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Homework Help: Basic Electricity question

  1. Oct 4, 2007 #1
    1. The problem statement, all variables and given/known data
    2 120 Volt bulbs (one 40 watt, one 100 watt) are connected to a variable power supply in series in a circuit. Which bulb will light first

    2. The attempt at a solution

    So, I'm pretty sure its the 40 watt bulb because it uses less current.

    B(40) = 0.3333A
    B(100) = 0.8333A

    Is this correct or... am i just guessing to much?
  2. jcsd
  3. Oct 4, 2007 #2
    It seems logically correct. Yes. Bulbs can light before their maximum current is applied to them though, meaning the bulbs can be dim and not need to reach .333A to start the first one.
  4. Oct 5, 2007 #3

    My initial response is that the current will always be the same everywhere at every instant in a non-reactive series circuit. Therefore you can expect a higher voltage drop across the higher resistance load, which in this case is the lower wattage lamp. I expect the lower wattage lamp to therefore glow well before the higher one shows any incandescence. Also when the voltage has been dialed up all the way, I expect the lower wattage lamp to still (hot cathode resistance at near design consideration) be way ahead of the higher wattage lamp by being near full brightness and a comparitively dim glow from the other. REM: the lower wattage, thus higher resistance lamp is the dominating, thus current controlling load, and it gets the lions share of the voltage. Also the hotter lamp's resistance will have increased more than the colder one, intensifying the difference between the individual voltage drops even more.
    Last edited: Oct 5, 2007
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