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Homework Help: Help with a circuit and Ohm's law

  1. Feb 3, 2010 #1
    help with a circuit and Ohm's law!!

    1. The problem statement, all variables and given/known data
    ei1qgl.jpg

    A) What is voltage V at terminal A with respect to ground?

    B) Imagine that a wire is connected between terminals A and B. What is the current flowing through the battery?

    C) With the wire connected between terminals A and B, what is the current flowing through the 2 Ω resistor?

    D) Imagine that, instead of a wire, a 6Ω resistor is connected between terminals A and B. What is the current flowing through the battery?

    2. Relevant equations
    V=IR


    3. The attempt at a solution
    For part A, I know that the ground is the symbol at the bottom, but I do not know how to calculate the voltage.
    For part B, I thought the answer would be 5A because V=IR gives 10=2I so I is 5, but that is incorrect.
    For part C, I applied the same reasoning in B and that was wrong again.
    For part D, I got 10=6I, which would give 1.67 but that's wrong.

    Can someone please help me understand the concepts? I think that would provide a good start. I thought I just had to apply Ohm's Law but is there something more I must do?? Thanks.
     
  2. jcsd
  3. Feb 3, 2010 #2
    Re: help with a circuit and Ohm's law!!

    Hi magnifik

    A) Do you know the potential at B? The potential across AB is the same at the potential across 2 Ω resistor

    B) and C) When there is wire connecting A and B, the current won't pass 2 Ω resistor

    D) You have to find the total resistance and find the total current
     
  4. Feb 3, 2010 #3
    Re: help with a circuit and Ohm's law!!

    A) I'm not sure what you mean by "potential"
    B) and C) Does that mean that it would just be equal to the current passing through the 3 Ohm resistor??
    D) for total resistance, would this circuit be series-parallel? not sure how to find the total resistance.. voltage is still 10V?

    thanks for the help. i'm still a little lost though.
     
  5. Feb 3, 2010 #4
    Re: help with a circuit and Ohm's law!!

    A) still unsure
    B) since you said it doesn't pass through the 2 Ohm resistor, there is still the current from the 3 Ohm resistor, which is equal to 3.33
    C) the answer should be 0, right? since you said it won't pass through that resistor at all
    D) i found total resistance to be 4.5 and solving for I in Ohm's Law, I got 2.22 but then I tried just finding the current from each resistor, added it up and got 10. I don't know if either one is right??

    am i making progress??????
     
  6. Feb 3, 2010 #5
    Re: help with a circuit and Ohm's law!!

    Hi magnifik

    A) Potential means voltage. Because B is grounded, the potential at B will be ...
    B) and C) yes that's right. Do you know why?
    D) I = 2.22 A is right. You can't find the total current by adding all the current passing through the resistors because the current passing through 3 Ω is 2.22 A, which is the same as total current. The total current passing through 2 Ω and 6 Ω will also be 2.22 A

    Let's try another. Can you find the total current passing through the circuit if it only contains 3 Ω and 2 Ω in series?
     
  7. Feb 3, 2010 #6
    Re: help with a circuit and Ohm's law!!

    A) 0 volts at B? not sure how to get A
    B & C) i can't say i know why. :T
    d) yaaaay!

    thanks so much for all the help!!!!!!!!! (:
     
  8. Feb 4, 2010 #7
    Re: help with a circuit and Ohm's law!!

    Hi magnifik

    A) Yes, potential at B = 0. Now find potential of AB, which is the same as potential across 2 Ω resistor. Then, potential of AB = potential of A - potential of B

    B) and C) Because we consider wire has a very low resistance so the current passing through it will be very large. almost all of the current. In ideal case, we consider all current passes through it.
     
  9. Feb 5, 2010 #8
    Re: help with a circuit and Ohm's law!!

    resolved
     
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