Working principle
A resistor is a passive two-terminal component that opposes the flow of electric current and converts the associated electrical energy into heat. Its value, resistance, is measured in ohms and depends on the resistive material, its length and its cross-sectional area. Unlike capacitors and inductors, a resistor stores no energy and introduces no phase shift between voltage and current, which makes it the simplest and most predictable component in any circuit.
Ohm's law
The relationship between the three quantities is Ohm's law: voltage equals current times resistance. Hold the resistance constant and current rises in direct proportion to applied voltage, which plots as a straight line through the origin — the signature of a linear, or ohmic, device. This is exactly the behaviour the simulator's characteristic curve shows, and it is what lets a resistor be used to set a precisely known current from a known voltage.
Power rating and heat
Power dissipated in a resistor is voltage times current, equivalently current squared times resistance. Because the current term is squared, doubling the current quadruples the heat. Every resistor carries a power rating — a quarter watt is common for through-hole parts — and exceeding it causes drift, discolouration and eventual failure. The rating assumes free air at room temperature, so a resistor in a hot enclosure has to be derated accordingly.
Types and applications
Carbon film resistors are cheap and general purpose, metal film types offer better tolerance and lower noise, and wirewound resistors handle high power. Variable versions include potentiometers and rheostats, while thermistors and light-dependent resistors change value with temperature or light. In circuits, resistors set bias points, limit LED and base currents, form voltage dividers and RC timing networks, and terminate transmission lines.