# Why are there at least two prongs on every electrical plug? [closed]

Why are there at least two prongs on every electrical plug? If power is traveling in one direction (from the wall to the appliance), why can't there be just one wire carrying it? Circuits need to be closed, but after all, current can flow through a capacitor, and that's an open circuit.

• A capacitor is a two-terminal device with a specifically constructed gap that's small. It's nowhere near the same as just having a dead-end with stray capacitance to ground on all of your appliances. – nanofarad May 4 at 20:58
• Power is traveling in one direction, but current isn't. And a capacitor is not an open circuit for AC. This is all very basic, so the problems you show here make this question "too broad" for an electrical engineering site. – Marcus Müller May 4 at 20:59
• A capacitor is not an open-circuit. Also, it has two wires, and the only way to make current flow through a capacitor is to use both wires. – mkeith May 4 at 22:02
• Current does not flow through a capacitor, the dielectric prevents this. The plates get cyclically charged and discharged by an alternating current, but only a very small leakage current crosses the dielectric. – Chu May 4 at 23:42
• @Chu AC and transient currents flow through capacitors. It is not really fruitful to think of it another way. When you think current doesn't really flow through a capacitor you also start to wonder how can you use KCL on a capacitor circuit and you think that charges may enter on one pin with no charges exiting via the other pin. That will get you all twisted up. So I stand by what I said. Current flows THROUGH a capacitor in response to changing voltage ACROSS the leads. – mkeith May 5 at 0:44

In a nutshell, to transfer power over a wire, you use a loop for current and electromotive force for voltage. You need two connections to the loop to supply the electromotive force that sends current down the wire.

To actually use the power, the load needs to convert some of the delivered energy to some other form, such as light, heat, mechanical movement, magnetic flux, charge, chemical change and so forth. If it doesn't do any of that no power is actually used.

And remember that capacitors, while they block DC, will carry AC currents just fine. And capacitors are all around us.

While it would seem possible to send power with a single wire and high voltage, what actually happens is that a loop is formed using earth ground as a return. A Tesla coil is an extreme example that uses an arc instead of a wire to send power, but nonetheless uses a ground return. Tesla coils make a pulsed arc, which can be carried easily to a DC connection to ground, or via capacitance.

A more practical ground-return example is early telegraph wires that used just a single wire, and relied on ground to complete the loop back between sender and receiver.

A Van de Graaff generator would seem to carry energy (charge) without a return current, except that you have to consider that the charge is building up on a collector (capacitor) that is referenced to ground, even if only indirectly. It's actually building up a difference in charge between the two ends of the machine. So it's inherently a two-terminal device making electromotive force.

That all said, it's also possible to transfer power as waves (particles? both?) over the air as electromagnetic fields, or as acoustic energy (sound). The receiver 'absorbs' this energy and converts it to another form. Practical example of each: microwave oven (RF waves), dynamic microphone (sound), you standing in the sunlight (light waves.)

• Nice answer. So Using photons, single "wire" power transmission is possible as you mentioned in examples. Imagine distributing power via a laser in an optical fiber. Currently it is not very efficient, so rarely done. If lasers and photodiode were more efficient and fibers less lossy, this question might not exist :) – tobalt May 4 at 22:11

The very simple analogy is water. You want water to perform work- so it has to flow, not just create pressure. Imagine a water mill wheel- you need water to flow. so you sort of need to create a path for water to come and a return path. Same here with electrons- you need them to flow (current) to do work (power), so you have to bring them and then let them go back. If you don't, they build up quickly and stop flowing.

However, if your supply is not electric- other possibilities exist. Microwaves can simply travel through waveguides, or light can simply travel in the air.

• Your open-circuit water mill is a bad analogy. A closed loop pumped water circuit might be better. The pump is the battery. The various radiators, etc., the resistors. You can even add in capacitors (pressure vessels with diaphragms) or inductors (long lenghts of pipe which cause water hammer when switched off suddenly). – Transistor May 4 at 21:43
• Hydraulic circuit would be an OK analogy. But might be less familiar to the OP. – mkeith May 4 at 22:03
• Bicycle chain??? – DKNguyen May 4 at 22:18
• Hydraulic models don't work because with electricity, the "pull" is just as important as the "push". Also, there's no such thing as a tank that will hold electrons. Try it: take hot+neutral only off a 5ma GFCI or 5ma RCD, and with only those 2 wires devise a circuit that will trip the GFCI (by causing currents on the 2 wires to differ by more than 5ma for about 10ms). You can't lol. – Harper - Reinstate Monica May 5 at 0:20
• @Transistor i tried hard to come up with something very simple and clear. If further question was asked by the op, I would ask to imagine the second wire connected to another power outlet to at least visually be similar to a mill. – Gregory Kornblum May 5 at 7:53