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Homework Help: How to calculate the voltage on a resistor at a power cable

  1. Sep 11, 2011 #1
    hallo everyone,

    i have here a simple problem that i cannot solve. This is the problem:

    A resistor of 46 Ω is connected to the end of a power cable.
    The voltage a the begin of the cable is 230 V.
    The length of the cable is 25 m
    The copper area is 1.0 mm2
    The specific resistance of copper is 0.0175.10-6 Ω.m
    What is the voltage on the resistor of 46 Ω [in V]?

    I calculted this by first determining the resistance of the cable and then adding(series) both the resistance of the cable and the resistor. After this i have calculated the current in the cable. With the current known the voltage of the resistor can easily be calculated using V=IR.

    This seems to be right to me but the answer is wrong. Does someone know what my mistake is and how this problem can be solved.

    thanks to everyone
  2. jcsd
  3. Sep 11, 2011 #2


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    Hi ronaldinho52 and welcome to PF. Your method seems to be OK. You probably went astray with the implementation. Can you show exactly what you did in more detail? We cannot help you without more details.
  4. Sep 11, 2011 #3
    Answer should be 227.833 Volts.
  5. Sep 11, 2011 #4
    So i first calculate the resistance of the cable:

    R(cable)=(0.0175x10e-6 x 25 m)/1e-6=0.4375 ohm
    Total resistance is R(cable)+R(resistor)=0.4375+46=46.4375
    Now the current in the serie can be calculated: 230/46.4375=4.953 amp

    So, the voltage across the resistance is 4.953 amp x46 ohm=227.83 Volts

    unfortunately this answer is wrong. This problem is part of a online homework system which i must work out every week. Maybe its an error in the system itself which gives an incorrect feedback.
  6. Sep 11, 2011 #5


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    I agree with your calculation and it seems that 1994Bhaskar does too. Perhaps you should ask the person who assigned this problem for clarification.
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