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Lights wired in series

  1. May 24, 2005 #1
    :surprised I need some help with a formula, I am constructing some aids for a class room demonstration on basic electricity. I am using lights to demonstrate series and parallel cicuits, also trying to explain some of the basic laws. My question, If I hook three 25 watt bulbs in series with a supply voltage of 123 VAC the displayed wattage (under power) is about 12 watts (I am using a meter). I would like to explain using an appropriate formula but I don't know how.
  2. jcsd
  3. May 24, 2005 #2


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    Where do you measure that Wattage...(in which point of the circuit)?VAC is that continuous or alternative current...?

  4. May 31, 2005 #3
    I am using a watt meter that allows me to select either Volts, Amps, Watts, or KWHR. It is AC.
  5. May 31, 2005 #4
    Hes asking between which nodes did you connect the multimeter
  6. May 31, 2005 #5

    Meir Achuz

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    If the resistance did not change with temperature (But, it does.) due to the current through it, then the resistance of the three bulbs in series would be 3 times the resistance of one bulb. The power (given by P=V^2/R) would be
    25/3=8 Watts. You read 12 Watts (>8), because there is 1/3 the current through each bulb. They stay cool and have a smaller resistance than a hot bulb.
  7. May 31, 2005 #6
    The bulbs are in series, they all take the same current.

    [tex] P_{res} = IV = \frac{V^2}{R} = I^2R[/tex]
  8. Jun 1, 2005 #7
    Thank you, that is very clear
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