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: Time varying resistance in circuits

  1. Jun 19, 2017 #1
    1. The problem statement, all variables and given/known data
    A battery of 10 volts is connected to a resistance of 20 ohms through a variable resistance R.The amount of charge which has passed through the circuit in 4 minutes,if the variable resistance R is increased at the rate of 5ohms per minute is:
    A)120 C
    B)120ln2 C
    C)120/ln2 C
    D)60/ln2 C
    2. Relevant equations
    V=IR(Ohms law)
    Integration of 1/(ax+b)=[ln(ax+b)]/a
    Unitary method:
    Rate of increase of resistance=5 ohms per minute or...1/12 ohms per second
    3. The attempt at a solution
    But,my answer isn't in the options...answer's B
  2. jcsd
  3. Jun 19, 2017 #2
    Your problem is the substitution of bounds into the integral
    What happened to the "lower bound"?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted