AC/DC & Electric Current
Understanding what current actually is, how DC and AC differ, and why both exist in the world.
What is Electric Current?
Current is the flow of electric charge — specifically, electrons moving through a conductor.
Formal definition: 1 Ampere = 1 Coulomb of charge passing a point per second.
A Coulomb is about 6.24 × 10¹⁸ electrons. So even a tiny current of 1mA is roughly 6 quadrillion electrons per second moving past a point. The scale is staggering.
Direction convention: Current is defined as flowing from positive to negative (conventional current). Electrons actually flow the other way — from negative to positive. This historical accident happened because Franklin guessed wrong about which charge carrier moves. We’ve kept the convention anyway.
DC — Direct Current
Charge flows in one direction, continuously.
- Voltage stays constant (or close to it)
- Produced by batteries, solar cells, regulated power supplies
- What you get out of a USB port: 5V DC
A battery maintains a potential difference between its terminals. As long as the circuit is closed, current flows from + to − through the external circuit, and internally from − to + through the chemical reaction.
DC graph: a flat horizontal line.
AC — Alternating Current
The direction of current reverses periodically — it alternates.
- Voltage swings between positive and negative peaks
- In India: 230V, 50Hz — the voltage completes 50 full cycles per second
- In the US: 120V, 60Hz
AC graph: a sine wave.
Why AC for the grid?
Transformers only work with changing magnetic fields — which means they only work with AC. A transformer can take 11,000V from a transmission line and step it down to 230V for your home. This is why AC won the War of Currents: you can transmit at high voltage (low current = low resistive loss) over long distances, then step down at the destination.
The 230V number
That’s the RMS (Root Mean Square) value — the equivalent DC voltage that would deliver the same power. The actual peak of the sine wave is:
V_peak = 230 × √2 ≈ 325V
Frequency and 50Hz
50 cycles/second means the sine wave completes a full oscillation in 20 milliseconds. This is why old fluorescent lights flickered — they’d dim and brighten 100 times a second (twice per cycle). Barely perceptible but there.
Practical Takeaway
For hobby electronics and microcontrollers — everything runs on DC, typically 3.3V or 5V. AC from the wall gets converted by a power supply (AC → DC). Understanding AC matters for:
- Safety (mains voltage kills — 230V AC is far more dangerous than 230V DC at the same level)
- Understanding power supplies
- Eventually: motor drivers, inverters, induction
Current and Safety
Even small currents through the body are dangerous:
| Current (mA) | Effect |
|---|---|
| 1 | Barely perceptible |
| 10–20 | Muscular contraction, can’t let go |
| 100+ | Ventricular fibrillation, potentially fatal |
It’s current through the body that kills, not voltage directly. But voltage determines how much current flows through your body’s resistance (roughly 1–100kΩ depending on skin condition). Mains voltage is more than enough to push lethal current through you.