EMZETT.Voltage, current and power: volts, amperes and watts
In short: Voltage (volt, V) is the “pressure” that drives charge. Current (ampere, A) is how much charge flows per second. Power (watt, W) is the product of both: P = U × I. Milliamperes (mA) are thousandths of an ampere.
In more detail: The water picture helps: voltage corresponds to water pressure, current to the amount of water per time, resistance to the narrowness of the pipe. How much energy a device needs follows from voltage times current.
In Depth
| Quantity | Unit | Symbol | Water picture |
|---|---|---|---|
| Voltage | Volt | U (V) | Pressure |
| Current | Ampere, milliampere (1 A = 1000 mA) | I (A) | Amount per second |
| Resistance | Ohm | R (Ω) | Narrowness of the pipe |
| Power | Watt (W), kilowatt (kW) | P | Work per time |
| Energy | Watt-hour (Wh), kilowatt-hour (kWh) | E | Work done |
Formulas:
- Ohm’s law: U = R × I
- Power: P = U × I (watts = volts × amperes)
- Energy: E = P × t (Wh = W × hours)
Practical examples:
- Wall socket: 230 V. A kettle of 2300 W draws 2300 / 230 = 10 A.
- USB: 5 V × 0.9 A = 4.5 W (USB 3.0). Fast charging: 20 V × 5 A = 100 W (USB-PD).
- Smartphone battery: 5000 mAh at 3.85 V ≈ 19.25 Wh. A 20 W charger fills it in about an hour.
- Motherboard and CPU: The CPU runs at about 1.0 – 1.4 V but draws over 100 A, giving ~125 W. Small voltage, big current.
- Power supply outputs: +12 V (CPU, graphics card), +5 V (drives, USB), +3.3 V (RAM, chipset).
- Main memory: DDR5 at 1.1 V saves power compared with 1.5 V (DDR3).
Why CPU voltage keeps dropping: Power loss grows roughly with the square of the voltage (P ≈ C × U² × f). Half the voltage gives a quarter of the heat.
Alternating and direct current: The wall socket delivers alternating voltage (AC, 50 Hz). Computer parts need direct voltage (DC). The power supply converts.
Safety: Mostly the current through the body is dangerous. From about 50 V AC and 30 mA it can become life-threatening. Mains voltage should be opened and serviced only by professionals; do not unscrew power supplies.
See also: Power supply (PSU) and 80 PLUS, Kilowatt-hour (kWh), Power supply and redundancy in the PC