Units: the zeros matter too

A calculator can apply a formula correctly while answering the wrong question. Entering 20 A when you meant 20 mA is a scale error, not a division error.

The unit and prefix work together

Amperes (A) measure current; volts (V), potential difference; ohms (Ω), resistance; watts (W), power; and farads (F), capacitance. A prefix multiplies the unit: kilo means one thousand, milli one thousandth, micro one millionth and nano one billionth.

1 kΩ = 1000 Ω
1 MΩ = 1 000 000 Ω
1 mA = 0.001 A
1 µF = 0.000001 F
1 nF = 0.000000001 F

Convert before substituting

For the RC time constant of 10 kΩ and 100 µF, first write 10000 Ω and 0.0001 F. Their product is 1 s. Simply multiplying 10 by 100 does not give a thousand seconds: the prefix factors are missing.

In OhmPlay, changing a unit selector preserves the physical quantity: 1 kΩ becomes 1000 Ω. Changing the number changes the circuit. Sliders have convenient exploration ranges; numeric fields allow a wider range within the stated limits.

Quick order-of-magnitude check

5 V across 1 kΩ gives a few milliamperes, not thousands of amperes. Before trusting the last decimal, ask whether the scale makes sense. At fixed voltage, a smaller resistance carries more current and a larger one carries less.

Exercise: convert 0.47 µF to nF.

0.47 × 1000 = 470 nF. The capacitor has not changed; only the way its value is expressed has changed.

Exercise: how much power does 1 kΩ dissipate at 2 mA?

P = (0.002 A)² × 1000 Ω = 0.004 W = 4 mW. Squaring 2 without converting units gives the wrong scale.

Try Ohm’s law ↗ Try the RC capacitor tool ↗