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!

Potential difference and ohms law

  1. Feb 8, 2004 #1
    A 3Ohm and a 1.5 ohm resistor are wired in paralles and the combination is wired in series to a 4 ohm resistor and a 10 V emf device. The potential difference on across the 3 ohm resistor is?

    I reduced the 2 in parrallel to 1.875 ohms and then addes the 4 ohm, which gave me a total resistance of 5.875 ohms. Then measured the current using ohms law, which gave me 1.7 amps. Then multiplied that times the 3 ohms to get 5.1 V.

    But that is not one of the answer choices, where did I go wrong.

    these are the choices: 2V,6V,8V,10V,12V

    Thanks
    Nautica
     
  2. jcsd
  3. Feb 8, 2004 #2

    jamesrc

    User Avatar
    Science Advisor
    Gold Member

    Wouldn't the equivalent resistance be 1? making the total resistance 5 ohms? Then you'd have a (10/5)*1 volt drop across the 2 resistors in parallel and a (10/5)*4 volt drop across the 4 ohm resistor.

    Look at the resistors in parallel again:

    [tex] R_{eq} = \left(\frac 1 3 + \frac 1 {1.5} \right)^{-1} = \left(\frac{1+2}{3}\right)^{-1} = 1 [/tex]
     
  4. Feb 8, 2004 #3
    nice, maybe I need to go back to 3rd grade math.
     
  5. Feb 8, 2004 #4
    I am still not coming up with the answer.

    If the total resistance is the 1 ohm, which i reduced, plus the 4 ohm in series. That gives me a total resistance of 5 ohms. Then I plug that into ohms law, which is V=IR and it gives me a total current of 2 amps.

    To find the potential difference on the 3 ohm, I would have to cut the current in half, which is 1 and then use ohms law, which is V=IR so 1 x 3, which gives me 3, but that is wrong.

    Nautica
     
  6. Feb 8, 2004 #5

    ShawnD

    User Avatar
    Science Advisor

    I'm getting the answer as being 2V


    Resistance of parallel things

    [tex]R = (3^-^1 + 1.5^-^1)^-^1[/tex]

    [tex]R = 1 \Omega[/tex]

    resistance total

    R = 1 + 4
    R = 5[tex]\Omega[/tex]

    total current:

    [tex]I = \frac{V}{R}[/tex]

    [tex]I = \frac{10}{5}[/tex]

    I = 2A

    voltage drop over the 4[tex]\Omega[/tex] resistor:
    V = IR
    V = (2)(4)
    V = 8V

    voltage remaining:
    V = 10 - 8
    V = 2V
     
  7. Feb 8, 2004 #6
    So how is that split between the remaining 3 and 1.5 ohm on the parrallel
     
  8. Feb 8, 2004 #7

    ShawnD

    User Avatar
    Science Advisor

    It isn't. When dealing with a parallel, the voltage is the same in each path. That's why hooking lightbulbs in parallel makes power consumption much higher than hooking them in series.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Potential difference and ohms law
  1. Ohm's Law and power (Replies: 1)

  2. Ohm's Law (Replies: 4)

  3. Ohms law (Replies: 1)

  4. Ohm's law (Replies: 3)

Loading...