# Homework Help: AC circuit

1. May 3, 2014

### influx

25/3162.3 = 7.9 x 10-3 (approximately).

Why have they written the mA unit next to the angle rather than next to the 7.9? Is this the convention or?

Also:

Where did they get the angles 90 and -90 degrees? I know that you can write it in the form r<theta, but I am unsure how they chose 90 and -90?

Last edited: May 3, 2014
2. May 3, 2014

### Staff: Mentor

Yes, it's the convention to place the units of the overall quantity to the right of the quantity. The quantity itself is (7.9 x 10-3 ∠71.56°) Amps. It's a phasor comprising both magnitude and angle.

On the complex plane the "y-axis" is the imaginary axis. Purely imaginary values lie along that axis. Angles are always measured from the positive x-axis (the real axis), so all phasors that are purely imaginary have angles of +/- 90°.

3. May 3, 2014

### influx

You mean -71.56° right? Apart from that it makes sense. Thanks!

Last question, do you happen to know why they have used v = 100 (in the last step of the calculation on the right) rather than the given value v = 100sin250t ?

Last edited: May 3, 2014
4. May 3, 2014

### Staff: Mentor

The calculation is made using phasors, so only the magnitude is required. The "sin250t" is the time domain representation of how the the voltage varies over time. It's phasor representation is "100", and the rotation of the phasor about the origin over time is implied.

You might want to review phasor representation for AC currents and voltages.