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Homework Help: Circuit/Resistor Problems

  1. Apr 23, 2013 #1
    Hey guys so I'm new here but really need help with just with a confirmation on the answer on this question.

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

    Two resistors R1 and R2 are connected in series and these are connected in series to a battery. If 20% of the power coming from the battery is dissipated through the R1 resistor, determine the ratio R1/R2.

    2. Relevant equations

    R=V/I (just moved around Ohms Law)

    3. The attempt at a solution

    Well since the resistor is in a series you normally would add them together. Since you lose 20% of the power through the first resistor you have 80% left. So I'm not positive but i figured you would just divide 20/80 and you end up with 0.25. Thanks for the help I appreciate it.
    Last edited by a moderator: Apr 24, 2013
  2. jcsd
  3. Apr 23, 2013 #2


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    Science Advisor
    Gold Member

    Be careful with that reasoning. Power dissipation is VI = R I^2. Set up the equation for current in terms of R1 R2 and the voltage of the battery V. Write the ratio of powers expressed in terms of the resistances and the current and the value of V should cancel as should the currents. You'll get an expression for the ratio in terms of the Resistances R1 and R2. You can then solve that equation for R1 i.t.o. R2 or vice versa and work out their ratio. It may end up being as your intuition reasoned but you will be using fact and not intuition. (Or it may be completely different. Don't assume, Do The Math!)
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