Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AC dissipation

  1. Mar 18, 2010 #1
    This is probably a dumb question, but I only need to know for a program I would like to write.

    Basically, on average (I realize there are a lot of factors involved, but a rough estimate is all I need), how much energy dissipates over a specific length of a conductor (power lines, whatever they are made of)? (I don't know what an appropriate length would be, as I have no idea at all how far AC can travel. I have a vague idea of DC, but that is only because I am into networking.)

    Also, since W=V*I, and I'm assuming that I shouldn't decrease, then V is what decreases?

    Thank you.
  2. jcsd
  3. Mar 18, 2010 #2
    You would need the resistance of the conductor

    R = ρ * l / A

    Where ρ is the specific resistance of the material (1.72×10^-8 Ωm for copper), l is the length of the conductor and A is the cross-sectional area.

    This conductor will dissipate P = I²R watts if you send a current of I amperes through it. For time-varying current, replace I with the rms-value.
  4. Mar 18, 2010 #3
    Put this in google and read about transmission loss.
    " edison ac wiki "
  5. Mar 19, 2010 #4


    User Avatar
    Science Advisor

    The voltage drop in a cable can be found (approximately) with this equation:

    [tex]V = IRcos\phi + IXsin\phi [/tex]


    V is the voltage drop in the circuit (line to neutral)
    I is the current flowing in the conductor
    R is the line resistance for one conductor, in ohms
    X is the line reactance for one conductor, in ohms
    [itex]\phi [/itex] is the angle whose cosine is the load power factor

    This is the generally accepted approximate voltage drop formula from IEEE. There is an exact one but it's not usually necessary.

    Note that the formula gives line to neutral voltage drop. Hence, if you have a single phase system you'll multiply by 2 for the total voltage drop; if it is a three phase system you multiply by 1.73.

  6. Mar 19, 2010 #5
    There are also additional ac losses due to eddy currents generated in the interior of solid conductors (like power lines). See discussion on skin effect at


    Bob S
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook