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I am now studying phasors and my teacher told us that when you find in an exercise, for example the current and that current is for example \$ -2 + 3j \$ , then, when you want to find the angle, which is \$ \theta = arctan(-1.5) \$, you have to add to the result angle 180 degrees to find the final angle.

My question is, how do I know when to add and when not to add 180 degrees to the angle?

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The one-argument arctangent function (atan()) can only return answers in quadrants I and IV. If you know that the point is in quadrant II or III (i.e. the real component is negative) then you need to transform the result via rotation by \$\pi\$ around the origin.

The two-argument arctangent function (frequently known as atan2()) does not have this caveat.

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  • \$\begingroup\$ So I only need to do the transformation when the real component is negative.Thank you very much! \$\endgroup\$ – Xizi Sep 6 '15 at 10:48

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