Determining power across a certain device.

  • #1
120
0

Homework Statement



The voltage across and current through a device are given by:

v(t)= 5cos(4∏t)V
i(t) = 0.1cos(4∏t)A

determine the instantaneous power p(t) at t=0 and t = 0.25s

Homework Equations



p=vi

The Attempt at a Solution



for t=0 I got p=0.5W

for t=0.25 I got p = 0.5W

the textbook says at t=0 it should be 0. Does power always equal 0 at t=0 or did i mess up on my calculation?
 

Answers and Replies

  • #2
gneill
Mentor
20,925
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Are you sure that both v and i were specified as cosines, and that neither included a phase angle?
 
  • #3
120
0
I copied it exactly the way it's written in the textbook.
 
  • #4
gneill
Mentor
20,925
2,866
I copied it exactly the way it's written in the textbook.

Well, very strange then; the textbook's answer for t=0 appears to be incorrect.
 

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