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Energy in a circuit

  1. Aug 29, 2009 #1
    1. The problem statement, all variables and given/known data

    2. The voltage and current at the terminals of the circuit element in Fig. 1 are zero for t<0. For t>0 they are

    V= e^(-500*t)-e^(-1500*t) V
    i= 30-40e^(-500*t)+10e^(-1500*t) mA

    a) Find the power at t = 1 ms.
    b) How much energy is delivered to the circuit element between 0 and 1 ms?
    c) Find the total energy delivered to the element.

    Figure 1. An ideal basic circuit element

    2. Relevant equations

    W=integral (P dt)

    3. The attempt at a solution

    i solved part a by simple substituting the time in both V and i equations then i multipled them giving me a result of 3.1 W, in the second part i used the integral where i substitued the answer i got in the previous part and set the integral to be from 0 to 1 ms but it turns to be wrong !!

    need your help tanx in advance
  2. jcsd
  3. Aug 29, 2009 #2


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    Voltage and current are variable and so is their product, the power. You must perform the multiplication and integrate it.
  4. Aug 30, 2009 #3
    i did so as you said but i got a negative wrong answer

    can you explain more in depth
  5. Aug 30, 2009 #4


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    Staff Emeritus
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    Homework Helper

    If you show your work, somebody can help figure out where things went wrong.
  6. Aug 30, 2009 #5


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    Both voltage and current start with the value 0 at t=0 and them they become positive. The current rises monotonically to 40 and after some time (well after 1 ms) the voltage starts to decay to zero, but still positive.
    It is not possible that their product can be negative.
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