Electricity and magnetism were considered separate phenomena until the 19th century. Hans Christian Oersted discovered that a compass needle deflects near a current-carrying wire. Michael Faraday showed that a changing magnetic field creates an electric current. James Clerk Maxwell unified these observations into four elegant equations that predict electromagnetic waves, including light.
Coulomb’s law describes the force between two electric charges. Like charges repel, opposite charges attract. The force is proportional to the product of the charges and inversely proportional to the square of the distance. Our Coulomb’s Law Calculator computes electrostatic forces. An electron and a proton in a hydrogen atom exert about 8.2 times 10 to the negative 8 newtons on each other. Tiny in absolute terms, but enormous on the atomic scale.
Electric fields surround charged objects. The field points away from positive charges and toward negative charges. When you rub a balloon on your hair, you transfer electrons, creating a charge separation. The balloon sticks to the wall because the electric field induces an opposite charge on the surface. Static electricity is annoying in winter but useful in photocopiers and laser printers.
Magnetic fields are created by moving charges. A straight wire carrying current produces circular magnetic field lines around it. A coil of wire, or solenoid, concentrates the field inside and creates something similar to a bar magnet. Electromagnets are everywhere. Junkyard cranes use them to pick up cars. MRI machines use superconducting magnets to image the body. Speakers use them to vibrate cones and produce sound.
Faraday’s law of induction is the principle behind generators and transformers. Move a magnet through a coil, and a current flows. This is how power plants generate electricity. Spin a coil in a magnetic field, and the changing flux induces a voltage. The same principle in reverse drives electric motors. Motors and generators are essentially the same device running in opposite directions.
Maxwell’s equations predict that accelerating charges emit electromagnetic waves. These waves travel at the speed of light because, well, light is an electromagnetic wave. Radio, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays are all the same phenomenon at different frequencies. Your phone communicates via radio waves. Your microwave heats food with waves at about 2.45 GHz.