1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: RC Circuits - Graph

  1. Jul 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A simple type of blinking light circuit can be constructed using a neon lamp. The circuit has a 4.0uF capacitor in parallel with a neon lamp. When the voltage is low in the RC portion of the circuit, the lamp does not conduct electricity. Therefore, it is effectively not there from an electrical point of view. The RC circuit will then charge from the 110 V power supply. However, when the voltage across the capacitor reaches 75 V, the neon will ionize very quickly and the neon lamp will become a very good conductor, and will immediately discharge the capacitor. The energy stored in the capacitor will be given off as a flash of orange light, making this a useful circuit. After the flash, the charging process will start once more since the voltage will again be low.

    a) Determine the flash frequency with the resistance value shown.
    b) Make a sketch of the voltage across the capacitor versus time in such a circuit, showing several periods.

    2. Relevant equations

    time constant = RC
    V = Vo(1- e^ t/RC)

    3. The attempt at a solution

    For part a)
    time constant = RC = (25 * 10^3 ohm) * (4.0 * 10^-6 F) = 0.1s

    t = -ln (1- 75V/110V) * 0.1s
    = 0.114s

    frequency = 1/t = 8.8 Hz

    For part b)
    I've attached a sketch but I'm not sure if this is correct. Any help would be great!

    Attached Files:

  2. jcsd
  3. Jul 1, 2013 #2
    Your sketch is pretty good, if anything it may fall faster than you have shown because it says 'will immediately' discharge the capacitor.
  4. Jul 1, 2013 #3
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted