Electricity & Magnetism

What this part of the map covers

Electricity and magnetism were once studied as two separate mysteries. This branch shows how they turned out to be two sides of the same thing, and how that discovery powers the modern world. It starts with electric charge, follows it into currents and magnets, and ends with induction, the effect behind every generator.

Electric charge

Electric charge is a property carried by the particles inside atoms: electrons and protons. There are two kinds, which Benjamin Franklin named positive and negative.^[1] Charges of the same sign push each other away, and charges of opposite sign pull together.^[1]

Electrons carry negative charge, and protons carry exactly the same amount of positive charge.^[1] An object becomes charged when electrons are added to it or taken away from it. Charge can be moved from one object to another, but the total amount in the universe never changes.^[1]

Electric current

When charge flows, it’s called an electric current: the rate at which charge flows past a point.^[2] Current is measured in amperes (amps). One ampere is one coulomb of charge passing a point every second,^[2] which is about 6.24 billion billion elementary charges each second.^[8]

Currents cover a huge range. A handheld calculator can run on as little as 0.3 milliamps, while starting a truck’s engine takes about 180 amps.^[2] A hair dryer takes around 15 amps, and a lightning bolt 100,000 amps or more.^[8] Since 20 May 2019, the ampere itself has been defined using the charge of a single electron or proton, a fixed constant of nature.^[8]

Energy and power

Electrical devices turn electrical energy into other forms, such as light and heat. The power a device uses, in watts, is the current through it times the voltage across it: P = I × V.^[3] Electricity bills count energy in kilowatt-hours, and one kilowatt-hour equals 3.6 million joules.^[3] Efficient devices make a real difference: a 20-watt LED bulb can replace a 100-watt incandescent bulb that gives the same light.^[3]

Magnetism

Magnetism has been known since the time of the ancient Greeks.^[4] Every magnet has a north pole and a south pole. Like poles repel and opposite poles attract.^[4]

In 1600 William Gilbert, physician to Queen Elizabeth I, published De Magnete, in which he argued that Earth itself is a giant magnet. He also coined words we still use, such as “electricity” and “magnetic pole”.^[10] Modern physics puts it the same way: Earth acts like a very large bar magnet.^[4]

One force, two faces

In 1819 the Danish physicist Hans Oersted noticed that a compass needle moved whenever current flowed in a nearby wire.^[4] An electric current, in other words, creates magnetism. André-Marie Ampère went further and proposed that electric currents are the source of all magnetic effects.^[4]

In 1831 Michael Faraday found the reverse effect. Pushing a magnet into or out of a coil of wire produces a voltage in the coil, and the faster the motion, the bigger the voltage.^{[5, 12, 11]} This is electromagnetic induction. Generators in power plants use it: a coil turns inside a magnetic field, driven for example by falling water, steam from burning fuel, or the wind.^[6]

Induction still powers the modern world. According to the US Energy Information Administration, generators driven by turbines produce nearly all of the electricity in the United States. Solar panels, which work differently, made about 3% of utility-scale electricity there in 2022.^[11]

Where to go next

• Electric Charge: the property behind it all, and why things get “static”.
• Magnetism: poles, compasses and Earth’s magnetic field.
• Electromagnetic Induction: generators, transformers and the power grid.
• Over in Technology & Engineering, Electronics shows how these ideas are put to work in circuits and chips.