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Homework Help: Electric Circuit Help (Emf and Internal Resistance)

  1. Nov 22, 2007 #1
    1. Find the current in, and the potential difference across, each resistor in the circuit shown below:

    The circuit is a simple parallel circuit with 2.0 ohms at the first resistor, 10.0 ohms at the second, and 20.0 ohms at the third

    The problem also gives you Emf which is 12.0 V and the internal resistance "r" which is 0 ohms




    2. Voltage = Current*Resistance
    Possibly: Voltage = Emf - Current*Resistance
    Req (total resistance) = [(1/Resistance 1) + (1/Resistance 2) + (1/Resistance 3)]-1




    3. I would attempt the problem but I've tried so many times and I'm just lost. I was absent the day this question was answered in class.

    I thought you were supposed to find the Req first then use the voltage given to find the current in the system, but thats not helping me at all. I don't understand Emf or internal resistance so I'm not sure of its purpose.


    I hope this helps. I'm on a laptop so I can't scan any images at all.
     
  2. jcsd
  3. Nov 22, 2007 #2

    stewartcs

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    Science Advisor

    In parallel circuits the voltage through each branch is the same. Use Ohm's Law to find the current.
     
  4. Nov 23, 2007 #3
    I don't believe that's correct.

    [tex]\frac{1}{R_{eq}}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_2}[/tex]

    [tex]R_{eq}=\frac{R_1 R_2 R_3}{R_1 R_2 + R_1 R_3 + R_2 R_3}[/tex]

    This isn't really needed to find the current through each resistor.

    Ohm's law, applied in this case:

    [tex]V = I_1 R_1 = I_2 R_2 = I_3 R_3[/tex]

    Therefore...

    [tex]I_1=\frac{V}{R_1}[/tex]
    [tex]I_2=\frac{V}{R_2}[/tex]
    [tex]I_3=\frac{V}{R_3}[/tex]

    You can use [tex]R_{eq}[/tex] to see if you got it right...

    [tex]V=I R_{eq}[/tex]
    [tex]I=\frac{V}{R_{eq}}=V\frac{R_1 R_2 + R_1 R_3 + R_2 R_3}{R_1 R_2 R_3}[/tex]

    and...

    [tex]I = I_1 + I_2 + I_3[/tex]
     
    Last edited: Nov 23, 2007
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