Why do transmission lines use two wires instead of one?

[fran1942]

Hello, a beginners question here.
Can someone please tell me why transmission lines use two wires instead of just one ?

 

[Drakkith]

It takes two lines to complete the circuit. With only one line the current wouldn't be able to get back to the source, which is required in a circuit.
Think of a loop of pipe that contains water. If you put a pump in there the water goes round and round. If you do the same in a straight pipe that's capped off, the water has nowhere to go, and doesn't flow. A similar effect happens with electricity.

 

[davenn]

Yes and no...

dont forget that there are were many transmission lines both power and communications
that used just 1 wire and an Earth return

Dave

 

[baiber]

If you do the same in a straight pipe that's capped off, the water has nowhere to go, and doesn't flow. A similar effect happens with electricity.

 

[Drakkith]

Yes and no...

dont forget that there are were many transmission lines both power and communications
that used just 1 wire and an Earth return

Dave

True. Per wiki on Single-Wire Earth Return:

SWER is a good choice for a distribution system when conventional return current wiring would cost more than SWER’s isolation transformers and small power losses. Power engineers experienced with both SWER and conventional power lines rate SWER as equally safe, more reliable, less costly, but with slightly lower efficiency than conventional lines.

So I would say that the 2nd wire completes the circuit because it is more efficient than using the Earth as the return.