Electric charge
Matter contains positive and negative electric charges. Protons carry positive charge. Electrons carry negative charge. Like charges repel, while unlike charges attract. Charge is measured in coulombs (C).
Current
Electric current is the rate at which charge passes a point. Its unit is the ampere (A). One ampere means one coulomb passes each second. Conventional current is shown from positive to negative. Electron movement in a metal is in the opposite direction.
I = Q / t
Voltage
Voltage is the difference in electric potential between two points. It provides the push that can drive current through a circuit. Voltage is measured in volts (V). A 13.8 V supply is common in amateur radio stations.
Resistance and conductance
Resistance opposes current flow and is measured in ohms (Ω). Conductance describes how easily current flows. It is the reciprocal of resistance and is measured in siemens (S).
G = 1 / R
Conductors and insulators
Copper and aluminium are good conductors. Rubber, glass and many plastics are insulators. Resistance depends on material, length, cross-sectional area and temperature. A long, thin wire has more resistance than a short, thick wire of the same material.
Passive and active devices
Passive components include resistors, capacitors, inductors and transformers. They do not provide signal gain by themselves. Active devices include diodes and transistors. They can control current and support rectification, switching or amplification.
Electrical power and energy
Power is the rate of energy transfer. It is measured in watts. Energy is the total amount used over time. It is measured in joules or kilowatt-hours.
P = VI | E = Pt