Instantaneous Current Calculation in Phasor Form | Phasor Physics Problem

[Moneer81]

Hello,

came across this simple problem: Let w = 2000 rad/s and t = 1 ms. Find the instantaneous value of each of the currents given here in phasor form: a) j10 A; b) 20 + j10 A

so for part a, I changed the current (0 + j10) to 10e^0 or simply 10, then my instantaneous value should be 10cos(wt + phi) = 10cos(2+0) = -4.16 A but the answer in the book is -9.09 A. Did I do this right?

 

[berkeman]

Why did you phase shift the j10 current to 0 phase before calculating the components at t = 1ms?

Your currents will have the form I = Ir + j Ii

So if you start out with the current I = 0 + j10, then there is a starting phase shift at time t=0, and you will start the phasor rotating from there, not from the real axis.

 

[bigjoe5263]

moneer,

it should be 10cos(wt + pi/2), this will give you the correct answer..

the angle of j10 is 90 degrees, not 180 so it should be pi/2 radians.

 

[berkeman]

moneer,

it should be 10cos(wt + pi/2), this will give you the correct answer..

the angle of j10 is 90 degrees, not 180 so it should be pi/2 radians.

Welcome to the PF, bigjoe. Just a reminder that we do not give out answers here on the PF for homework and coursework questions. We provide tutorial help, but not complete answers. This question is old enough that I'm pretty sure the original poster (OP) has figured it out, so I won't delete the final answer in your post (this time).