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Homework Help: Resistor in circuit

  1. Jan 17, 2010 #1
    In an alarm clock, there needs to be a delay of 1 minute between closing switch and circuit being active. A 2200microF capacitor is charged through a resistor from a 5V supply. The pd across capictor must rise to 4.3V for alarm to become active.

    Calculate size of resistor.

    The answer is 14000ohms. I need to use the equation: V=Vo * e^(-t/RC) with t = 60s

    but im not sure what to use for the value of the 2 V's.
  2. jcsd
  3. Jan 17, 2010 #2


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    V is the voltage across the capacitor and Vo is the voltage across the power source.
  4. Jan 18, 2010 #3

    4.3 = 5 e^(-t/RC)

    but this doesnt give R=14000 ohms?
  5. Jan 18, 2010 #4


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    Hi Masafi :smile:

    No, for a capacitor being charged from zero, it's …

    I(t) = (V0/R)e-t/RC

    V(t) = V0(1 - e-t/RC)

    (so the current becomes effectively zero, and the voltage across the capacitor becomes effectively V0, as t -> ∞)
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