The Long Shorts of Bi-directional Flow


I didn’t correctly understand some of what is written below yesterday. There may possibly have been ‘an incident’ which provoked some hurried research to cure bamboozlement. My unbamboozling led to this post. I clearly can’t be trusted to offer safe electrical advice, so please consider this to be a discussion document, so people who ‘get’ electricity can tell me how Wrong I am before I put this information anywhere dangerous.

Direct Current (DC)

Convention: Current flows, in one direction, from +ve to -ve terminals of a battery.

Physics (closer to ‘The Truth’): Negative electrons flow from -ve to +ve terminals of the battery.

Quantum Theory: Something else happens that I don’t yet understand well enough to attempt to describe. For most practical purposes, we don’t care. Welcome to thinking like a physicist. There may be some order of magnitude estimations later. If that is inconvenient to you, please choose to ignore them. See how easy it is?

Provision of DC

DC is most often provided from a battery with two terminals, one marked Positive (+) and one marked Negative (-). The common analogy is say that this is like a pump pushing water around a circuit of pipes. This is helpful for most purposes but is what confused me in my thinking about AC.

Alternating Current (AC)

In addition to the possibility of an Earth wire, which is a safety feature, single-phase AC systems have 2 connections, known as ‘Live’ and ‘Neutral’. In a normal UK domestic supply, ‘Neutral’ is at or close to an electrical potential of 0 Volts. ‘Live’ has a varying potential that, when graphed, is the shape of a sine wave. It usually has an average, Root Mean Square (RMS) value of about 220-240 Volts. UK Voltage used to be 240V and mainland Europe was 220 Volts. The standard now is 230V with wider error margins. In reality, not much will change until key equipment in the electrical grid is replaced.

An RMS 230V AC supply would have the same heating effect (energy) as a 230V DC supply.

Provision of AC

AC can be generated by a device called an alternator, which relies on an electric coil revolving in a magnetic field. Movement through the field causes a current to be induced in the coil, if the 2 ends of the coil are connected into a circuit. This is how a power-station works.

‘Flow’ of AC

When an electrical load such as a light bulb or a fan-heater is switched-in between the Live and Neutral connections e.g. Switching on at the plug socket of a table-lamp with no other switch, the potential difference between the Live and Neutral wires will cause current to ‘flow’ through the lamp and the lamp’s bulb will light up. In an AC circuit though, this is not a one-way flow but a 50 times per second tidal flow (assuming the 50Hz EU standard for mains electricity.) The Neutral side of the plug is connected back to earth at the supply end, at close to 0V, while the Live side oscillates between +240V RMS and -240V RMS as the alternator spins round. The potential difference between this value and 0V causes electrons to flood back and forwards between the Live and Neutral, across the bulb. AC circuits have a flow but it is in quickly alternating directions. The movement of ‘current’ from Live to Neutral is a theoretical concept that has a similar effect to the movement of ‘current’ from +ve to -ve in a DC circuit (actually the one-way movement of electrons in the opposite direction.)

Short Circuit

Electrical systems and electrical household goods are designed with safety in mind, while attempting to minimise installation costs by not requiring thicker wiring. The design of a household electrical system assumes there will always be some kind of appliance connected between the live and neutral ‘sides’ of the circuit that will have a significant electrical resistance and limit current. This prevents too many electrons flooding across, under the unconstrained influence of the potential difference and overloading some part of the circuit, leading to over-heating or even fire. Such a ‘short-circuit’ is prevented by an old-fashioned fuse-box or modern circuit-breaker. If there is an unexpected load then the fuse will ‘burn out’ or ‘blow’ (at an appropriate load) first or the circuit-breaker will trip for the appropriate part of the ring-main, to prevent damage.

My Error

Was that all obvious to you? I’ve decided to embarrass myself by explaining the bit I misunderstood.

My mental model of the ‘2-way water pump’ concept was that the 1 way pump of the DC circuit was replaced by 2 one-way water pumps that worked in turn, to push water one way then the other.

My current understanding is that both ends of the pipe go into reservoirs. On the Live side is a single pump which alternatively pushes then pulls, or blows then sucks, to give a pressure differential between the two sides, causing the water to move. If you ignore lots of stuff you know about fluids, this model works too.

An alternative model for my misunderstanding of AC would be: 2 large water tanks, connected by a pipe. In the pipe is a flow meter that spins to show water flow. This is analogous to a light bulb. For my original understanding, the tanks were on a see-saw. Water flowed from the top tank to the bottom, whether from the positive or negative side. I can now see that the Neutral tank always stays on the same level and the Live tank is moved up (+ve) and down (-ve.) The relative difference between the tanks is the same but the Live side is always responsible for the potential energy difference.

Under this new model, DC has fixed tanks and stops when the top tank runs out (or you replenish the battery by either replacing it with new tanks full of water or charge its ‘battery’ by pumping water uphill.)

I realise now that I had to ignore quite a lot of evidence to keep believing in my Wrong model. Us humans are good at that.

Safety Advice

Get an authorised electrical professional to do anything more than change a light bulb or a plug, like I ALWAYS do.

Be particularly careful when mixing water and electricity.


2 thoughts on “The Long Shorts of Bi-directional Flow

  1. I’ve spent hours in the past reading things like and and and ….

    (The ELI5 posts are normally quite good when you realise you have no understanding about how something works and need to start at the real basics first!! 🙂 )

    1. The problem with explaining complicated things to 5 year olds is that you tend to make simplifications. I wrote this to work out which of the simplifications in my past has made my mental model wrong. It isn’t intended to help someone safely change a bathroom fan but to fix me. I shared it in case someone else was broken in a similar way.

      I think my bad model was the result of such a simplification by a physics teacher, pre-16. I’ve since done A level and half a degree in physics and worked in IT in the energy industry, talking to electrical engineers about MegaVars shifting frequency, without my wrong model causing me any problems. I discovered along the way that many people follow rules, without any understanding. No surprise there, I guess.

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