Usual prefixes
k = 10³
m = 10⁻³
µ = 10⁻⁶
n = 10⁻⁹
m = 10⁻³
µ = 10⁻⁶
n = 10⁻⁹
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage | U | V |
| Current | I | A |
| Resistance | R | Ω |
| Power | P | W |
| Energy | W | J |
Elec Class
learn electricity hands-on
Lesson · chapter 1 of 15
To convert, replace the prefix with its power of 10: k = 10³, m = 10⁻³, µ = 10⁻⁶. So 2.2 kΩ = 2,200 Ω and 20 mA = 0.02 A.
The result is worked out before your eyes, line by line, just like on the board.
1I type the value
2I choose the target prefix
Each step is worth 3 decimal places: k = 10³, m = 10⁻³, µ = 10⁻⁶.
3The result
Converted value0.25 A
Divide by 10³: the decimal point moves 3 places to the left.
The definition, the key points to remember and everyday examples.
Every electrical quantity is measured in the International System: voltage in volts (V), current in amperes (A), resistance in ohms (Ω), power in watts (W), energy in joules (J).
Prefixes avoid zeros: k (10³), M (10⁶), m (10⁻³), µ (10⁻⁶), n (10⁻⁹), p (10⁻¹²). Scientific notation m × 10ⁿ keeps 1 ≤ m < 10; only keep the significant figures the measurement justifies.
You already know that 1 km = 1,000 m and 1 m = 1,000 mm. Electricity uses exactly the same prefixes: 1 kΩ = 1,000 Ω, 1 A = 1,000 mA. The value doesn't change, only the way it's written. Where it falls short: electricity goes much further (µ, n and p for capacitors, M and G for large powers), and centi, so common for lengths, is almost never used.
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage | U | V |
| Current | I | A |
| Resistance | R | Ω |
| Power | P | W |
| Energy | W | J |
| Quantity | Symbol | Unit |
|---|---|---|
| Quantity of electricity | Q | C |
| Energy | W | J |
Four steps to solve a problem, then an example solved line by line.
Read the letter in front of the unit (k, M, m, µ, n) and mind the case: m (milli) is not M (mega).
4.7 kΩ becomes 4.7 × 10³ Ω; 33 nF becomes 33 × 10⁻⁹ F.
Work out the formula with the converted values, without any prefix.
Pick the prefix that gives a number between 1 and 1,000 (0.0035 A is written 3.5 mA) and keep three significant figures.
Convert 4.7 kΩ to ohms, 250 mA to amperes and 0.000 22 F to microfarads, then write 47 000 Ω in scientific notation.
Divide by 1,000: 350 mA = 0.35 A. The other way round, multiply by 1,000: 0.02 A = 20 mA.
A million: 1 F = 10⁶ µF. Conversely, 470 µF = 470 × 10⁻⁶ F = 0.00047 F.
It's the milliamp-hour, a quantity of electric charge: 2,000 mAh = 2 Ah, that is 2 A for 1 h or 0.2 A for 10 h. In coulombs: 2 × 3,600 = 7,200 C.
Put the decimal point after the first non-zero digit and count how many places it moved: 0.00068 A = 6.8 × 10⁻⁴ A and 2,300,000 W = 2.3 × 10⁶ W.
It's the Greek capital letter omega: "omega" sounds like "ohm", and an O would be confused with a zero. On diagrams, you often see 4k7 for 4.7 kΩ: the letter replaces the decimal point.
The volt (V) for voltage, the ampere (A) for current, the ohm (Ω) for resistance, the watt (W) for power and the joule (J) for energy. Strictly speaking, only the ampere is an SI base unit; the others are derived from it (1 V = 1 W/A).
The formulas of the course (U = R·I, P = U·I…) are written for volts, amperes and ohms. With kilo-ohms or milliamperes the result is off by a factor of 1 000.
As many as the least precise datum: a 5 % resistor does not justify a current with six decimals. In practice, three figures.
From the simplest to the most complete, each problem comes with a step-by-step solution.
Pass real objects around (resistors, capacitors, a phone battery, a charger, an appliance's energy label): each group writes down the values printed on them, converts them to SI units and scientific notation, then compares with another group.
Using Elec Class in class