Determining power across a certain device.

[november1992]

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?

 

[gneill]

Are you sure that both v and i were specified as cosines, and that neither included a phase angle?

 

[november1992]

I copied it exactly the way it's written in the textbook.

 

[gneill]

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.

 

[rezeew]

Your calculation for both t=0 and t=0.25s are correct. The textbook may be referring to the average power over a certain time period, which would be 0 at t=0 since the device has not yet started functioning. However, when looking at the instantaneous power at a specific moment in time, it can have a non-zero value. It is important to distinguish between average power and instantaneous power when analyzing a device.