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Homework Help: Finding resistance, electric engineering problem

  1. Nov 15, 2014 #1
    1. The problem statement, all variables and given/known data
    Hi there. I have to solve this problem, and I'm having some doubts.

    It is desired to build an electric engine of 0.9[HP], which will be connected to 220[V]. This will be done by a Cu wire with a cross section of 1.2 [mm²]. Supposing an efficency of 90% compute how long should the wire be.

    3. The attempt at a solution
    Well, I wasn't sure on how to consider the circuit for this. I thought of putting the engine with a resistor in series. Then I have:

    ##R=\rho_Cu \frac{L}{A}##

    I wanted some guidance for this, because I'm not totally sure on what to do. I think that the exercise can be donde in a variety of forms, I thought of using a resistance in parallel, but I don't know if that would be fine. I thought of considering one resistance corresponding to the wire that goes from the battery to the engine, and then another resistance corresponding to the wire going from the engine to the battery again, and that would complicate things too much I think, because the configuration of the series resistances would depend on how long is the wire in each part.
  2. jcsd
  3. Nov 16, 2014 #2


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    Staff: Mentor

    Hi Telemachus. I think I understand the question in general terms. But could you attach a jpeg of the question as it's presented on your assignment sheet?
  4. Nov 17, 2014 #3


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    Science Advisor
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    Gold Member

    I agree with NascentOxygen. Need to see the question as originally written.

    Are they asking you to calculate the length (resistance) of the wire used to connect the motor to the 220V supply OR the length (resistance) of the motor windings?
  5. Nov 17, 2014 #4
    Hi. I'm sorry, probably the problem is that my english is not that good. The original question is written in spanish. The resistance I'm asked for is the resistance of the wire used to connect the engine (and how long the wire should be, from the data provided by problem). I think I must use that the engine uses 90% of the energy disipated, which is 0.9HP, and the rest of the energy is disipated by the wire. But I wasn't sure on how to consider the circuit for this. I thought of consider the wire as a resistor in parallel with another resistor given by the engine. I know from the data how much energy each resistor disipates, and I could solve the problem that way. The other way I thought was to consider the wire and the engine in series, the thing is that if I close the circuit, I can have different lenghts for the wire 'before' and 'after' the engine. So I can have many different configurations for the circuit in series, which makes things more complicated, but I think that a model in series would better adjust to a real situation.
    Last edited: Nov 17, 2014
  6. Nov 17, 2014 #5


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    Staff: Mentor

    It makes no difference whether the connecting cable is before the motor or after it, or part before and part after, this is still a series connection. Does your question paper have a diagram of the arrangement? If not, then I expect the arrangement can be represented as two resistors in series and with 90% of the power going to one of these.
    Last edited: Nov 17, 2014
  7. Nov 17, 2014 #6
    Great. No, it has no diagram. But I see that your answer is right. After thinking of it, I wasnt having in mind something obvious, which is that series resistors are just added, so, it really doesnt matter if the wire is before or after the engine. Thanks.
    Last edited: Nov 17, 2014
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