Why your wall socket and your phone battery use different kinds of electricity, what RMS and frequency mean, and how chargers and inverters convert between them.
| DC | AC | |
|---|---|---|
| Direction | Constant | Reverses periodically |
| Voltage | Steady | Swings positive and negative |
| Typical sources | Batteries, solar, USB | Mains supply, generators |
| Easy to step up/down | Needs electronics | Simple transformer |
| Typical uses | Phones, laptops, LEDs, cars | Homes, motors, long-distance power |
Frequency is how many full cycles AC completes each second, measured in hertz (Hz). India and much of the world use 50 Hz, while the US uses 60 Hz. The time for one cycle is the period.
Mains at 50 Hz:
T = 1 / 50 = 0.02 s = 20 ms per cycle
The direction reverses 100 times every second.
AC voltage is constantly changing, so we quote one effective number, the RMS (root mean square) value. It is the DC voltage that would deliver the same heating power to a resistor. For a sine wave:
A 230 V mains supply:
Vpeak = 230 × 1.414 ≈ 325 V
That is why insulation and capacitors in mains equipment must be rated well above 230 V.
Power loss in a cable is I² × R, so lower current means far less wasted heat. Since P = V × I, raising the voltage lowers the current for the same power. Transformers raise and lower AC voltage easily and efficiently, which made AC the winner for long-distance transmission.
Delivering 1 MW (1,000,000 W):
At 230 V: I = 1,000,000 / 230 ≈ 4,348 A
At 11 kV: I = 1,000,000 / 11,000 ≈ 91 A
The current is about 48 times lower, so the cable loss is about 2,300 times lower (48²) for the same cable.
A 1,000 W load run from a 12 V battery through an inverter that is 90% efficient:
DC input power = 1000 / 0.9 ≈ 1,111 W
Battery current = 1111 / 12 ≈ 92.6 A
That heavy current is why low-voltage inverter systems need thick cables. Our Wire Gauge calculator helps size them, and the Battery Life calculator estimates runtime.
Ohm's Law works directly for AC on purely resistive loads such as heaters and incandescent bulbs, using RMS values. With capacitors and inductors, resistance becomes impedance (Z) and current can lag or lead the voltage. See the Ohm's Law guide for the basics.
Try each one before you open the answer.
120 × 1.414 ≈ 170 V.
T = 1 / 60 ≈ 0.0167 s, or 16.7 ms.
I = 500 / 12 ≈ 41.7 A. A real inverter would draw more because of losses.
DC current flows in one direction at a steady voltage, like from a battery. AC current reverses direction many times a second, so its voltage swings up and down.
AC voltage can be stepped up or down easily with transformers. High voltage means lower current, which cuts the heat lost in long transmission lines.
It is the RMS value, the steady DC voltage that would heat a resistor equally. The actual voltage peaks at about 325 V.
A charger steps the voltage down, rectifies it to DC with diodes, then smooths and regulates it to a steady output such as 5 V.
Working with a transformer or an RC filter? Try our related calculators.
Open the Transformer Calculator