Question
Differentiate between alternating current (AC) and direct current (DC). Explain how AC is generated in a power plant using an AC generator.
(NCERT Class 10, Chapter 13 — Magnetic Effects of Electric Current)
Solution — Step by Step
| Feature | AC (Alternating Current) | DC (Direct Current) |
|---|---|---|
| Direction of flow | Reverses direction periodically | Flows in one direction only |
| Magnitude | Varies continuously (sinusoidal) | Remains constant |
| Frequency | 50 Hz in India (changes direction 100 times per second) | 0 Hz (no change) |
| Source | AC generators, power plants | Batteries, solar cells |
| Transmission | Easy over long distances (voltage can be stepped up/down) | Difficult over long distances (high losses) |
| Usage | Household appliances, industrial machinery | Electronics, charging, electroplating |
An AC generator works on electromagnetic induction (Faraday’s law): when a coil rotates in a magnetic field, the changing magnetic flux through the coil induces an EMF (voltage).
The setup: a rectangular coil (called the armature) rotates between the poles of a strong magnet. As the coil spins, the angle between the coil and the magnetic field keeps changing — so the magnetic flux changes continuously.
As the coil rotates through one full turn:
- At 0° (coil parallel to field): flux change is maximum → EMF is maximum
- At 90° (coil perpendicular to field): flux change is zero → EMF is zero
- At 180°: EMF is maximum again, but in the opposite direction
- At 270°: EMF is zero again
This produces a sinusoidal (wave-like) output. The current flows in one direction for half the rotation, then reverses for the other half. That’s why it’s “alternating.”
- Armature — the rotating coil
- Field magnets — provide the magnetic field
- Slip rings — two rings connected to the coil ends; rotate with the coil
- Brushes — stationary contacts that collect current from the slip rings and send it to the external circuit
Why This Works
The fundamental principle is Faraday’s law: , where is the magnetic flux. When the coil rotates at constant angular velocity , the flux varies as , so the induced EMF is — a sine wave.
In power plants, the coil isn’t rotated by hand — it’s driven by turbines. These turbines are spun by steam (in thermal/nuclear plants), water flow (in hydroelectric plants), or wind. The mechanical energy of the turbine is converted to electrical energy by the generator.
Alternative Method — Understanding through a DC motor comparison
An AC generator and a DC motor are essentially the same device run in reverse. A motor converts electrical energy to mechanical energy; a generator converts mechanical energy to electrical energy. The key structural difference: an AC generator uses slip rings (gives AC output), while a DC generator uses a split-ring commutator (converts AC to DC output by flipping the connection every half turn).
Common Mistake
Students often confuse slip rings (used in AC generators) with split-ring commutators (used in DC generators/motors). Slip rings are complete circles — they allow the current to alternate. A split-ring commutator is a ring cut into two halves — it reverses the external connection every half rotation, converting AC into DC. Mixing these up is a guaranteed way to lose marks on this topic.