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Homework Help: Solving Umax with Pmax and Ohms

  1. Aug 20, 2013 #1
    Brief personal intro, I have last time studied physics about 12 years ago.

    I am now starting in 2 weeks time in a university and there is mandatory physics course so I am trying to relearn and learn as much as I can before that time.

    For that reason I am trying to solve text book tasks from my old physics books.

    I have never been good at maths nor physics atleast not calculations. Thus I appreciate thorough-Step by Step explanations so I can understand the solution.

    Thank you for your time

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

    How large voltage U , can be attached to the circuit , when both the resistance have an Pmax of 0,25 W ?

    R1 = 120Ω

    They are in series.

    Answer is :
    Umax = 11

    2. Relevant equations

    P = U * I
    U = R * I

    3. The attempt at a solution

    0,25*120= 0,002083333333
    0,25*180= 0,003888888889

    0,002083333333 + 0,003888888889=0,00347222222

    0,00347222222*120 = 0,416666664
    0,00347222222*180 = 0,624999996

    4,16666664+6,24999996= 10,41666666
  2. jcsd
  3. Aug 20, 2013 #2
    Since they're in series, they have the same current through them. If you start with a low current and slowly ramp it up, do you know which of the resistors will reach its thermal limit first?
  4. Aug 21, 2013 #3
    No , it is not mentioned on the task nor which direction the current will go
  5. Aug 21, 2013 #4
    Just assume that you can control the current through the resistors, I promise you that I'll get to the point shortly :smile:

    Say you have a current of 1 A through the resistors. How much power is then dissipated in each resistor?
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