Electrical units and prefixes
Every quantity in a formula has a letter that says what it is and a unit that says what it is measured in. Voltage uses the letter U but the unit V (volt); resistance uses R and the unit Ω (ohm).
The core units
- V volt measures voltage
- How hard charge is pushed between two points: the electrical "pressure" difference.
- A ampere measures current
- How much charge flows past a point each second. Often called the amp.
- Ω ohm measures resistance
- How strongly something opposes current. 1 Ω lets 1 A through for every 1 V of push.
- W watt measures power
- How fast energy is used, or turned into heat.
- F farad measures capacitance
- How much charge a capacitor stores for each volt across it. One farad is huge, so real capacitors are in pF, nF and µF.
- H henry measures inductance
- How strongly a coil resists a change in its current. 1 H makes 1 V when the current changes by 1 A each second.
- Hz hertz measures frequency
- How many times something repeats each second.
- s second measures time
- Time. In electronics most things happen in ms (thousandths), µs (millionths) or ns (billionths) of a second.
- C coulomb measures charge
- An amount of electric charge: about 6 × 10¹⁸ electrons. Careful: the letter C in a formula means capacitance, but the unit C means coulomb.
- J joule measures energy
- An amount of energy. A battery stores energy; power is how fast it is used.
Prefixes: a ladder in steps of 1000
| Prefix | Name | Means |
|---|---|---|
| T | tera | a million million (trillion) : 1 TΩ insulation resistance |
| G | giga | a thousand million (billion) : 2.4 GHz Wi-Fi |
| M | mega | a million : 1 MΩ input resistance · 16 MHz clock |
| k | kilo | a thousand : 4.7 kΩ pull-up · 10 kHz |
| - | (none) | one : 5 V · 2 A · 1 s |
| m | milli | one thousandth : 20 mA LED current · 2 ms bounce |
| µ | micro | one millionth : 100 µF capacitor · 1 µs rise time |
| n | nano | one thousand-millionth (billionth) : 100 nF decoupling cap · 20 ns switching |
| p | pico | one million-millionth (trillionth) : 22 pF crystal cap · 200 pF bus capacitance |
| f | femto | one thousand-million-millionth : 5 fJ, the energy in a tiny capacitor |
How units combine
- A × Ω = V from Ohm’s law, U = R × I. E.g. 0.01 A through 470 Ω drops 4.7 V.
- V ÷ Ω = A from the same law turned round, I = U ÷ R. E.g. 5 V across 1 kΩ pushes 5 mA.
- V × A = W from power, P = U × I. E.g. 12 V at 2 A is 24 W.
- A² × Ω = W from P = I² × R (put U = R × I into P = U × I). E.g. 0.1 A through 100 Ω makes 1 W of heat.
- W × s = J from energy is power for some time. E.g. 2 W for 10 s is 20 J.
- A × s = C from charge is current for some time. E.g. 1 mA for 1 s moves 1 mC.
- C ÷ V = F from capacitance, C = Q ÷ U. E.g. 1 µC stored at 10 V means 100 nF.
- Ω × F = s from the RC time constant, τ = R × C. E.g. 10 kΩ × 100 µF = 1 s.
- H ÷ Ω = s from the RL time constant, τ = L ÷ R. E.g. 10 mH ÷ 10 Ω = 1 ms.
- 1 ÷ s = Hz from frequency is "times per second", f = 1 ÷ T. E.g. a 1 ms period repeats at 1 kHz.
- H × A ÷ s = V from a coil’s voltage, U = L × dI/dt. E.g. 1 mH with current changing 1 000 A/s makes 1 V.
- V ÷ V, Ω ÷ Ω = no unit from gains, ratios, duty cycle and efficiency. E.g. 10 V out ÷ 1 V in = gain of 10.