An electrical resistor is a fundamental circuit component designed to introduce opposition to the flow of electric current. This opposition, known as electrical resistance, is measured in ohms (\(\Omega\)). Resistors are used in nearly all electronic devices to regulate current flow, divide voltages, and protect sensitive components. They function by dissipating electrical energy as heat, which limits the movement of charge through the circuit.
The Core Concept of Ohm’s Law
The behavior of most basic circuits is governed by Ohm’s Law, which mathematically relates the three main variables: voltage, current, and resistance. Voltage (\(V\)) is the measure of electrical potential difference, providing the “push” that drives the electric charge. Current (\(I\)) is the rate of flow of the electric charge itself.
The law states that the voltage across a conductor is directly proportional to the current flowing through it, expressed by the equation \(V = I \times R\). If resistance (\(R\)) remains unchanged, increasing the voltage will cause the current to increase proportionally. Conversely, increasing the resistance while holding voltage constant will cause the current to decrease.
This relationship is often understood through the water analogy. Voltage is like water pressure, current is the flow rate, and resistance is the width of the pipe. High pressure (voltage) forces a high flow rate (current) through a fixed width (resistance). If the pipe narrows, resistance increases, and the flow rate decreases, even if the pressure stays the same.
Characteristics of an Ohmic Resistor
An ohmic resistor is a device that strictly obeys Ohm’s Law, meaning its resistance value remains constant across a wide range of applied voltages and currents. This constancy is the defining characteristic of an ohmic device and is maintained only when the temperature is kept constant. For an ideal ohmic resistor, doubling the applied voltage results in an exact doubling of the current.
This proportional relationship between voltage and current is described as being linear. When the current (\(I\)) is plotted against the voltage (\(V\)) on a graph, the result is a perfectly straight line that passes directly through the origin (zero voltage and zero current). The slope of this line is directly related to the resistance of the component; a steeper slope indicates a lower resistance, while a flatter slope indicates a higher resistance.
Common components like carbon-film and metal-film resistors are engineered to be highly ohmic within their operational limits. These fixed-value resistors are designed so that their material properties provide a steady, predictable opposition to current flow. Their predictable behavior makes them foundational for designing stable electrical circuits.
Ohmic Versus Non-Ohmic Behavior
Not all electrical components maintain a constant resistance; those that do not are called non-ohmic devices. These components violate Ohm’s Law because their resistance changes depending on operating conditions, such as applied voltage or temperature. The relationship between voltage and current is not linear, meaning a graph of their current-voltage relationship produces a curve rather than a straight line.
A common example of a non-ohmic device is an incandescent light bulb, which uses a thin tungsten filament. As current flows through the filament, it heats up dramatically, and this rise in temperature causes the resistance of the metal to increase significantly.
Another example is a semiconductor diode. It only allows current to flow easily in one direction, exhibiting a very high resistance when the voltage is reversed.
Thermistors are specialized non-ohmic resistors whose resistance is intentionally designed to change with temperature. This makes them useful in sensors and temperature regulation systems.