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Find resistance and power

  1. Dec 19, 2012 #1
    1. The problem statement, all variables and given/known data

    resistance_zpsabfe4bea.jpg

    2. Relevant equations

    current = (emf)/(resistance)

    power = (Capacitance)(Voltage)/ (2)

    3. The attempt at a solution

    i might be misunderstanding the question but when in part A when it says 5V across it, does it mean if the emf was 5 V instead of the given 12 V?

    current = (5V)/(270 ohm)
    current = .04 Amperes

    current is same in a capacitors

    .04 = 5V(Resistance_2)
    Resistance = 120 ohms
     
  2. jcsd
  3. Dec 19, 2012 #2

    SammyS

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    No. It does not mean that you replace the 12V with 5V.

    Do you know Kirchhoff's Circuit Laws?

    In this series circuit, the sum of the voltage drops across the resistors is equal to the emf of the battery.

    According to Ohm's Law, the voltage drop across a resistor of resistance, R, which has current, I, flowing through it is I∙R .
     
  4. Dec 19, 2012 #3
    charges gain energy at emf and lose energy in resistors. what you stated previously would mean the voltage drop is 12 V. i thought you would only use kirchhoff's law if there is more than one emf or if the circuit is connected in a complex way

    let see at emf there is a gain of 12 V. part a says 5 V across so thats a 5 V voltage drop in R_2. that would also mean a 7 V drop in R_1. since a gain of 12 V added with (-12 V drop) must sum to zero based on the kirchhoff's law
     
    Last edited: Dec 19, 2012
  5. Dec 19, 2012 #4

    SammyS

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    A voltage drop of 5V across R2 means there is a voltage drop of 7V across R1 because the source emf is 12V .
     
  6. Dec 20, 2012 #5
    yes that is right. apparently I need to draw a CW(Clock wise loop to represent the state of emf) in this region of the circuit

    i noticed I also need to find power dissipation which are in joules. which would mean I cannot used the stored energy equation. how would I go about finding power dissipation?
     
    Last edited: Dec 20, 2012
  7. Dec 20, 2012 #6

    SammyS

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    Do you have a textbook or notes? This is very basic stuff.
     
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