We are talking about AC voltage not DC voltage. An AC motor will not run in the opposite direction if you reverse the black and white wires.

Try it on a house hold fan. Trim the fat end the of the plug so it will fit into the outlet the other way and see if you can get it to run backwards.

The black wire is POWER. The white wire is neutral and the green is ground.

The black wire is one leg of the 220 Volts coming into your house.

(If you have a single phase hookup you will have three wire coming off the power pole to your house. L1,L2, and ground. Place a meter between L1 And L2 and you will get around 220 volts. Place the meter between L1 and ground or L2 and ground and you will get around 110 volts.)

The white wire is connected to the same bus bar in the main box, as your ground (green wire).

Touch the white wire to the green wire and nothing will happen.

Touch the black wire to either the white or green wire and you will get a large electrical arc and a tripped breaker or burned fuse.

Touch the white or green wire to your hand with bare feet on a concrete floor- Nothing.

Do the same thing with the black wire and you will get knocked on your ass.

You will also probably hear someone say from far away " Dose anyone know how to do CPR".

If you need to wire in an on/off switch do it on the black wire. If you do it on the white wire the circuit will still have power in it relative to the ground you are standing on. So even when the switch is off and the light is off there is still power going to the circuit. If you forget and start to work on the light you could get a bad shock or worse. CPR? Anyone here know CPR?

Be careful and stay safe.