Determining power across a certain device.

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Discussion Overview

The discussion revolves around determining the instantaneous power across a device given the voltage and current as functions of time. The focus is on evaluating the power at specific time points and reconciling discrepancies with textbook answers.

Discussion Character

  • Homework-related

Main Points Raised

  • One participant presents the voltage and current equations and calculates the instantaneous power at t=0 and t=0.25s, obtaining 0.5W for both times.
  • The same participant questions whether power is always zero at t=0 based on the textbook's assertion that it should be.
  • Another participant asks for clarification on whether the voltage and current functions were specified correctly, particularly regarding the absence of phase angles.
  • A subsequent reply reiterates that the equations were copied exactly from the textbook, suggesting confusion over the textbook's answer.
  • One participant expresses skepticism about the textbook's correctness regarding the power at t=0.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there is disagreement regarding the correctness of the textbook's answer for the power at t=0.

Contextual Notes

There is uncertainty regarding the definitions and assumptions related to the voltage and current functions, particularly concerning phase angles, which may affect the calculations.

november1992
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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?
 
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Are you sure that both v and i were specified as cosines, and that neither included a phase angle?
 
I copied it exactly the way it's written in the textbook.
 
november1992 said:
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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