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Calculate the maximum current

  1. Sep 29, 2005 #1
    have looked thought all my notes and books and can not find the formal on how to do this question. I'm slowly going out of my mind any help would be great

    Calculate the maximum current a 140 ohms, 1/2 watt resistor can have flowing through it safely.
    Enter answer: mA

    Then the 2nd part of the question is

    What is the maximum e.m.f. which may be applied to the resistor above without causing overheating?

    Enter Part 2 answer: V
  2. jcsd
  3. Sep 29, 2005 #2


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    Do you know the relationship between ohms, amps, volts and watts? If so, using the formulas you can get it.
  4. Sep 29, 2005 #3

    P = I^2 * R

    I = sqrt(P/R) = sqrt(0.5/140) = 59.7 mA
  5. Oct 10, 2005 #4
    emf? I have not encounter this question before :confused:
  6. Oct 8, 2007 #5
    I have a question almost 100% identical to this one...
    I know the first part, but is part two simply E=IR ??

    I just have never seen e.m.f. being used to refer to voltage... and cant find any help anywhere on it
  7. Oct 8, 2007 #6
    emf stands for electromotive force. Same thing as voltage.

    In the first part it tells you what is the watt rating of the resistor. Power is determined by P = I^2 *R or P= V^2 / R or V=I*R.

    If you have the value of the resistor and the watt rating you can easily figure out the answer.

    PART 2: V or E is the variable to be found. Since we only have P and R let us use the relationship P = V^2 / R
    0.5W = V^2/ 140 ohms

    if the watt rating of a resistor is 0.5W that means the resistor will only be able to handle so much power until it burns out. So let us complete this problem.
    sqroot(0.5W*140 ohms) = V or E
    V or E = 8.366V

    So if you apply 8.366V, you are on the border line of the max voltage the resistor can take.( in reality a resistor's watt rating is given a 10-20% margin I believe).
    If you divide 8.366V by 140 ohms you get 59.76 mA which the maximum current the resistor can handle. How do you know this is the max current? well use P=V*I to check.
  8. Oct 8, 2007 #7
    thanks a lot for you help ^_^

    I only asked if I should use E = IR because the first part is calculating current, and once you have that voltage is easy

    just wish my textbook/teacher had told me that electromotive force is the same thing as voltage, haha
  9. Oct 8, 2007 #8
    True, you can use the current you calculated. But if you made a mistake in getting the current, the error would carry on.

    Always recheck your values to see if they satisfy the equations.
  10. Oct 10, 2007 #9
    They're trying to phase out the term. Voltage isn't a force so EMF is a misnomer.
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