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Homework Help: Calculate the voltage drop of the train (Picture included!)

  1. Mar 24, 2012 #1
    Hello guys, so I had a homework and I couldn't understand the point of my teacher. The question goes like this:

    http://desmond.imageshack.us/Himg3/scaled.php?server=3&filename=21976607.jpg&res=medium [Broken]

    So the question is,
    "Calculate the voltage drop over the TRAIN when it is at the position indicated"

    Alright so he asked for the voltage that goes to the train, therefore,

    R=The resistor the current will go through before it hits the train which is 0.140

    V=300*0.140 ----> V=42
    So the Voltage that will go through the train is going to be 1500-42 ------> v=1458

    My teacher said it's wrong, I should calculate the total resistance of the circle which is 0.140+0.044 and then multiply by the current, but I don't understand! the question asked about the resistance over the train only not over the whole circle, anyone can help?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Mar 24, 2012 #2


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    Staff: Mentor

    I think you mean that he asked for the voltage over the train, that is, the voltage someone on the train would measure if he had a meter and measured from the "top" of the train (overhead wire) to the "bottom" of the train (track rail). That is the potential drop across the train. It happens to be what's left of the total potential (1500V) when all the drops due to other resistances in the whole circuit path are accounted for. Your teacher is correct.
    Last edited by a moderator: May 5, 2017
  4. Mar 24, 2012 #3


    Thank you for your help, now I understand it

    If we calculate the total resistance of the whole circle, it will be

    R total=V/I ----> R total =1500/300 ----> R total =5
    Now by taking out the resistor 0.140 and the resistor 0.044 from the total resistance, it will be 4.816, this is the resistance of the engine of the train, now calculating the Voltage
    V=IR -----> V= 4.816 * 300 ------> Voltage drop over the train engine is going to be 1444.8 which is the same answer as my teacher gave me

    Thank you very much for your help!
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