# Homework Help: Converting phasor to time domain

1. Dec 14, 2016

### CoolDude420

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

I am extremely confused. Everywhere I go to look on how to convert phasors into time domain I get a different answer. I am trying to convert -j to the time domain. My notes says to find the Im(phasor). I have explained in the image above what my lecture notes say, what the question is and what my answer is.

Here is the provided answer in the notes:

Can anyone provide me with a universal method ?

2. Dec 14, 2016

### Staff: Mentor

First, sketch the given phasor on the complex plane, with x-axis Real and y-axis Imaginary.

3. Dec 15, 2016

### CoolDude420

4. Dec 15, 2016

### Staff: Mentor

Right.

There is a standard way to describe angles, on a graph. What angle would you associate with the line you have drawn? (For this angle, we need both magnitude and sign.)

Last edited: Dec 16, 2016
5. Dec 15, 2016

### CoolDude420

Measuring from the positive side of the X axis, I would say an angle of -90degrees.

6. Dec 15, 2016

### CoolDude420

Angle should be -90 degrees I think. or - pi/2 rads

7. Dec 15, 2016

### rude man

Note that (1) the frequency (1000 Hz here) is not included in the phasor.
(2) you have assumed 1V peak. More generally, a phasor is Vpkexp(jθ) ⇔ Vpksin(ωt + θ).
But you have the phase right; -j = exp(-jπ/2). This is an identity. And -jπ/2 = -90 deg so the phase is -90 deg and the time expression is what you wrote above.

8. Dec 16, 2016

### Staff: Mentor

Right. That's the angle/phase taken care of.

What is the magnitude (i.e., length) of the line you drew on that graph?

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