Dictionary

The four fundamental equations of electromagnetism that describe how magnetic and electric influences (in physics language: electric and magnetic fields) are produced: Electric fields are produced whenever there are electric charges or, alternatively, when magnetic fields change over time. Magnetic fields are produced whenever there are electric currents (moving electric charges), but also whenever electric fields change over time. The fact that electric and magnetic fields can exist without the presence of charges or currents, simply by mutual excitation where a change in the magnetic field produces an electric field and vice versa, is the basic phenomenon behind electromagnetic waves.