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!

Inductor and bulb in an circuit

  1. Feb 13, 2017 #1
    1. The problem statement, all variables and given/known data
    the problem statement is provided in the attached file

    2. Relevant equations
    logic based , basic principle of inductors.

    3. The attempt at a solution
    after switch is opened the more the current passes through the bulb , more will be the brightness .
    earlier in the steady state , current through each bulb will be E/R ( assuming power supply of emf E and each bulb of resistance R).
    after the switch is opened the inductor will oppose the change in current in its branch and hence same amount of current will flow as it was flowing the inductors earlier ,
    so current in bulb B and C would remain same but in Bulb A current flow will be doubled (2E/R).
    hence brightness of bulb A should increase and rest should remain same .

    BUT in the answer it is given take brightness of bulb C should increase and rest should remain same.
    WHAT is wrong in my approach?

    Attached Files:

  2. jcsd
  3. Feb 13, 2017 #2


    User Avatar
    Homework Helper
    Gold Member

    For convenience, here's the rotated picture.
  4. Feb 13, 2017 #3
    oh yeah thank u
  5. Feb 13, 2017 #4


    User Avatar
    Homework Helper
    Gold Member

    I agree with your answer and reasoning. The brightness of A increases while B and C do not change.
  6. Feb 14, 2017 #5
    but why is the answer saying it otherway around
  7. Feb 14, 2017 #6

    rude man

    User Avatar
    Homework Helper
    Gold Member

    EDIT: I agree with the OP and TSny.
    (Current in A reverses & doubles.)
    Last edited: Feb 14, 2017
  8. Feb 14, 2017 #7


    User Avatar
    Homework Helper
    Gold Member

    I replaced the switch with a variable resistor that has zero resistance at t = 0 but increases fairly rapidly after t = 0. I then used Mathematica to solve the differential equations for the currents. It gives the following graph for the currents as a function of time:
    The blue curve is the current in bulb A, the orange curve is the current in bulb B, and the green one is bulb C. All currents are equal to 1.0 at t = 0.

    The current in A rapidly reverses direction and momentarily becomes greater in magnitude than its initial value. The currents in B and C never become greater than their initial values.

    When I tried to make the variable resistor increase more rapidly (to better mimic the switch), Mathematica had trouble solving the equations. But the result above does at least show the general behavior.
  9. Feb 14, 2017 #8
    that great thank you for the explanation .
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: Inductor and bulb in an circuit
  1. Circuit w/ Light Bulbs (Replies: 15)

  2. Light bulb circuit (Replies: 1)