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Series circuit resister help needed

  1. Sep 13, 2011 #1
    1. The problem statement, all variables and given/known data
    Here are pictures of the questions:

    Q1:
    http://i.imgur.com/3s9Ma.jpg


    Q2:
    http://i.imgur.com/0YChq.jpg


    3. The attempt at a solution
    Q1: As you can see, I have put down 1V above the 5V and 6V to the left. Is this right?

    Q2: I have no idea what I've done.
     
  2. jcsd
  3. Sep 13, 2011 #2

    berkeman

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    Staff: Mentor

    Sorry, the questions are not clear. What are you given, and what are you asked to find?

    Also, you seem to show places where there are different voltages on the top and bottom of a pair of parallel connected resistors. If two resistors are in parallel, they will have the same voltage across them.
     
  4. Sep 13, 2011 #3
    Right, sorry, I though it was clear enough that I have to find the voltages that were unknown. If the voltages of two parallel resisters are the same, then in the first picture, there are two parallel resisters, so they'll be 5V each, does that mean that the one I filled in 6V should be 1V?
     
  5. Sep 13, 2011 #4

    berkeman

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    Do all the resistors have the same value of resistance?
     
  6. Sep 13, 2011 #5
    Yes they do.
     
  7. Sep 13, 2011 #6

    berkeman

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    If they are all the same resistance value, and the configuration is one resistor in series with two parallel resistors, then you do not get 1V and 5V.

    The total will add up to the 6V source voltage that you show, but the voltages across the resistors are not 1V and 5V.

    What math are you using to calculate the voltage division?

    BTW, the two circuits look the same to me. Are they supposed to be different configurations?
     
  8. Sep 13, 2011 #7
    Oh yeah, I just realised I had put a duplicate up. Here's the actual Q2:

    http://i.imgur.com/nqPrY.jpg

    Well, my teacher said that the total voltage of the circuit is equal to the voltages of the separate resisters added up.
     
  9. Sep 13, 2011 #8

    berkeman

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    You are not getting the right answers for the resistor voltages. What equations are you using?

    The fundamental equation is V=IR. The voltage across parallel components is the same. The current through series components is the same.
     
  10. Sep 13, 2011 #9
    We're only using V=IR, the question ask to complete the voltage readings on the voltmeters. Oh, it's D.C. by the way, if that changes anything. So basically, I know that the voltage changes in a series circuit but I'm not sure what the volt readings should be. Also if two resisters are in a parallel circuit but in an overall series circuit, they have the same voltage readings right? So how does the Q1 circuit work out?
     
  11. Sep 13, 2011 #10

    berkeman

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    What is the overall resistance of two identical resistors in parallel?
     
  12. Sep 14, 2011 #11
    They're the same right?
     
  13. Sep 14, 2011 #12

    berkeman

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    I don't know what you mean by that statement.

    If you put two 100 Ohm resistors in parallel, what is the resulting combined resistance?
     
  14. Sep 14, 2011 #13
    200 Ohm, but what I don't get is, if you look at the first picture, the voltages of 2 resisters in parallel are the same, the current changes. So, if I label one of the parallel resisters 5V, then is it right?
     
  15. Sep 14, 2011 #14

    berkeman

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    No, not 200 Ohms. When you put resistors in series, their values add. So two 100 Ohm resistors in series make a total of 200 Ohms.

    What do you get when you put two 100 Ohm resistors in parallel, and why?
     
  16. Sep 14, 2011 #15
    To be honest, I don't know, that's kind of why I'm asking. I really need to understand the answers to the questions I'm asking.
     
  17. Sep 14, 2011 #16

    berkeman

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    It is strange that your teacher is not giving you the tools to figure out these questions.

    Please read this introductory page at wikipedia:

    http://en.wikipedia.org/wiki/Resistor

    Now you should be able to tell us what the value of two parallel 100 Ohm resistors is...
     
  18. Sep 14, 2011 #17
    Ok, it's a 100 Ohms. Right, so will the voltage be the same as well?
     
  19. Sep 14, 2011 #18

    berkeman

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    No. Two 100 Ohm resistors in parallel do not still measure 100 Ohms.

    What is the formula from the wikipedia page for the parallel combination of resistors? What do you get when you plug in 100 Ohms for the value of the two parallel resistors?
     
  20. Sep 14, 2011 #19
    Is it 1/Req = 1/R1 + 1/R2? If so, 1/Req = 1/100 + 1/100 which is 0.02.
     
  21. Sep 14, 2011 #20

    berkeman

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    So if 1/Req = 0.2, what does Req equal?

    And knowing what the parallel resistance is compared to the single resistor, what would you expect the voltage drops to be going around that circuit? They do need to add up to the total battery voltage applied, and the voltage drops will ratio with the effective resistances of each stage...
     
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