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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)
    I=dq/dt
    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
    20170619_232022-1.jpg
    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"?
     
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