You can do a calculation perfectly and still get it wrong if you forget to convert your units. Examiners love to give values in kilojoules, milliamps or minutes, so here’s how to deal with them.
The prefixes you need to know
- giga (G) = × 1,000,000,000 (10⁹)
- mega (M) = × 1,000,000 (10⁶)
- kilo (k) = × 1000 (10³)
- centi (c) = ÷ 100 (10⁻²)
- milli (m) = ÷ 1000 (10⁻³)
- micro (μ) = ÷ 1,000,000 (10⁻⁶)
- nano (n) = ÷ 1,000,000,000 (10⁻⁹)
So 3.5 kW = 3500 W, and 250 mA = 0.25 A.
Don’t forget time
Most physics equations need time in seconds. Multiply minutes by 60 and hours by 3600.
Worked example
A 2.2 kW kettle is switched on for 3 minutes. How much energy does it transfer?
- Convert power: 2.2 kW = 2200 W
- Convert time: 3 minutes = 3 × 60 = 180 s
- Equation: E = P × t
- Substitute: E = 2200 × 180
- Answer: E = 396,000 J (or 396 kJ)
If you’d forgotten to convert, you’d have got 6.6, which is wrong by a factor of 60,000!
The tricky ones: area and volume
Because 1 m = 100 cm:
- 1 m² = 100 × 100 = 10,000 cm²
- 1 m³ = 100 × 100 × 100 = 1,000,000 cm³
A common density conversion: 1 g/cm³ = 1000 kg/m³. Water has a density of 1 g/cm³, which is 1000 kg/m³.
Top tips
- Before you start any calculation, underline every value and check its unit.
- Convert everything to standard units (m, kg, s, A, V, W, J) first.
- Learn to use the standard form button on your calculator (often labelled ×10ˣ or EXP).
Leave a comment