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Current and Watts

  1. Jul 15, 2011 #1
    Can you determine current given only the watts?

    Ex) if you have a lightbulb with 120 Watts, can you determine the current (charge) that flows through it in a second, for example?
  2. jcsd
  3. Jul 15, 2011 #2


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    In a low voltage circuit, you need a lot of current to produce the same power as for a high voltage circuit.
    Remember - Power (Watts) = Volts (Volts) X Current (Amps)
  4. Jul 15, 2011 #3
    From the way the question is phrased, no was the correct answer.
    You need any 2 (Voltage/Current/Resistance/Power) to determine an unknown in basic circuits.

    However, if you MEANT (since you referenced a 120W light bulb) how much current will the bulb draw on a standard 120 VAC circuit, then that is simply P=V*I ---> I = P/V = 120W/120V = 1 Amp.

    A 120 W rated bulb will not actually draw/dissipate 120 W of power unless it is powered by the expected voltage (ie a 12V battery will not push 10 amps through it).
  5. Jul 15, 2011 #4


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    In general no, you need also voltage:
    Power = voltage times current.

    In the lightbulb example you probably know the voltage implicitly i.e. it's a lightbulb used with 120V AC house lights. In that case you would solve to find the current is 1 amp (1 coulomb per second).

    current = power / voltage
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