Magnetic fields
A magnet has north and south poles. Like poles repel and unlike poles attract. Magnetic field lines show the direction of the field. The field is stronger where the lines are closer together.
Permanent magnets and electromagnets
A permanent magnet keeps its magnetism after the magnetising force is removed. An electromagnet creates a magnetic field when current flows through a coil. More turns or more current usually produce a stronger field. An iron core can increase the field.
Electromagnetic induction
A changing magnetic flux can induce a voltage in a conductor or coil. This is the principle used in generators and transformers. The induced voltage opposes the change that produces it. This is described by Lenz's law.
Self-inductance
A changing current in a coil creates a changing magnetic field. That field induces a voltage in the same coil. The induced voltage opposes the change in current. Inductance is measured in henries (H).
Mutual inductance
When two coils are close, a changing current in one coil can induce a voltage in the other. This is mutual inductance. It is the operating principle of a transformer and many tuned RF coupling circuits.
Inductors used in radio circuits
Air-core coils are common in RF tuning circuits because they avoid some core losses. Ferrite-core coils are used in compact RF and intermediate-frequency circuits. Iron-core inductors are more common at lower frequencies. Chokes oppose AC or RF current while allowing the required DC path.
Capacitance
A capacitor has two conducting plates separated by an insulating dielectric. It stores energy in an electric field. Capacitance is measured in farads (F). Ceramic, film, electrolytic and variable capacitors serve different circuit needs.
Capacitor connections
Capacitors in parallel add directly. Capacitors in series combine by adding their reciprocals. In parallel, the voltage across each capacitor is the same. In series, the charge on each ideal capacitor is the same.
Parallel: CT = C1 + C2 + ... | Series: 1/CT = 1/C1 + 1/C2 + ...
Inductor connections
For ideal inductors without mutual coupling, series inductances add. Parallel inductances combine by the reciprocal rule. Coupling between coils changes the result, so the simple formula must not be used when mutual inductance is significant.
Stored energy
A capacitor stores energy in an electric field. An inductor stores energy in a magnetic field.
Capacitor: E = ½CV² | Inductor: E = ½LI²