Alternating current (AC) is a current whose magnitude and direction change periodically, normally tracing a sinusoidal waveform around a zero mean. The supply reverses direction twice in every cycle, so the net charge transported over one full cycle is zero.
Learning objectives
- Describe how an alternating current differs from a direct current
- Relate peak, RMS and frequency values of a sinusoidal supply
- Read an AC waveform on an oscilloscope or a true-RMS meter
- Explain why transformers and rotating generators favour AC
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
The mains supply is AC because it is easy to generate with a rotating machine and easy to change voltage with a transformer. A 50 Hz supply completes fifty full cycles every second; 60 Hz is the other common frequency.
Intermediate How it behaves in a circuit
Because the average of a sine wave is zero, AC magnitudes are usually quoted as RMS values: Vrms = Vp/√2. A 230 V RMS mains waveform therefore has a peak of about 325 V.
Advanced Limits, assumptions, engineering detail
AC is preferred for bulk transmission because transformers change voltage with low loss, but very long links may use high-voltage DC instead, since capacitive charging current and reactive losses dominate beyond a certain distance. The choice is an engineering trade-off, not a property of AC itself.
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Frequency | f | hertz (Hz) |
| Period | T | second (s) |
| Peak voltage | Vp | volt (V) |
| RMS voltage | Vrms | volt (V) |
| RMS current | Irms | ampere (A) |
Worked examples
Peak to RMS
Problem. A sinusoidal mains voltage has a peak value of 325 V. What is its RMS value?
Solution. Vrms = Vp / √2 = 325 / 1.4142.
Cycles in a time interval
Problem. A 50 Hz supply feeds a load for 0.25 s. How many complete cycles occur?
Solution. T = 1 / f = 1 / 50 = 0.020 s. Number of cycles = 0.25 / 0.020.
Peak from an RMS rating
Problem. An AC adapter is rated 12 V RMS. What peak voltage appears across the load?
Solution. Vp = Vrms × √2 = 12 × 1.4142.
Practical applications
- Mains distribution and household supply
- AC motors and generators
- Transformer-based voltage conversion
- Automotive alternators before rectification
- Variable-frequency motor drives
- Induction heating and welding
Automotive applications
An alternator generates three-phase AC as the engine turns, then a diode bridge rectifies it to the DC the vehicle network needs. Some sensors and injector drivers switch AC-like signals, and electric vehicles use an inverter to create three-phase AC for the traction motor.
Industrial applications
Three-phase AC powers most industrial motors, pumps, fans and compressors. Variable-frequency drives rectify the incoming AC and synthesise a new waveform at the required frequency, which is why drive output is rich in harmonics and needs filtering and shielded cables.
Measurement procedure
- Select the AC voltage or AC current function; a DC range reads roughly zero on a symmetrical sine wave.
- Use a true-RMS meter for distorted waveforms; averaging meters are calibrated for sine waves only and under-read.
- Measure frequency with the meter Hz function, or read the period on an oscilloscope and compute f = 1/T.
- Check the CAT rating of the meter and the probe rating against the circuit before touching mains terminals.
- Use a differential probe or an isolation transformer when scoping live mains equipment.
Common mistakes
- Treating the peak value as the RMS value: 325 V peak is 230 V RMS, and power calculations use RMS.
- Confusing frequency with period: 50 Hz means a 20 ms cycle, not 50 ms.
- Assuming AC is always a clean sine wave; drives, rectifiers and dimmers produce distorted current.
- Using a DC-averaging meter on AC and concluding that the voltage is zero.
Safety considerations
- Mains AC can be lethal: isolate, lock off and verify the circuit is dead before working on it.
- Use test equipment with the correct CAT rating for the point of measurement.
- Capacitors in AC equipment can stay charged after isolation; discharge them and verify before touching terminals.
- Do not rely on a single indicator; prove the tester on a known live source before and after the check.