What is electricity? Charge, current and voltage, explained

Electricity is the set of effects produced by electric charge, a basic property of the particles that make up atoms. Charge at rest gives static electricity, the spark from a doorknob. Charge moving through a wire is an electric current, which carries energy from a power station or battery to whatever is plugged in. Four quantities describe how it works: current, voltage, resistance and power.

In short

• Electric charge comes in two kinds, positive and negative. Like charges repel, unlike charges attract, and electrons carry the negative charge that moves in wires.
• Voltage is the push, current is the flow of charge, and resistance is what opposes it: V = I × R.
• Power is voltage times current, P = V × I. Energy used is power times time, which is what an electricity bill counts in kilowatt-hours.

What is electric charge?

Every atom has a nucleus of positively charged protons, surrounded by negatively charged electrons. The two charges are exactly equal in size and opposite in sign, so a whole atom is neutral. Charges of the same sign repel and opposite signs attract, and the force between them is enormously stronger than gravity: it is what holds atoms and molecules together.

Charge is measured in coulombs. Since 20 May 2019 the charge of one proton, the elementary charge, has been fixed at exactly 1.602176634 × 10⁻¹⁹ coulombs, and the ampere is defined from it, as NIST explains. One coulomb is the charge of about 6.24 × 10¹⁸ electrons.

Static electricity is charge that has built up and stays put. Rub a balloon on your hair and electrons move from hair to balloon; the balloon is now negative, your hair positive, and they attract. Lightning is the same thing on a vast scale: charge separates inside a storm cloud until the air breaks down and a current flows in a flash.

What are current, voltage and resistance?

Current is the rate at which charge flows past a point, measured in amperes, or amps (A). One amp is one coulomb a second, about 6.24 × 10¹⁸ electrons passing every second. In a metal wire, the charge that moves is the outer electrons of the metal's atoms, which are free to wander.

Voltage, measured in volts (V), is the push that drives the current. Strictly, it is the energy each coulomb of charge gains or gives up between two points: 1 volt means 1 joule per coulomb. A 9 V battery gives every coulomb that passes through it 9 joules of energy, which the circuit then turns into light, heat or motion.

Resistance, measured in ohms (Ω), is how much a material opposes the current. Electrons moving through a wire keep bumping into the metal's atoms, losing energy as heat. Copper has low resistance; the thin filament or heating coil in an appliance has much more, which is the point: that is where the energy is released.

The three are tied together by Ohm's law: V = I × R, voltage equals current times resistance. Put the other way, I = V ÷ R: more push gives more current, more resistance gives less. And power, the rate at which energy is delivered, in watts (W), is P = V × I.

The water-pipe picture, and where it breaks

A common way to picture a circuit is water in pipes. Voltage is the water pressure, current is the flow rate, resistance is a narrow section of pipe, and a battery is a pump. It gets a lot right: higher pressure pushes more flow, a narrow pipe allows less, and the pump doesn't create water, it moves it round.

It breaks down in three places. First, the electrons themselves move astonishingly slowly. OpenStax's physics text puts their typical drift speed at around 10⁻⁴ m/s, a tenth of a millimeter a second, so an electron would take hours to travel a meter of wire. Yet a light comes on the instant you flip the switch, because the push travels through the wire at a large fraction of the speed of light, moving every electron at once. Second, cut a water pipe and water pours out; cut a wire and the current simply stops, because charge needs a complete loop. Third, in the alternating current from a wall socket, the electrons don't flow along at all: they shuffle back and forth 50 or 60 times a second, and energy still flows one way, to the appliance.

A worked example: one kettle on 120 V and 230 V

Take a 2,000-watt electric kettle. In North America, household outlets supply about 120 V; in Europe and much of the world, about 230 V.

1. Current, from P = V × I, so I = P ÷ V. At 120 V: 2,000 ÷ 120 = 16.7 A. At 230 V: 2,000 ÷ 230 = 8.7 A. In practice 16.7 A is more than many North American household circuits are built to supply, which is why kettles sold there are usually rated lower, nearer 1,500 W.
2. Resistance of the heating element, from R = V ÷ I. At 120 V: 120 ÷ 16.7 = 7.2 Ω. At 230 V: 230 ÷ 8.7 = 26.5 Ω. A kettle built for one voltage has a different element from one built for the other.
3. Energy to boil 1 liter of water from 20 °C. Heating 1 kg of water by 80 °C takes about 1 × 4,186 × 80 = 334,880 joules, a calculation explained in the note on temperature. At 2,000 joules a second, that takes 334,880 ÷ 2,000 = 167 seconds, just under 3 minutes, if no heat escapes.
4. Energy in kilowatt-hours, the unit on a bill. One kilowatt-hour (kWh) is 1,000 watts for one hour, 3.6 million joules. So 334,880 J ÷ 3,600,000 = 0.093 kWh.

The cost of each boil is then 0.093 × the price of a kWh in your area. The same rule works for any appliance: cost = power in kilowatts × hours of use × price per kWh. Notice that at 120 V the kettle draws nearly twice the current for the same power. The heat lost in a wire grows with the square of the current (P = I² × R), which is one reason long-distance power lines run at hundreds of thousands of volts: high voltage, low current, less waste.

Where does electricity come from?

Electricity isn't a primary energy source like coal or sunlight; the U.S. Energy Information Administration calls it an energy carrier, made by converting another source. Most of it is made by generators, which spin coils of wire past magnets, and that is no coincidence: electricity and magnetism are two sides of one force, as the note on magnets explains. Steam from burning fuel or from a nuclear reactor turns the turbines in most power stations; wind and falling water turn them directly. Solar panels are the main exception, turning light straight into current with no moving parts, one of the renewable sources.

Batteries make current by chemistry: a reaction at one terminal releases electrons and a reaction at the other takes them in, so electrons flow through any wire connecting the two.

Generators naturally make alternating current, and transformers can raise or lower its voltage easily, which is why grids carry AC. In the 1880s and 1890s Thomas Edison backed direct current while George Westinghouse and Nikola Tesla backed alternating current; AC won for the grid. Electronics, from phones to computers, run on direct current, so their chargers convert it.

What do people get wrong about electricity?

The most common mistake is that electricity is used up. It isn't: the charge goes round the loop and comes back. What a device uses is energy, carried by the charge and turned into light, heat or motion. A second is that the electrons in a wire race along; they crawl, and only the signal is fast. A third is that voltage alone is what's dangerous. What harms the body is current through it, and current depends on voltage and on resistance, which is why the same voltage is far more dangerous to wet skin than dry.

Check yourself

• Without looking back: what is the difference between voltage and current, in one sentence each?
• If electrons drift so slowly, why does a light come on the moment the switch is flipped?
• Why do long-distance power lines carry electricity at such high voltages?

Sources

• Ampere: Introduction — National Institute of Standards and Technology
• 9.2 Model of Conduction in Metals, in University Physics Volume 2 — OpenStax, Rice University
• 9.4 Ohm's Law, in University Physics Volume 2 — OpenStax, Rice University
• 9.5 Electrical Energy and Power, in University Physics Volume 2 — OpenStax, Rice University
• Electricity explained — U.S. Energy Information Administration
• Electricity — Wikipedia