1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

RC circuit in a camera flash

  1. Jan 29, 2015 #1
    1. The problem statement, all variables and given/known data
    Consider the following circuit in a camera flash where a light bulb is in series with a charged capacitor:
    a) Derive, but do not solve, the governing differential equation for the circuit. (Hint: treat the resistor and the light bulb as a single resistance.)
    b) Assume Rlight = 5 ohms If at time t = 0 the switch is closed, the solution to the differential equation from part a) states that a current will develop in the circuit which will light the bulb according to: i(t) = Q/RC*e^(-t/RC) where Q is the initial charge present on the capacitor. We want the capacitor to discharge quickly to produce a short flash for photography. If we want i(t) to fall to half its maximum value in 0.01 s and the capacitor has a value of 160 muF, what should R be?
    c) Calculate the value of the ratio of the instantaneous power dissipated in the resistor to the instantaneous power dissipated in the light bulb, i.e. Powerresistor/Powerlight.


    **Image can be found at http://www.chegg.com/homework-help/...ies-charged-capacitor-derive-solve-g-q3553019
    2. Relevant equations
    v=iR
    v=1/C integral idt
    P=i^2*R

    3. The attempt at a solution
    For a)

    I got 0 = R*di/dt + i(t)/C
    I found an expression for total voltage and differentiated it w.r.t. time

    b) I plugged in numbers and got that R = 85.2 ohms (but this seems large???)

    so:

    1/2Q/RC=Q/RC*e^(-t/RC)
    1/2 = e^(-t/RC) -- R=Rt-Rl = 85.2 ohms

    for c)

    Presistor = i^2*Rr
    Plight = i^2*Rl

    Dividing, i^2 cancel so: Rr/Rl = 17.04???
     
  2. jcsd
  3. Jan 29, 2015 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    It is correct. That circuit is not efficient!
     
  4. Jan 30, 2015 #3
    Thank you! Would the power ratio also be correct? I wasn't 100% that the i^2 would just cancel..
     
  5. Jan 30, 2015 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    It is correct. The same current flows both through the resistor and bulb, so their square is also the same, and cancel :)
     
  6. Jan 30, 2015 #5
    Perfect! Thank you so much! :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: RC circuit in a camera flash
  1. Flash in cameras (Replies: 13)

  2. RC circuit complete (Replies: 2)

Loading...