Converting from sin to cos appropriately with phasors

[wellmoisturizedfrog]

TL;DR

Difficulty understanding when to add pi/2 vs when to subtract pi/2.

My transmissions line class often features problems where the voltage is expressed as a sin, not a cos. Obviously a phase shift of pi/2 is sufficient to convert between the two. However, I have trouble understanding when adding pi/2 is appropriate as opposed to subtracting pi/2. As per my understanding, both should be sufficient to achieve the desired conversion, but my professor says otherwise. While I understand that the angle should reflect the position of the phasor in the complex domain, I still feel as though I am missing something. Could anyone offer a concrete clarification of this matter?

 

[PeroK]

Can you be more specific? Note that:
$$\sin(x +\frac{\pi}2) = \cos(x)$$

 

[FactChecker]

You might get better help if you post an actual problem, with full details, and ask that question. For any homework-type problem, you need to show as much of your own work as possible. There is a specific format for homework-type problems.

 

[DaveE]

I still feel as though I am missing something. Could anyone offer a concrete clarification of this matter?

$sin(\Theta - \frac{\pi}{2}) = -cos(\Theta)$
$sin(\Theta + \frac{\pi}{2}) = cos(\Theta)$
etc.

Can you explain a bit more about what you are unsure of?

 

[Baluncore]

TL;DR Summary: Difficulty understanding when to add pi/2 vs when to subtract pi/2.

However, I have trouble understanding when adding pi/2 is appropriate as opposed to subtracting pi/2. As per my understanding, both should be sufficient to achieve the desired conversion, but my professor says otherwise.

One will convert sin() to cos(), the other will do the same, but will invert the signal, by the net phase shift of pi.

 

[sophiecentaur]

TL;DR Summary: Difficulty understanding when to add pi/2 vs when to subtract pi/2.

but my professor says otherwise.

I wonder if he really said that or if you mis- interpreted him (i.e. just in one particular example). The 'timing of events (phases) can sometimes be very relevant but not always.