What Electricity Is

A single AA battery can sit in a drawer for a year and do nothing. It holds 1.5 volts the whole time, ready, and spends none of it. Lay a short piece of copper wire across it so one end touches the flat terminal and the other touches the bump, and within seconds the wire turns warm, then hot, and the battery starts draining fast. Nothing was added to the battery. It had everything it needed the entire time it sat in the drawer. The only thing that changed is that the wire gave the charge a path to move through.

That warm wire is the whole subject of this lesson. Electricity is not a substance that pours out of a battery the way water pours from a bottle. It is what happens when electric charge gets a complete path to move along, and the four words that describe that movement, voltage, current, resistance, and power, are the vocabulary every circuit in every later lesson is built from.

Charge that moves

Everything electrical starts with charge. Charge is a basic property of matter, the same kind of built-in property as mass. Some particles carry it. The one that matters for wires is the electron, a tiny particle that sits in every atom and carries a small negative charge. A copper wire is packed with electrons that are loosely held and free to drift from atom to atom. Left alone they jostle in place and go nowhere in particular.

Give those free electrons a reason to move in one direction and the aimless jostling becomes a flow. That flow is current. Current is the movement of electric charge through a material, and it is measured in amperes, almost always shortened to amps. One amp is a precise amount: one coulomb of charge passing a point every second, where one coulomb is about 6.24 times 10^18 electrons. So a wire carrying one amp has roughly six billion billion electrons crossing any given point in it each second.

Current is a rate, not a quantity sitting still. It only means something while charge is actually moving. A battery on its own carries no current, the way a river with a dam across it carries no flow. The current is the flowing itself.

One detail trips up almost everyone, so it is worth stating plainly and then setting aside. The electrons in a wire drift from the negative terminal of a battery toward the positive one. But by a convention fixed long before anyone knew electrons existed, current is drawn and described as flowing the other way, from positive to negative. That backwards convention is baked into every schematic and every equation in this course. The math comes out identical either way, so the standard move is to follow the convention, treat current as flowing from plus to minus, and not lose sleep over the electrons drifting the opposite direction underneath.

Voltage is the push

Free electrons will not move in any organized way without something driving them. That driver is voltage. Voltage is the difference in electrical pressure between two points, the push that sets charge moving from one place to another. Its unit is the volt. A battery labeled 1.5 volts maintains a 1.5 volt difference between its two terminals, and that difference is exactly the push that shoves current around any path connected across it.

The precise meaning of a volt is energy per unit of charge: one volt is one joule of energy handed to every coulomb of charge that moves. A higher voltage pushes harder on each unit of charge. This is why a 9 volt battery drives more current through the same wire than a 1.5 volt battery does. It leans on the charge with six times the pressure.

Voltage is always a difference between two points, never an amount sitting at one point on its own. A single terminal does not have a voltage in isolation, the way one end of a table does not have a height difference. It only makes sense to talk about the voltage between the two terminals, or between one point and some agreed reference. This matters more than it sounds, and forgetting it is behind a lot of confused early measurements.

A pipe carrying water is the analogy people reach for, and it holds up well enough to be worth using. Voltage is the water pressure, the force pushing the water along. Current is the flow rate, how much water actually moves past a point each second. Higher pressure pushes more water through the same pipe, exactly as higher voltage pushes more current through the same wire.

Resistance is the opposition

Nothing lets charge through for free. Every material pushes back against current to some degree, and that opposition is resistance. Resistance is measured in ohms, written with the Greek letter omega. A material with high resistance lets only a trickle of current through for a given push. A material with low resistance lets a flood through for the same push. Copper has very low resistance, which is exactly why wire is made of it. Air has enormous resistance, which is why charge does not normally leap out of a socket into the room.

In the water picture, resistance is how narrow the pipe is. A wide pipe barely fights the flow. A pinched, narrow pipe lets only a thin stream through even under high pressure. Squeeze the pipe and the flow drops without touching the pressure at all. Raise the resistance in a circuit and the current drops without touching the voltage.

One ohm has an exact definition that ties all three quantities together: one ohm is the resistance that allows exactly one amp of current to flow when pushed by one volt. That single sentence is the seed of the most useful equation in electronics, and it arrives a few paragraphs from here.

A circuit has to be a loop

Charge will not flow unless it has a complete path back to where it started. A circuit is that complete path: an unbroken loop from one terminal of the source, through whatever the current passes on the way, and back to the other terminal. Break the loop anywhere and the current stops everywhere, instantly and completely.

This is why the AA battery in the drawer does nothing. Its two terminals are not connected, so there is no loop, so there is no current. The moment the copper wire bridges both terminals the loop closes and current flows around it: out of one terminal, through the wire, and back into the other. Lift one end of the wire and the loop opens and the current dies at once. There is no such thing as current pouring out of a battery and off into empty air. Charge flows in a ring or it does not flow at all.

Ohm's law ties them together

Voltage, current, and resistance are not three unrelated facts. They are locked together by one equation, the single most used relationship in all of electronics. It is Ohm's law:

V = I times R

Voltage equals current times resistance. V is voltage in volts, I is current in amps (the letter I, for intensity, is the traditional symbol), and R is resistance in ohms. Knowing any two of the three fixes the third, because the equation rearranges into whichever form the problem needs:

I = V / R     current, when voltage and resistance are known
R = V / I     resistance, when voltage and current are known

Worked example. A 5 volt source is connected across a total resistance of 100 ohms. The current is I = V / R = 5 / 100 = 0.05 amps, which is 50 milliamps. A milliamp is one thousandth of an amp, and small circuits are usually measured in milliamps rather than whole amps. Raise the resistance to 1000 ohms and the current falls to 5 / 1000 = 0.005 amps, or 5 milliamps. The push never changed. The tighter opposition simply let less through, exactly as a pinched pipe would.

Run it the other way. Suppose a meter reads 0.02 amps flowing while a 9 volt battery drives the loop. The resistance in the path must be R = V / I = 9 / 0.02 = 450 ohms. Ohm's law pins down an unknown without ever measuring it directly, which is most of what troubleshooting a circuit actually comes down to.

Power is the rate of work

Current moving through resistance does work. It delivers energy, and that energy shows up as heat, light, motion, or sound depending on what the current is passing through. The rate at which a circuit delivers energy is power, measured in watts. One watt is one joule of energy per second.

Power has its own short equation that follows directly from the two quantities already in hand:

P = V times I

Power equals voltage times current. A 5 volt source pushing 0.05 amps delivers P = 5 times 0.05 = 0.25 watts. That quarter of a watt is a small, tame amount of heat. The warm wire from the opening is the same idea run to an extreme.

A copper wire has very low resistance, perhaps 0.1 ohm for a short piece. Lay it straight across a 1.5 volt battery and Ohm's law predicts the current: I = V / R = 1.5 / 0.1 = 15 amps, if nothing else held it back. In practice the battery's own internal resistance drags that number down, but a large current still forces its way through, and the power dumped into a few centimeters of metal, P = V times I, turns straight into heat. That is why the wire gets hot fast and the battery drains in minutes.

A battery and a wire, on their own, can only make heat. Everything else a circuit does, limiting current on purpose, lighting a lamp, reading a sensor, comes from parts placed deliberately in the loop, each one chosen for the resistance or the behavior it adds. The next lesson, components and schematics, introduces those parts, the resistor first among them, along with the shorthand drawings that let a whole circuit fit on a single page without a photograph of a single wire.