Magnetism is a core phenomenon underlying motors, transformers, inductors, and nearly all modern electronics. Current, materials, and electromagnetic laws govern how magnetic fields behave. A magnetic field is created when a current flows through a conductor. Magnetic fields arise only when charges are in motion. Static voltage (no field) and current flow (field present). The amount of current determines magnetic field strength, doubling the current doubles the magnetic field strength. It is the proportional relationship between current magnitude and magnetic field intensity.
The magnetic property of materials is permeability, which is the ratio of magnetic flux density to the magnetizing force . High‑permeability materials, such as iron and ferrites, concentrate magnetic fields and are used in cores. Reluctance is the opposition to the creation of magnetic lines of force in a magnetic circuit, compared to electrical resistance, but for magnetic flux. Lenz’s Law states that induced currents create magnetic fields that oppose the change that produced them. A simple example: a conductor moving through a magnetic field or a collapsing field in an inductor. Back EMF is a voltage that opposes the applied EMF. It is Lenz’s law: back EMF is the electrical manifestation of the magnetic opposition, where it appears: in motors, inductors, and transformers.