Unraveling Resistors in AC Circuits: Operational Principles and Illustrative Examples

Resistors play a fundamental role in AC (alternating current) circuits. When it comes to their working in AC circuits, the basic principle remains similar to that in DC circuits in some aspects. Resistors oppose the flow of electric current. In an AC circuit, as the voltage continuously alternates its polarity and magnitude sinusoidally with a specific frequency, the resistor restricts the flow of electrons in accordance with Ohm's law, which states that the current (I) through a resistor is equal to the voltage (V) across it divided by its resistance (R), i.e., I = V/R.

The key difference in AC is that both the voltage and current are constantly changing. However, the resistance value of a resistor, which is a property depending on its physical construction and material, remains constant regardless of the alternating nature of the current. So, for each instantaneous value of the AC voltage, the resistor instantaneously determines the corresponding current value.

For example, consider a simple household lighting circuit where an incandescent bulb is connected in series with a resistor. The AC voltage from the mains supply (usually 110V or 220V with a frequency of 50Hz or 60Hz depending on the region) drives current through the circuit. The resistor here can be used to limit the current flowing through the bulb to prevent it from burning out due to excessive current. If the bulb has a rated power and voltage, by calculating the appropriate resistance value, we can ensure it operates within its designed parameters.

Another example is in an audio amplifier circuit. AC signals representing sound waveforms pass through various resistors. These resistors are used for tasks like voltage division to set proper biasing levels for transistors within the amplifier. They help shape the amplitude and frequency response of the audio signal, ensuring that the amplified sound is clear and distortion-free. Resistors in such circuits interact with other components like capacitors and inductors to precisely control the flow and characteristics of the AC audio signals, allowing for high-fidelity sound reproduction.