# How do we know the direction to draw the arrows for voltage drops and current direction when drawing a circuit diagram for an ac circuit?

In an ac circuit the polarity of the voltage source changes periodically. So too will the current. However my circuit course has the voltage source labelled with a positive and negative as seen in the schematic below:

simulate this circuit – Schematic created using CircuitLab

This software used to draw the schematic also provides the voltage source icon with the positive and negative signs on it. Using the polarity of the source we can draw the direction of current flowing towards the resistor in the loop. My main source of confusion is that ac alternates the direction of current therefore why do we draw current in one direction and polarise the sources?

• You can have a DC offset in which case the +/- signs make sense. Otherwise you can ignore the polarity. Commented May 10, 2019 at 13:26

If you had no polarity signs you would have a 180 degrees ambiguity on the phase of involved quantities.

Say you have a sine ruled voltage source, at t=0 is voltage rising or falling? The positive terminal is the answer.

The same applies to all the quantities, voltages and currents around the circuit .

There can indeed be cases where this doesn't matter at all, e.g. a single generator but it generally does matter.

It's arbitrary, except where it isn't.

As you've observed, the current usually flows in both directions depending on which part of the cycle you're in. Your circuit could be drawn equally well with the signs reversed and produce the same results. However, we need to label the two signals with something, and indicate that they have opposite polarity, so we assign a "+" and "-". The same goes for things like differential digital signals like USB D+ and D-.

However, quite often one side of the AC circuit is coupled to ground, either directly or indirectly, and that is then coupled to every other AC circuit in the building. Ground is conventionally labelled "-".