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Calculate the current between point A and B

  1. Aug 13, 2015 #1
    1. The problem statement, all variables and given/known data

    Forum_Image_Handler_2.png
    What is the current between points A and B?
    The current I is unknown.
    The voltage is 12V.
    R1 = 3 Ohm
    R2 = 6 Ohm
    R3 = R4 = 4 Ohm

    2. Relevant equations
    U= R*I

    3. The attempt at a solution
    Total resistance from both branches:
    1/R = 1/(R_1+R_3) + 1/(R_2+R_4)
    1/R = 1/7 + 1/10
    R = 1/(1/7 + 1/10) = 1/17/70 = 70/17 Ohm

    Total current from both branches:
    I = U/R = 12/(70/17) = 72/35 A

    Here I get lost. Even If I can calculate the current that goes through each resistance (which I don't know how to do.) I don't know how I should determine how much current that goes from point A to B and/or vice versa.
     
  2. jcsd
  3. Aug 13, 2015 #2
    Edit : First - Explain why you took R1 and R3 in parallel , and why you took R2 and R4 in parallel .

    I noticed you had made a mistake . Hence the edit .
     
  4. Aug 13, 2015 #3
    Please note my edited post .
     
  5. Aug 13, 2015 #4
    How do I determine the current in either? Can I use that 1/R = 1/R1 + 1/R2
    Do you refer to this:

    1/R = 1/(R_1+R_3) + 1/(R_2+R_4)

    Is this wrong?
     
  6. Aug 13, 2015 #5
    Well , I'd rather you tried answering that yourself .

    When do you use the formula for parallel ( What is a parallel connection ) ?
    What is the potential difference between A and B ?
     
    Last edited: Aug 13, 2015
  7. Aug 13, 2015 #6
    calculate your TOTAL current

    Have two lots of parallel resistors

    1&2 R = 1/ (1/R1 + 1/R2)

    3&4 = 1/ (1/R3 + 1/R4)

    From this, you can work out the total resistance, and the total current.

    Once you have the total current, you should be on your way
     
  8. Aug 13, 2015 #7
    If you have two resistors in parallel , then current through either will be total current into the ratio of other resistor to total resistance , i.e. , if Ra and Rb are in parallel , ia = i*Rb/( Ra + Rb ) .
     
  9. Aug 13, 2015 #8

    SammyS

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    Yes, it's wrong.

    1/R = 1/(R_1+R_3) + 1/(R_2+R_4 is the result you get if R1 and R3 are in series, R2 and R4 are in series., and then the two series combinations are in parallel. That would be the case if there was no connection between points A and B.

    Why would you use 1/R = 1/R1 + 1/R2 ?
    (Yes, this is a correct step, but why?)​
     
  10. Aug 13, 2015 #9
    As you said there are a connection between the two branches. I wasn't sure if R1 and R3 were in series or not because of this, but now I understand.

    When I do 1/R = 1/R1 + 1/R2 I get that the resistance is 2 Ohm. The resistance for R3 and R4 is 2 Ohm also.

    Total resistance for the whole circuit is 2 + 2 = 4 Ohm
    Total current = 12 V / 4 Ohm = 3 A

    Okay so the total current is 3 A. I have no clue how to go from here.
     
  11. Aug 13, 2015 #10
     
  12. Aug 13, 2015 #11

    SammyS

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    How much voltage is dropped across the R1 , R2 combination?

    How much voltage is dropped across the R3 , R4 combination?
     
  13. Aug 13, 2015 #12
    Both the R1 , R2 combination and the R3 , R4 combination has 12V, correct?

    If so, then according to Qwertywerty

    I can use the ratio between R1 and R2 and then use that same ratio with the voltage.
    So we have a ratio of 2:1. If my theory is correct, the voltage of R1 should be 4 and R2 should be 8. Is this correct so far?
     
  14. Aug 13, 2015 #13

    SammyS

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    Not correct.

    You have 3 A passing through 2 Ω in both combinations.

    How much voltage is dropped across the R1 , R2 combination ?
     
  15. Aug 13, 2015 #14
    We have U=R*I and across the R1 , R2 combination we have, as you said, 3 A passing through 2 Ω.

    The voltage should be 3*2 = 6V in both combinations since the combinations have equal resistance.

    Can I figure out the voltage for each resistance now with this information?
     
  16. Aug 13, 2015 #15

    SammyS

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    I hope so.

    If you know (find) the current through R1 and the current through R3, can you get the final answer ?
     
  17. Aug 13, 2015 #16
    You should be using the term potential difference / drop instead of voltage ; beacause voltage doesn't really make sense .
     
  18. Aug 13, 2015 #17

    SammyS

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    Picky - picky !
    It's very common usage.
     
  19. Aug 13, 2015 #18
    If you say so . Although I didn't get this -
    What voltage is being referred to here ? This seemed a bit confusing as the value 6V had already been found out .
     
  20. Aug 13, 2015 #19
    Yes I believe so. The difference in current between R1 and R3 should be the current that goes through A and B.

    I'm not quite sure about the voltage of R1 and R3 though. If I understand correctly R1 has 4V and R2 has 2V?
    R3 3V and R4 3V.

    If this is correct then R1 has the current 4/3 A and R3 3/4 A
     
  21. Aug 13, 2015 #20

    SammyS

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    Well, the word "voltage" was perfectly OK. However, OP should have included the word "drop" to to make it a "voltage drop" across each resistor.
     
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