1995

    User Avatar
    Homework Helper

    Yes.
     
  17. Jan 13, 2017 #16
    I understand, I had previously thought that the 15 and 10 OHM resistor had the same voltage as 14 OHMS but I was wrong. I understand that you need to find the total resistance which is 100 across the whole network. Then you divide 100 on the bottom by 20 OHMS and the top network you divide the 100 by 5 ohms which gives you a total of 25A. V=(25)(9) --> V=225v

    Thank you so much for your help, you really helped me understand the concept I owe you one!
     
  18. Jan 13, 2017 #17

    cnh1995

    User Avatar
    Homework Helper

    Right.
    You can also solve it by inspection only. Current through the lower network is 5A and voltage across both the networks is 100V. Knowing the equivalent resistance of the upper network as 5 ohm, current through the upper network will be 100/5=20A.
    So, total current will be 25A which is nothing but the current through the 5 ohm resistor on the left. So, voltage across that 5 ohm resistor is 125V. So the cell voltage will be 125+100=225V.
    You are welcome!:smile:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: How to find the cell voltage of this circuit?
  1. Finding voltages (Replies: 1)

Loading...