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Why Does AC Current Keep Reversing Instead of Flowing Steadily?

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Why Does AC Current Reverse Direction Periodically? Understanding Alternating Current generation, Frequency, and Working Principle of an AC generator

The answer to this question goes back to the generator where AC power is generated.

A simple voltage generator: 

Imagine a simple loop of coil is rotating between the two strong magnetic bars.

AC generator working



0 to 180 degrees,i.e. Half rotation.

You will notice that when the coil is perpendicular to the magnetic field, its output is minimum or zero due to the minimum linkage of the magnetic field. But as the coil rotates to become parallel to the magnetic field, its output increases and becomes maximum when it is parallel.

180 to 360 degrees, i.e. Full rotation.

The same cycle repeats, but now the voltage is negative. The following diagram explains it.

Generator coil rotation and corresponding sine wave output is shown

Hence, as the AC voltage reverses its polarity periodically, the load current will also change direction of flow periodically.

AC current direction:

AC current reversing direction periodically

FAQ Set:

Q1: Why does AC current reverse direction periodically?

AC (Alternating Current) reverses direction because the voltage generated by an AC generator alternates sinusoidally, causing current to flow first in one direction and then in the opposite direction.

Q2: How is AC generated?

AC is produced by rotating a coil within a magnetic field in an AC generator (alternator), inducing a voltage that changes polarity periodically according to Faraday’s law of electromagnetic induction.

Q3: What determines the frequency of AC?

The frequency (number of reversals per second) is determined by the rotational speed of the generator and the number of magnetic poles. Standard mains frequency is 50 Hz in India and 60 Hz in the USA.

Q4: What is the working principle of an AC generator?

  • A conductor (coil) rotates in a magnetic field

  • A voltage is induced across the coil

  • As the coil rotates, the induced voltage varies sinusoidally, reversing polarity each half cycle, producing alternating current

Q5: Why is AC used instead of DC for power distribution?

AC can be easily transformed to higher or lower voltages, reducing transmission losses over long distances, and its periodic reversal allows efficient operation of transformers and motors.

Q6: Does the current amplitude remain constant in AC?

No, AC voltage and current vary continuously in a sinusoidal waveform, reaching a maximum (peak) and minimum (zero) value during each cycle.

Q7: How does the periodic reversal affect electrical devices?

Devices designed for AC, like motors and transformers, rely on this reversal for efficient operation, while purely resistive loads (like heaters) are unaffected by direction changes.

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