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Homework Help: AC Current

  1. Nov 21, 2011 #1
    1. Question

    DC power can be transmitted using a wire over distances of a couple miles. On the contrary, AC power may be transmitted using a wire over much greater distances.

    a) What physical quantity needs to be minimized to maximize the reach of wired power transmission from a power station to a final user (house, factory, etc.)?

    b) What physical quantity was accordingly changed at the AC power station?

    c) Why would have this change not be acceptable for the final user at the end of the transmission line if the power station was instead transferring DC power?

    d) Any lamp powered in DC produces a yellow light, but produces a white light when powered in AC. This occurs although the average power (the average rate at which energy is dissipated by the heated filament) is exactly the same in the two cases. Why? (This requires some additional investigation on your part on what property confers a hot body a certain color.)

    2. Relevant equations

    Ohm's Law: I = V/R
    Resistance = pL/A

    3. Attempt at solution

    a) the physical quantity that needs to be minimized is the resistance.

    b) Voltage

    c) DC current can not be bumped or down in strength like AC current. DC Current loses power quickly because it travels only in one direction.

    d) the difference in color is due to the electrons, in the material heated, being excited and releasing a certain wavelength.

    Any input/corrections/help?
    Last edited: Nov 21, 2011
  2. jcsd
  3. Nov 21, 2011 #2


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    RE: a)

    Power loss is given by P = VI or I2R or V2/R Where:

    I = current flowing
    R = Resistance of connecting wires
    V = Voltage DROP along the way [not the supply voltage, not the PD across the house at the end]

    It is generally acknowledged that we wish to minimise I in power transmission situations - though we certainly want R to be as small as possible, but given that the wires from the generating plant to your house may by 200km long, there is a limit to how low the resistance can be..
  4. Nov 21, 2011 #3


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    Don't know if it's any help jlax31, but it's kind of interesting...


  5. Nov 21, 2011 #4
    Thank you PeterO.

    I'm still wondering about the answer to d). Is it because DC current doesn't provide a constant potential and is therefore less efficient than a constant current? Or is it because of the calcium inside the light bulbs that some how react with dc current..?
  6. Nov 22, 2011 #5


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    No idea really, however it is the DC that provides the constant current, and the AC that alternates. Perhaps it is the peak currents that cause peak temperatures - certainly the colour of AC lights changes with time.

    If you look at some street lamps through binoculars [great of you can look down on a city from an apartment] and swirl the binoculars, the basic image is very hard to see, but you see light trails. The purpose of the binoculars it to give you tunnel vision and it enables you to "swirl the image" without moving your head - just swinging the binoculars. A telescope should work too, but more people have binoculars that have telescopes, I find. You can see some of the trails look like dotted lines. The dotted trails are due to AC electricity supply. Great if you can see street lights and car lights at the same time, as cars run on a constant 12-14V DC. Fluorescent tubes are great for watching the flashing.
  7. Nov 22, 2011 #6
    Browsing the web here is what I found:

    "The color of light has nothing to do whether it is powered from dc or ac. Assuming that the lamp is incandescent, the color is determined by the metal of the filament, which I presume is tungsten, and the applied voltage. A tungsten filament changes color as it heats up from red to orange to yellow to white. At lower voltages, the filament may remain a color on the cooler end of the spectrum such as yellow.

    This is likely what you are observing. If the lamp is rated 120Vac, then, whether it is powered by 120Vac or 120Vdc, the lamp should burn with a fairly white light. If you energize it with a lower voltage whether ac or dc, the light will likely be yellow or orange, depending on voltage and the temperture to which the filament is ultimately heated. I suspect your dc source is a lower voltage than the lamps rated voltage."

    So low voltage may cause the heated filament to remain a color on the cooler end of the spectrum (yellow) in a dc current.
  8. Nov 22, 2011 #7


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    The 120V DC is 120 V at all times. 120V rms AC is sometimes nearly 170V, and that is going to make the filament a lot hotter! [for a little while, then cool off, then heat up, then cool off etc], but I suspect the rate at which it cools off is so slow that the filament is generally hotter with AC. Here, in Australia, the peak voltage occurs 100 times each second, while in the US it peaks 120 times per second. When you turn a lamp off it take an awful lot longer than 1/100 th of a second to cool [stop glowing]. btw 100 peaks per second, though half of them are a negative voltage - there are 50 cycles per second.
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