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Homewrk: electrical

  1. Nov 6, 2006 #1
    a cable with total resistance of 2 ohms used to transfer 15000W of electrical power with electrical potential of 10000 V.
    How much electrical energy is lose in the cable after transferring 2 hours of electrical power?

    ------------this is how my teacher taught me, but still confused----------
    P = VI
    15000 = 10000 I
    I = 1.5 A

    V = IR
    V = 1.5 * 2
    V = 3

    E = VIt
    E = 3 * 1.5 * (2 * 60 * 60)
    E = 32400 J

    What i don't understand is, since the question has provided me with 10000V, why should i find another voltage (1.5V)? It's impossible for a cable to have 2 completely different voltage (1.5V and 10000V) is it?
    My question is, what is 1.5V and 10000V?:yuck:
    Why cant i just use,
    E = Pt
    E = 15000 * (2 * 60 * 60)?

    HELP!!!!!!!!!!!!!!!!!! I know my English is not good enough... sorry for that.
  2. jcsd
  3. Nov 6, 2006 #2


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    The "1.5" is not a voltage, it is a current. 1.5 amperes (1.5 A) means that 1.5 coulombs of charge pass by a given point in the wire every second.

    You cannot use power alone to find transmission loss, because transmission loss depends only on current, not on power. If you use a higher voltage to transmit the same power, you will have smaller transmission losses.

    - Warren
  4. Nov 6, 2006 #3
    i'm sorry, erm... not 1.5, it's the 3V actually...(i'm confused!!)
    what is the voltage of 3 and 10000

    The question told me the voltage is 10000, y still need to use formula, V = IR to find the V which is 3V?

    What is the differences between the value of voltage provided by the question and the voltage get from the V = IR?

    My physic is terrible huh??...
    Last edited: Nov 6, 2006
  5. Nov 6, 2006 #4


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    The cable loses 3V over it's length. At one end, the voltage is 10,000V; at the other it's 9,997V.

    - Warren
  6. Nov 8, 2006 #5
    o... now I get it! thanks for your help...
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