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!

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

    P=V*i
    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

    CEL

    User Avatar

    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

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

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

    CEL

    User Avatar

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook