20. Alternating Current (AC) vs. Direct Current (DC)
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Electric current is broadly classified into two types based on how its magnitude and direction behave over time: direct current, which flows steadily in only one direction, and alternating current, whose direction periodically reverses.
Direct current (DC) flows in a single, constant direction through a circuit, and in the ideal case its magnitude also stays constant with time (as from a battery or cell). Alternating current (AC), by contrast, periodically reverses its direction of flow, and its magnitude varies continuously with time in a repeating (typically sinusoidal, wave-like) pattern, alternating between a maximum value in one direction and a maximum value in the opposite direction. The number of complete cycles (one full reversal and return) that occur per second is called the frequency of the AC supply, measured in hertz (Hz); the standard AC supply frequency used in India (and most of the world outside North America) is 50 Hz, meaning the current reverses direction 100 times every second.
Feature | Alternating Current (AC) | Direct Current (DC) |
|---|---|---|
Direction of flow | Periodically reverses direction | Flows in one constant direction only |
Magnitude over time | Varies continuously (typically sinusoidal wave) | Remains constant (ideal case), e.g. from a battery |
Typical source | AC generators / alternators at power stations | Cells, batteries, DC generators, solar cells |
Ease of voltage conversion | Easily stepped up or down using a transformer | Cannot be transformed by a simple transformer |
Transmission over long distances | Highly efficient (can use step-up transformers to reduce transmission losses) | Inefficient over long distances at low voltage; requires costly high-voltage DC (HVDC) technology for long-distance transmission |
Typical uses | Household and industrial power supply, running large motors | Batteries/cells, electronic devices, electroplating, electrolysis, charging |
Frequency (India) | 50 Hz | Not applicable (zero frequency, i.e. steady) |