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Discovered in the 19th century, the electromagnetic force (or electromagnetism) is a unification of the electrostatic force and the magnetic force. In the mid-20th century, this force was explained in a framework of quantum mechanics called quantum electrodynamics, or QED. In this framework, the electromagnetic force is transferred by particles of light, called photons.

The relationship between electricity and magnetism is covered in Chapter 5, but the basic relationship comes down to electrical charge and its motion. The electrostatic force causes charges to exert forces on each other in a relationship that’s similar to (but more powerful than) gravity — an inverse square law. This time, though, the strength is based not on the mass of the objects but on the electrical charge.

The electron is a particle that contains a negative electrical charge, while the proton in the atomic nucleus has a positive electrical charge. Traditionally, electricity is seen as the flow of electrons (negative charge) through a wire. This flow of electrons is called an electrical current.

A wire with an electrical current flowing through it creates a magnetic field. Alternately, when a magnet is moved near a wire, it causes a current to flow. (This is the basis of most electric power generators.)

This is the way in which electricity and magnetism are related. In the 1800s, physicist James Clerk Maxwell unified the two concepts into one theory, called electromagnetism, which depicted the electromagnetic force as waves of energy moving through space.

One key component of Maxwell’s unification was a discovery that the electromagnetic force moved at the speed of light. In other words, the electromagnetic waves that Maxwell predicted from his theory were a form of light waves.

Quantum electrodynamics retains this relationship between electromagnetism and light because, in QED, the information about the force is transferred between two charged particles (or magnetic particles) by another particle — a photon. (Physicists say that the electromagnetic force is mediated by a photon.)