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Voltage drops - i don't understand!

  1. Dec 5, 2007 #1
    1. The problem statement, all variables and given/known data
    Hi, this question is really confusing me and preventing me from completing the rest of the question. It is probably a very simple problem but i just dont understand so any help would be much appreciated.

    A metal- filament lamp,rated at 750W, 100V, is to be connected in series with a capacitor across a 230V, 50Hz supply. Calculate the voltage drop across the capacitance required, the current, the capacitance, phase angle between the supply voltage and current.

    2. Relevant equations
    P = IV
    V=IX X = capacitive reactance

    3. The attempt at a solution
    I think the current is given by
    I = 750W/100V?
    I dont know how to calculate the voltage drop.Thanks for any help and tips
  2. jcsd
  3. Dec 5, 2007 #2


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    Science Advisor
    Gold Member

    The voltage drop needed across the capacitor to keep the bulb happy is 230 - 100 = 130V. To find the capacitance, look for a capacitor equation relating the voltage to the rate of change of the current (it will involve the frequency).
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