Firstly, I think I get the concept of alternating current. Am I correct in thinking, that the positive phase of the sine wave represents current flow in one direction, whilst the negative phase represents current flow in the opposite direction? If this is the case, then, if the sine wave is perfect, (i.e. both the negative and positive phases are of equal magnitude and duration) how can there be a net current flow in either direction?

My second question relates to direct current. I understand that DC does not fluctuate. On figures that explain it, it is generally represented on a chart with a y axis from –X to +X, with the DC line generally residing in the positive half, and is therefore +X volts DC.

However, as there is a negative portion to this chart, am I correct in assuming you can have negative direct current, and this is why you sometimes see on circuit diagrams +Xv and –Xv. If this is the case then:

1. What exactly does negative DC represent?

2. Is this only relevant in dual power supply circuits. I.e., sometimes you just see circuits that have +XV and then 0V, which I always took to be positive and negative sides (of say a battery). In some (dual supply?) circuits, you see +xV, -xV and ground. If you were connecting a battery / power supply to this, how would you supply + and – DC, and where would ground go?

I just can’t get my head around how you can have negative DC, what it represents, and how it is different to + DC. Are there circuits with just –DC?

Sorry again if this is basic.

Thanks.