Electricity and Magnetism United
Magnetism and electricity, once thought separate, are two aspects of one electromagnetic force — a unification that is among physics' greatest achievements and the basis of motors, generators, and data storage.
The key discoveries:
- Moving charges create magnetic fields. A current-carrying wire produces a magnetic field circling around it. Electricity makes magnetism.
- Magnetic fields exert forces on moving charges. A charge moving through a magnetic field feels a force perpendicular to both its velocity and the field. This force is what spins an electric motor — current in a field experiences a force that produces rotation.
- Electromagnetic induction (Faraday's law). A changing magnetic field induces a voltage (and current) in a nearby loop. Crucially, it is the change that matters — a steady field induces nothing; only a changing flux drives a current. This runs in reverse of the motor: motion in a field generates electricity, the principle of every generator and of the read heads that recover data from a spinning disk.
So electricity and magnetism are deeply intertwined: currents make fields, changing fields make currents. Maxwell's synthesis showed that oscillating electric and magnetic fields sustain each other and propagate as electromagnetic waves — which is light (and radio, and every wireless signal). The screen you are reading, the Wi-Fi carrying this text, and the hard drive storing it all rest on electromagnetism.
Common pitfall: thinking a steady magnetic field induces a current, when only a changing field does. Faraday's law of induction depends on the rate of change of magnetic flux — a magnet sitting motionless near a coil induces nothing; the current appears only while the field is changing (the magnet moving, or the current producing it varying). "Field present" is not enough; "field changing" is the requirement.