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Find current in circuit

  1. Aug 27, 2015 #1
    How do I find the current across the 5Ω resistor?

    I worked out the supply current by simplifying the circuit down.
    I then went on to work out the voltage across the 3Ω resistor using the supply current.
    I then used that voltage to then work out the voltage across the 4Ω resistor.

    Now I'm stuck. How do I work out the voltages of the other 3 resistors and get the current across the 5Ω resistor?
     

    Attached Files:

  2. jcsd
  3. Aug 27, 2015 #2

    donpacino

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    do you know how current division work?
    or you can use ohms law to find the voltage drop across the first resistor
     
  4. Aug 27, 2015 #3
    I don't know how current division work, and what do you mean by using ohms law to find find the voltage across the first resistor?
     
  5. Aug 27, 2015 #4

    donpacino

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    you know the current through the 3 ohm resistor, so you should be able to calculate the voltage drop across it. then use the supply voltage to figure out the voltage across the 5 ohm resistor.
     
  6. Aug 27, 2015 #5
    I calculated the voltage across the 3Ω resistor which I show in the last picture. After I worked out the voltage across the 3Ω resistor, I subtracted it from the supply voltage to work out the 4Ω resistor.

    Can you explain how I work out the current and voltages for the rest of the resistors?
     
  7. Aug 27, 2015 #6

    donpacino

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    if you know the voltage across the 4 ohm resistor, then you know the voltage across the 5 ohm resistor. you also know the voltage across the series combination of the 2 and 1 ohm resistors.
     
  8. Aug 27, 2015 #7
    I've just started my engineering course in and I've not did this kind of stuff for a long time. Can you tell me the voltage across 5Ω resistor so I can understand what you mean.
     
  9. Aug 27, 2015 #8

    donpacino

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    the 5 ohm resistor and the four ohm resistor are in parallel....
     
  10. Aug 27, 2015 #9
    So the voltage across the 4Ω resistor is the same as the 5Ω resistor?
     
  11. Aug 27, 2015 #10

    donpacino

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    yes, due to the fact that they are in paralell.
    you made an error in your work

    edit: disregaurd what i said , you skipped a step in your work and i thought u made an error
     
  12. Aug 27, 2015 #11
    I'm sorry for going on and maybe I'm asking stupid questions, but I really need to know this

    I don't understand what you mean when you say that I've made a calculation error. Where did I do this? Which picture shows this?

    I know that after working out the voltage of the 5Ω resistor, it allows me to .work out the current and that's the question answered, but can you please explain how I then work out the voltages and currents of the 2Ω and 1Ω resistors.
     
  13. Aug 27, 2015 #12

    donpacino

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    thats fine, no need to apologize....

    you did not make an error. I thought you added 1/Rt and 3 ohm.

    so the 2 and 1 ohm resistors are in parallel with the 5 ohm resistor. SO you can either use voltage division, or use the same process you did before
     
  14. Aug 27, 2015 #13
    How do I do voltage division?
     
  15. Aug 27, 2015 #14

    donpacino

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    https://en.wikipedia.org/wiki/Voltage_divider

    its a shortcut for finding the voltage across individual resistors in a series chain.

    lets say you have 2 resistors R1 and R2 in series and a voltage Vin across them

    the current is Vin/(R1+R2)
    then to get the voltage across either of the resistors, you multiply the current by the resistance.

    so Vr1=Vin*R1/(R1+R2)

    the same goes for R2.
    make sense?
     
  16. Aug 27, 2015 #15

    donpacino

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    current division, which I talked about earlier, can be derived in a similar way...

    two resistors R1 and R2 in parralel with current Iin going into them

    the voltage is
    V=Iin*R1*R2/(R1+R2)

    then voltage over resistance gives you current

    so IR1=Iin*R2/(R1+R2)
     
  17. Aug 27, 2015 #16
    Okay, maybe a dumb question, but I understand voltage division now, but what would be the value of Vin for the 1Ω and 2 Ω resistor?
     
  18. Aug 27, 2015 #17
    tip

    draw circuits using only horizontal and vertical lines.


    another tip

    draw circuits going left to right (rather than as a loop) (this is a very common, commonsense way to draw circuits)

    start on the left with the positive terminal of the battery. imagine walking along the wire. thats a straight line. when you get to a junction you can turn left, right, or keep going. draw like this until you get to the negative terminal of the battery which is on the far right of your page. Now, what is in parallel and what is in series very clearly defined. This method is very useful as circuits become more complex and it is essential to know what is in parallel with what just by glancing at the diagram.


    this way you can get a much clearer picture of what is going on, something like this, but draw it for yourself.

    when it is like this you can quikly work out the total current; and the current flowing through each resistor
     

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  19. Aug 27, 2015 #18

    donpacino

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    vin is the voltage across the two resistors.... so what is the voltage across the two resistors?
     
  20. Aug 27, 2015 #19
    Is the voltage across the two resistors 42.09v? or is it just 60v like the supply voltage. i dont really know
     
  21. Aug 27, 2015 #20

    donpacino

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