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Homework Help: Electricity - i need help

  1. Dec 20, 2007 #1
    [SOLVED] Electricity - i need help

    hi! I need some assistance regarding my assignment. hope there's anyone who could help me out. the questions are:

    1.a. a 12 ohms resistor is connected in parallel with a series combination of resistors of 8 ohms and 16 ohms. if voltage drop across the 8 ohms resistor, is 48 volts. Determine the total impressed voltage and total current. Vt=? It=?

    1.b. Relevant equations
    V = IR; I=V/R

    1.c. The attempt at a solution
    so... first i have to get the total resistance to get the total current and voltage right? (Please correct me if i'm wrong. thanks! :D)

    R12 = 8ohms + 16ohms = 24ohms

    1/RT = 1/R12 + 1/R3
    1/RT = 1/24 + 1/12ohms
    1/RT = 1/8 ohms

    Rt = 8 ohms

    <i> here's the part where i'm totally confused </i>
    It = Vt/Rt
    It = 48Volts / 8 ohms
    It = 6 A

    Vt = 48? (i really dont know how to solve for the total voltage)

    2.a. 2 resistors 1.95ohms and 0.05ohms are connected in parallel and takes a total current of 50V. What is the current in each resistor?

    2.b. Relevant equations
    I1 = It

    I1 = V1/R1

    2.c. The attempt at a solution

    I1 = 50V / 1.95ohms
    I1 = 25.64 A

    I2 = 50V / 0.05ohms
    I2 = 1000 A
  2. jcsd
  3. Dec 20, 2007 #2


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    With regards to the first question all you need to do is apply Kirchoff's laws.
  4. Dec 20, 2007 #3


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    For part (a) you are giving the voltage across one resistor, that is in series with another resistor.. what can you say about the current through both resistors??
  5. Dec 20, 2007 #4
    first of all, i wanna say thank you for helping me out. but honestly, i'm really am having big problems when it comes to getting the total current or total voltage...

    with regards to your question... hmmm...

    current through the both resistor in question 1.a

    the one that is in series which is the 8 and 16 their current is 2A

    and the one that is in parallel which is 12 ohms is 4A

    i used the formula I = E/R

    whew.... i hope this is right...
  6. Dec 20, 2007 #5


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    No I don't think it is, sorry.

    Hootenanny mention Kirchoff Laws, one of them states that the sum of the currents entering a node MUST equal the sum of the currents exiting the node... so in your answer you're saying that the current through the series (8ohm and 16 ohm) part is 2A, yet in you're first attempt you said the current through the (8 Ohm) resistor was 6A.. hopefully you see something wrong with this!!

    If you wouldn't mind showing us how you calculated the 2A and 4A values. .

    Remember the 48V is supplied across the 8ohm resistor in the series, this is NOT the total voltage across the entire circuit!
  7. Dec 21, 2007 #6

    whew... just like what i posted before i'm really am having a hard time understanding this that's why i have to repeat this subject :frown:

    anywayz... here's how i calculated it

    48V / 12ohms = 4A

    :cry: i really cant solve it.... i dont know how i got the 2A anymore... sorry... i'm really am a slow learner when it comes to physics. its like the subject hates me. sorry... :frown:
  8. Dec 21, 2007 #7


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    Firstly, can you calculate the current through the 8ohm resistor and secondly, what do you know about the current through two resistors in series? [You must have learn this result in class]
  9. Dec 21, 2007 #8
    this is what i know...

    first i have to get the total resistance...
    so in series i just have to add them up. with parallel i add them like this 1/R1 + ...+1/Rn

    then the total current...
    which is It = Vt / Rt

    getting the current one by one is the one i really dont understand at all. Anywayz, i have passed my assignment already... so i cant undo it anymore... :frown:

    thanks for all the help... i tried to attempt to do what you've told me to do but i just dont understand it. i tried to check some in the net but it made me more confused...

    i just hope that my prof will just passed me by being a good kid. hahaha... since Christmas is near.
  10. Dec 21, 2007 #9


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    Passing the course isn't everything. Just encase you want to work through the problem, the current through both of the resistors will be the same.
  11. Dec 21, 2007 #10
    Draw the circuit and put the values in

    Code (Text):
     |                     |
    Work out the current through the 8Ω first.
    You are given the voltage (48V) that this uses. So your answer to I=V/R is the current through this resistor alone.

    Then you need to work out (maybe you already have this answer... in front of you) what current goes through the 16Ω resistor, one you have that you can work out the voltage it uses.
    Then you need figure out how to work out the voltage through the 12Ω... to get the current through the 12Ω resistor.
    Then lastley get total voltage and total current...
    (hint if you have 3 wires a,b,c all connected together at one point. Ia + Ib = Ic)
    Last edited: Dec 21, 2007
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