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: The voltage(V) volts in a circuit

  1. Aug 21, 2010 #1
    1. The problem statement, all variables and given/known data
    The voltage(V) volts in a circuit at time(t) is given by

    V(t)=18(1 - e -4t/9)

    Find the expression for dV/dt.

    Hence evaluate dV/dt at

    (a)t=0 and (b)t=5 seconds

    2. Relevant equations

    Just wondering if this is right and if not where did i go wrong

    3. The attempt at a solution


    = -9(1-e-4/9t)

    (a) t=0 -8(1-e-4/9(0))


    (b)t=5 -8(1-e-4/9(5))

  2. jcsd
  3. Aug 21, 2010 #2
    [tex]\frac{d}{dt}V(t) = \frac{d}{dt}18 - \frac{d}{dt}18e^{-4t/9}[/tex] what's the derivative?
  4. Aug 21, 2010 #3
    not to sure.Im sorry but im not that good at this hoping to get better!
  5. Aug 22, 2010 #4

    [tex]\frac{d}{dt}V(t) = \frac{d}{dt}18 - \frac{d}{dt}18e^{-4t/9} [/tex]

    Whats the derivative of 18 with respect to t?

    What about 18exp(-4t/9) with respect to t?

    I seperated the parts of the equation using the fact that [tex]\frac{d}{dt}[v(t) + w(t)] = \frac{d}{dt}[v(t)] + \frac{d}{dt}[w(t)][/tex] you can generally remember this rule as "the derivative of the sum is the sum of the derivatives".

    Does this make sense?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook