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Electrical circuits confirmation

  1. May 12, 2014 #1
    1. The problem statement, all variables and given/known data
    Hi, I just need some confirmation on this question on whether it is correct or not.

    2. Relevant equations

    P= V^2/R.
    Based on that circuit I was asked to complete the following table and then find the power of the circuit.

    3. The attempt at a solution
    To find power I did the total voltage squared, divided by the total resistance. However, I was having second thoughts on the total resistance. Is it just all of them added together? Because I was taught that it was just the voltage divided by the current.

    P=3^2/10. P=9/10. P=0.9W. Is this correct?
    Last edited: May 12, 2014
  2. jcsd
  3. May 12, 2014 #2

    Here are the pictures.

    Attached Files:

  4. May 12, 2014 #3


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    Staff: Mentor

    You have to add all of the individual I².R terms (or E²/R or V.I products). You cannot simply sum all the resistor values!

    There is a check you can do, to confirm you have the right answer. Can you see it?
  5. May 12, 2014 #4
    Oh okay, so everything is alright then? And I am not sure about the check. I know that P=V^2/R as well as P=V(I). So to find the power for the whole circuit I used the total values and both equations gave me the same answer.
  6. May 12, 2014 #5


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    Staff: Mentor

    That will exactly equal the power drawn from the battery, 3.0x0.3 W (EDIT)

    You were not given the value of R3, but had to work it out?
    Last edited: May 12, 2014
  7. May 12, 2014 #6
    I'm really confused, sorry. :frown: Where did the 30 come from? And I worked it out by doing 0.48V/0.06A. Did I do that part wrong?
  8. May 12, 2014 #7


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    Staff: Mentor

    Oh, the decimal point went missing. That should be 3.0x 0.3 Watts. That's the power delivered by the battery. It equals the power dissipated by all the resistors.

    0.48V/0.06A is correct for R3.
  9. May 13, 2014 #8
    Okay, thank you!
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