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HELP PLSSS - Physics

  1. Apr 2, 2004 #1
    HELP!!! PLSSS!! - Physics

    Hi All,
    I badly need helpwith the below 2 questions... as for question 2, I worked out part a-c but I'm unable to complete part d.... pls help and correct me if I am wrong.

    really appreciate it!


    1. A 125 W heater is designed to operate with an emf of 120 V.
    Find the approximate power dissipated in the heater if the potential difference is decreased to 112 V. (Unit: W)

    can somebody help me up with this?

    2. A 5.2 F capacitor is charged to 1.4E2 V and is then connected across a 4.5E2 resistor. Answer the following questions:
    All answers must have three significant digits unless otherwise specified.

    (a) What is the initial charge on the capacitor? (Unit: microcoulombs, uC)
    answer: 728 uC

    (b) What is the initial current just after the capacitor is connected to the resistor? (Unit: milliamperes, mA)
    answer: 311 mA

    (c) What is the time constant of this circuit? (Unit: milliseconds, ms)
    answer: 2.34 ms

    but I'm Stuck on this question...
    (d) How much charge is on the capacitor after 5.1 ms? (Unit: microcoulombs, uC)
    can somebody help me up with this??

    thanks again!
  2. jcsd
  3. Apr 2, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    For #1: First find the resistance of the heater using the rated values for power and voltage. Now find the new power dissipated when the voltage is reduced.

    For #2d: You found the time constant Τ, now use it. The charge at time t is: Q = Q0e(-t/Τ).
  4. Apr 4, 2004 #3

    Hi Doc Al,

    thanks so much! you're a gem!
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