Tired of smiling and nodding along while your electrical engineering buddies debate the finer points of electromagnetic theory? If you're taking a critical eye to the definitive guide to batteries, you've got to understand what electricity is to begin with. Here's a crash course on the fundamental force that's driving our digital revolution.
For the countless permutations of matter and the infinite variety of ways it can behave, all of creation is governed by a quartet of fundamental forces. We hardly notice the Strong Nuclear force, which binds quarks together into neutrons, or the Weak Nuclear force, which accounts for many forms of radioactive decay, because they operate on the subatomic level. On the macro scale, conversely, we can easily appreciate the effects of Gravity every time we step out of doors and are not flung off the surface of the Earth, but other than keeping us grounded, it doesn't do a whole lot. Electromagnetism, also known as quantum electrodynamics, is the driving force of the world as we know it. Responsible for the bonds that weld individual atoms together into molecules, virtually every physical phenomenon we can sense without technological aids, be it from heat from a fire or light from the sun—not to mention all of chemistry—can be attributed to the Electromagnetic force.
Electromagnetism exists as either a magnetic or an electrical field and relies on another intrinsic property of atoms—their electrical charge. Charge, both the positive and the negative varieties, is a property of subatomic particles such as protons and electrons. It exists in discrete units, like photons, and is measured in Coulombs (that's the amount of charge moved by one ampere in once second).
The property of electromagnetism also determines whether particles exhibit electrostatic attraction or repulsion—similar charges repel one another, opposite charges attract one another. The strength of this attraction is governed by Coulomb's Law, which states that electrostatic force between two particles is proportional to the product of their charges, and inversely proportional to the square of their distance from one another.
Normally, an atom contains the same number of positively-charged protons in its nucleus as has negatively-charged electrons orbiting it. This results in a stable and neutral net charge of 0. However, if a few electrons were to be dislodged from their respective atoms via chemical reactions, these subatomic particles will drift a short distance and resettle on nearby atoms that are themselves short an electron. Now if a large contingency of electrons happen to flow through, say, a metal wire, well you've got yourself an electric current.
An electric current is a movement of electric charge. The magnitude of flowing current is measured in amperes, and amperes are calculated by timing how long it takes one coulomb of electrons to pass a set point in an electric circuit. One coulomb of electrons (6.241 × 1018) per second makes one ampere.
Now, the direction of this flow is a little tricky to understand, since an electric circuit technically has two currents—negative charge moving in one direction and an equal, positive charge moving in the other. For simplicity, scientific tradition arbitrarily dictates that a "conventional" current flows in the same direction as the positive charge—opposite that of the electrons—towards the most negative part of the circuit, the ground. However, since the flow of positive charge in one direction forces negative charge to flow in the other direction, the circuit just operates normally.
There are two types of current. Direct current, borne from Thomas Edison's workshop, always flows unidirectionally from the power source to the ground. Alternating current, the brainchild of Nikola Tesla, regularly reverses the direction of its flow over time. While DC currents can be transformed into AC using an inverter and the process can be reversed with the help of a rectifier, their unique flow properties make AC and DC currents useful on very different scales.
DC's steady unidirectional current is very handy over the short distances of circuits, electric vehicles, and renewable resources such as solar cells. But it cannot easily step its voltage up or down like AC can. In a DC power transmission system, the 100V created by a distant power plant and distributed across the power grid will be the same 100V coursing through your house. This is massively inefficient over any appreciable difference because resistance grows within great lengths of transmission wires. That means much of the electrical load is lost as waste heat. As such, DC power plants would need to be built close to the communities they serve in order to be economically feasible.
In an AC power transmission system, however, the high-voltage current passing through transmission lines is stepped down to as little as a tenth of the original with the help of transformers and then rectified to DC for use in the home. This allows customers to run devices on much smaller, safer voltages and allows power plants to be located further away from customers while transmitting power without nearly as much of it lost to waste heat.
The shortcomings of DC power over long distances is due to a pair of electrical forces—voltage and resistance. Voltage, measured in volts, is defined as "the practical meter-kilogram-second unit of electrical potential difference and electromotive force equal to the difference of potential between two points in a conducting wire carrying a constant current of one ampere when the power dissipated between these two points is equal to one watt and equivalent to the potential difference across a resistance of one ohm when one ampere is flowing through it." In English, that means voltage measures the electric potential difference between two points—essentially, the amount of work or total energy needed to push one ampere of power between between them, divided by the magnitude of the charge. It can be thought of as the amount of electrical pressure, or tension, on the transmission line.
Resistance, on the other hand, determines how hard a current will have to work to move through a line—think of it as electrical friction—and is measured in ohms. One ohm is equivalent to the resistance generated within a circuit where one volt of potential difference produces one ampere of current. Every material known to science has some degree of resistance—save of course for super-conducting materials. Longer lines exhibit higher resistance while wider ones are less resistive—just as water flows more easily through wider pipes, so does electricity.