If you brush your hair several times, your hair may stand on end and be attracted to the brush. Why? The movement of the brush imparts electrical charges to each hair and the identically charged individual hairs repel each other. Similarly, if you place identical poles of two bar magnets together, they will repel each other. But set the opposite poles of the magnets near one another, and the magnets will attract each other. These are familiar examples of electromagnetic force; opposite charges attract, while like charges repel.
Scientists have studied electromagnetism since the 18th century, with several making notable contributions.
- In 1785, famed French physicist Charles Coulomb described the force of electrically charged objects as directly proportional to the magnitudes of the charges and inversely related to the square of the distances between them. Like gravity, electromagnetism has an infinite range.
- In 1819, Danish physicist Hans Christian Oersted discovered that electricity and magnetism were very much related, leading him to declare that an electric current generates a magnetic force.
- British-born physicist and chemist Michael Faraday weighed in on electromagnetism, showing that magnetism could be used to generate electricity in 1839.
- In the 1860s, James Clerk Maxwell, the Scottish math and physics whiz, derived equations that described how electricity and magnetism were related.
- Finally, Dutchman Hendrik Lorentz calculated the force acting on a charged particle in an electromagnetic field in 1892.
When scientists worked out the structure of the atom in the early 20th century, they learned that subatomic particles exerted electromagnetic forces on each other. For example, positively charged protons could hold negatively charged electrons in orbit around the nucleus. Furthermore, electrons of one atom attracted protons of neighboring atoms to form a residual electromagnetic force, which prevents you from falling through your chair.
But how does electromagnetism work at an infinite range in the large world and a short range at the atomic level? Physicists thought that photons transmitted electromagnetic force over large distances. But they had to devise theories to reconcile electromagnetism at the atomic level, and this led to the field of quantum electrodynamics (QED). According to QED, photons transmit electromagnetic force both macroscopically and microscopically; however, subatomic particles constantly exchange virtual photons during their electromagnetic interactions.
But electromagnetism can't explain how the nucleus holds together. That's where nuclear forces come into play.