How Electricity Works

Everything is made of atoms, and atoms contain tiny particles with electric charge. Protons in the center are positive. Electrons around the outside are negative. Opposite charges attract and like charges repel. That push and pull is the force behind all electricity.

In metals like copper, some electrons are only loosely held, so they can drift from atom to atom. That's why metals are conductors. Rubber, plastic, glass and dry wood hold their electrons tightly, so they're insulators. A power cord is both: copper inside to carry the current, plastic outside to keep it away from you.

Current is charge flowing past a point. It's measured in amperes (amps, A). One amp means one coulomb of charge passes each second, and a coulomb is about 6.24 billion billion electrons.

Electrons are slow, the signal is fast. Each electron in a wire only drifts along at a fraction of a millimeter per second. The light comes on instantly because the wire is already full of electrons: push on one end and they all start moving at once, like marbles in a full tube.

Which way does current flow? Long before electrons were discovered, scientists picked "positive to negative" as the direction of current. Electrons actually move the other way. Circuit diagrams still use the old "conventional current" direction, and the math works the same either way.

For current to flow there has to be a closed loop, a circuit, from one side of the source back to the other. A switch simply opens a gap in the loop. Open the gap and the flow stops everywhere in that loop.

The classic way to picture a circuit is water in pipes. It isn't perfect, but it gets the main ideas right:

Voltage is the difference in electrical "pressure" between two points. A battery uses a chemical reaction to keep one terminal at a higher voltage than the other. A AA battery gives 1.5 V, a car battery about 12 V, and US wall outlets about 120 V. Voltage is always measured between two points, which is why a bird can sit on a single power line safely: both feet are at the same voltage, so almost no current flows through it.

Resistance is how much a material fights the flow. Every part of a circuit has some. It goes up when a wire is:

• Thinner, like a narrow pipe. That's why big appliances use thick cords.
• Longer, because charge has farther to squeeze through.
• Hotter, for most metals.
• Made of a poorer conductor. Copper and silver are excellent. The tungsten in an old-style bulb is much worse, which is the point: resistance turns electrical energy into heat and light.

Resistance isn't bad. A toaster, a space heater and an incandescent bulb all work because of resistance. The trick is to put it where you want heat, and keep it low in the wires.

In 1827 Georg Ohm showed that voltage, current and resistance are linked by one simple rule. More push means more flow. More resistance means less flow.

Know any two and you can find the third. A handy trick: cover the one you want in the triangle V over I × R. Cover I and you see V over R.

Real bulbs bend the rule a little. Ohm's law is exact for an ideal resistor. A bulb filament's resistance rises a lot as it heats up, so a cold bulb draws a big burst of current for a split second when switched on. The playground below treats each bulb as a fixed resistor to keep things clear.

Your home is wired in parallel. That's why every outlet gives the same 120 V, and turning off the lamp doesn't turn off the fridge. It also means each thing you plug in adds more current to the circuit. Plug in too much and the total current trips the breaker.

Old-style holiday light strings were wired in series, which is why one dead bulb could black out the whole string. Batteries in a flashlight are also in series: two 1.5 V cells in a row give 3 V.

Why use AC for the grid? Because transformers can easily raise or lower AC voltage. Power plants step it up to very high voltages for long-distance lines, which lets the same power travel with much less current. Less current means less energy wasted as heat in the wires. Transformers near your home step it back down. This advantage is why Westinghouse and Tesla's AC system won out over Edison's DC in the 1890s.

Most of your electronics actually need DC, so their chargers and power supplies convert AC to DC. That's what the "brick" on a laptop cord does.

120 V is an average. AC voltage is quoted as an effective (RMS) value that delivers the same heating as that much DC. The actual peaks of US household power reach about 170 V. US homes also get 240 V for big loads like dryers, ovens and many water heaters.

Power is how fast electrical energy is used, measured in watts (W). It's voltage times current:

Your power company bills you for energy, which is power × time, in kilowatt-hours (kWh). A 1,500 W heater running for 2 hours uses 3 kWh. To see what that costs and which appliances are the biggest users, read Watts, kWh and your electricity bill.

P = I² × R explains why the grid uses high voltage, and why loose or undersized wires get hot: double the current through the same resistance and you get four times the heat.

How electricity hurts people

It's current through your body that does the damage, and it takes surprisingly little. Your skin's resistance drops a lot when it's wet, so the same voltage pushes much more current through you. OSHA gives these rough effects for 1 second of current from hand to foot:

A standard household breaker is 15 or 20 amps, that is 15,000 to 20,000 mA. OSHA notes that fuses and breakers protect wires and equipment from overheating, not people. That's the job of a GFCI (ground-fault circuit interrupter), the outlet with Test and Reset buttons. It shuts off power when it detects as little as about 5 mA leaking somewhere it shouldn't, in as little as 1/40 of a second.

• Keep electrical devices away from water, and never touch them with wet hands.
• Use GFCI outlets in bathrooms, kitchens, garages, basements and outdoors. Press Test and Reset once a month.
• Don't overload outlets or daisy-chain power strips. Warm plugs, flickering lights or a burning smell mean trouble.
• Replace cords that are frayed, cracked or pinched. Never run cords under rugs.
• Turn off the breaker before any electrical repair, and confirm it's off with a voltage tester.
• Stay at least 10 feet from overhead power lines, and assume any downed line is live. Call 911 and the utility.

If someone is being shocked, don't touch them. Shut off the power at the breaker or unplug the device first. If you can't, call 911 and wait. Once the power is off, call 911 and start CPR if they aren't breathing normally. See First aid basics.