Today in class, there was a question about AC vs. DC in homes. What did we originally use? Why did we settle on AC? Why does North America use 110V while Europe uses 220V (and why 60Hz vs. 50Hz)?
The most important question: why AC? This topic is addressed in a 4-5 page discussion in Geoffrey West's brilliant book "Scale". Edison had the first power distributions; they were small DC networks with a single generator. People loved them, and they rapidly expanded from a small number of city blocks to over a dozen. They started to have reliability problems -- problems that did not arise in those first small networks. In short, they would not scale.
Networks needed to branch to step down the voltages from the high ones produced by generators to the ones used in homes. Those branches are crucial; there must be containment/damping of changes in the demand near the endpoints of the distribution network. If there is a high-power device at a factory that is suddenly switched off, that can create a spike of the already-flowing current in the circuits. DC networks behave badly at those distribution junctures; the voltage spike can continue to travel upstream like a wave and destroy both power company equipment and customer devices.
You can see an analogue of this in the plumbing older houses when a fully-open faucet to a bathroom tub is suddenly turned off. Suddenly terminating the energy of the flowing water can create a pressure wave in the pipes: a loud booming sound. This is called water hammer; it can literally destroy the pipes in a house. In modern homes, water hammer is contained by placing water hammer arrestors in the plumbing. These devices are a vessel partially filled with water partially with pressurized air; they absorb the energy of the shockwave in the air-filled part of the chamber. The energy is dissipated as heat -- just like a resistor. The alternative means to deal with water hammer is to gently decrease the flow of water into a tub; this is probably what our grandparents did (and their children had to learn).
While Edison was developing a DC grid, Nicola Tesla was developing an AC grid -- first powered by turbines at Niagara Falls. A properly-designed AC grid does not propagate spikes upstream through the network; there is proper impedance matching at the nodes. Scaling is possible by applying Oliver Heaviside's equations for electrical impedance.
After the section on the electrical grid, author Geoffrey West describes our arterial network. Our arteries can be modeled with [mechanical] impedance; energy storage is provided by the elastic expansion and contraction of the arterial walls. [Side note: this is why "hardening of the arteries" is bad: it decreases our vital capacity for energy storage and release in our pulsing arterial network.] Back-flow must never happen, because back-flow would create eddies in the arteries and precipitate blood clots. Nature knows the precise dimensions of arteries and their branches; it maintains this precise geometric relationship as we grow from the developing embryo to adulthood.
In our circulatory network, the arteries are AC, but most of the pumping energy has petered out by the time we get to the capillaries. In other words, the capillaries of our network -- its end-nodes -- are essentially DC. This is similar to many of our electronic devices: they connect to an AC source but the electronic circuits are DC; the power is first converted by a power supply. Home design is rapidly adapting to simply provide DC to our electronic devices -- avoiding the need for a separate power supply in each device. AC outlets started adding DC USB-A sockets a few years ago. Newer outlets are now providing higher-power USB-C connectors. In the main grid, we must use AC to avoid propagating the turbulence of power transitions. In the home, we generally prefer DC for most of our electrical devices. This mirrors the design of nature.
North America standardized on 110V because we already had many incandescent electrical bulbs; those fragile filaments could not tolerate any higher voltage. Europe doubled the voltage, because it generates less heat in the grid; they figured out how to step down the voltage in the home when required. North America seems to have chosen 60Hz simply because Nicola Tesla liked that number; someone in Europe (apparently Emil Rathenau of AEG) liked 50Hz. Once a particular frequency was chosen by the "first" business, it was essential that all other installations use that same frequency. Power supplies for laptop computers and other devices -- converting AC to DC -- are fully capable of dealing with a range 110-240VAC and 50Hz-60Hz.
