IGCSE Physics / Practical and maths skills
Units, prefixes and standard form
Edexcel International GCSE Physics (4PH1), chapter 29: Practical and maths skills, specification point M1B, M3C. Free lesson.
Read the revision notes · IGCSE Physics past papers
Prefixes change the size of a unit, while standard form makes very large or very small numbers easier to write. Convert quantities into the required units before substituting them into a physics equation.
What the prefixes multiply by
A prefix is placed before a unit and multiplies it by a particular amount. Giga, mega and kilo make the unit larger, while milli, micro and nano make it smaller. Giga (G): × 10⁹, for example GHz. Mega (M): × 10⁶, for example MW, MJ and MHz. Kilo (k): × 10³, for example kg, kW, kΩ and kPa. Milli (m): × 10⁻³, for example mA and ms. Micro (μ): × 10⁻⁶, for example μm. Nano (n): × 10⁻⁹, for example nm.
Convert units before using an equation
Equations require quantities in particular units. For example, power, energy and speed equations need time in seconds. Convert first, then substitute the new value into the equation. To change hours into seconds, multiply by 60 to get minutes, then multiply by 60 again to get seconds. To change grams into kilograms, divide by 1000.
Convert areas and volumes carefully
Area and volume conversions use the conversion for length more than once. Since 1 m = 100 cm, a square metre contains 100 × 100 square centimetres. For volume, the conversion is used three times. To convert cm² into m², divide by 10 000. To convert cm³ into m³, divide by 1 000 000. Pressure calculations need area in m², so convert the area before substituting it.
Change prefixes into base units
A prefix must be converted before you use a quantity in an equation that expects a different unit. For example, V = I × R needs current in amps, not milliamps. 250 mA = 0.25 A. Using 250 as though it were amps gives an incorrect voltage.
Write small numbers in standard form
Standard form writes a number as A × 10ⁿ, where 1 ≤ A < 10. Move the decimal point until there is one digit before it. If the original number is smaller than 1, the power is negative. 656 nm = 0.000000656 m. Moving the decimal point seven places to the right gives 6.56, so the power is −7. On a calculator, enter 6.56, press the ×10ˣ key, then enter −7. Do not type × 10 as well.