Introduction

What is Alternating Current?

So far, you have studied direct current (dc) — where charges flow steadily in one direction. In dc circuits, the voltage and current are constant in time.

However, most real-world electricity behaves differently. The voltage supplied to your home does not stay constant; it varies sinusoidally with time. This time-varying voltage is called an alternating voltage (ac voltage) , and the current it produces is called an alternating current (ac current) .

Why is AC used everywhere?

Almost all electrical devices we use today run on ac. The main reasons are:

• Efficient voltage conversion: AC voltages can be easily stepped up or down using transformers. This is crucial because:
  • Power is transmitted at very high voltages (to reduce energy loss over long distances).
  • It is then stepped down to safe, usable levels for homes and offices.
• Economic transmission: High-voltage ac can be transmitted over long distances with much lower losses than dc.
• Unique circuit properties: AC circuits exhibit special behaviours (like resonance and phase shifts) that are exploited in devices such as radios, televisions, and induction motors.

Key Idea: Sinusoidal Nature

The defining feature of ac is that the voltage (and current) follows a sine function with time. This simple harmonic time dependence is what makes ac analysis both elegant and powerful.

Historical Context

The development of ac power systems is largely credited to Nicola Tesla (1856–1943), a Serbian-American inventor. He conceived the rotating magnetic field, which is the foundation of all ac machinery. His inventions include:

• The induction motor
• The polyphase system of ac power
• The Tesla coil (used in radio, TV, and other electronic equipment)

The SI unit of magnetic field, the tesla (T) , is named in his honour.