physics//electromagnetism

Electromagnetism is the branch of physics that describes how electric charges push and pull on each other through electric and magnetic fields, and it is the physics behind every circuit, motor, transformer, radio and sensor an engineer builds. Three ideas carry most of it: a charge creates an electric field, a moving charge (a current) also creates a magnetic field, and a magnetic field that changes creates an electric push of its own, which is electromagnetic induction.

Electromagnetism is the branch of physics that describes how electric charges push and pull on each other through electric and magnetic fields, and it is the physics behind every circuit, motor, transformer, radio and sensor an engineer builds. Three ideas carry most of it: a charge creates an electric field, a moving charge (a current) also creates a magnetic field, and a magnetic field that changes creates an electric push of its own, which is electromagnetic induction.

The practical side is the electric circuit: a closed path where a voltage source pushes current through components that resist it, store it or turn it into heat, motion or light. Almost every device in this notebook is a circuit plus a few fields doing something useful.

Charge, current, change.

A still charge gives an electric field, a moving one adds a magnetic field, and a changing magnetic field gives a current back; engineering lives on that last loop, because it lets energy move without any wire touching.

The fields are real enough to be engineered. A dielectric carries an electric field but no current, which is how a capacitor works; a coil concentrates a magnetic field, which is how an inductor and a transformer work.

When the fields change fast enough they detach from their source and travel on their own as an electromagnetic wave: light, radio and the signals a magnetometer or an antenna picks up are the same phenomenon at different frequencies.

Some of it is still classical folklore at the edges. Why rubbing two materials charges them, static electricity, is understood in outline and still argued over in detail.