Homework Question (Calculating current by taking charge derivative)

Discussion in 'Homework Help' started by fishguts, Sep 10, 2009.

1. fishguts Thread Starter Member

Sep 3, 2007
22
0
I have a question about the following homework problem.

The charge enters the positive terminal of an ekement is given by q(t) = -12e^-2t mC. The power delivered to the element is p(t) = 2.4e^3t W. Computer current in the element the voltage across the element and energy delivered to the element in the time 0 < t < 100ms

I know that current is equal to the derivative of charge

i = dq(t) / dt

But when I take the derivative of q(t) i get 24e^-2t which does not give me a constant. Do I plug in 100 ms into 24e^-2t and subtract that from 24e^-2t evaluated at 0?

Thanks for any help!

2. mik3 Senior Member

Feb 4, 2008
4,846
70
Integrate voltage*current between 0s and 100ms to find the energy.

3. someonesdad Senior Member

Jul 7, 2009
1,577
142
What leads you to believe the current should be a constant? Once you have the answer to that, you'll realize you already have the correct answer to the question about the current.

4. jit26 New Member

Sep 12, 2009
11
0
Hi,
I am not sure but try this way.

p(t)=v(t)*i(t)
p(t)=v(t)*dq(t)/dt
p(t)*dt=v(t)*dq(t)

now intergrate t=0 to t=100ms with respect to t. not sure if i am right or wrong.