Units – iGCSE Science (Single Award) Edexcel Revision

Units of measurement are vital in science to provide clarity and consistency.
Energy is measured in joules (J). It’s worth noting that 1 joule is equivalent to 1 newton meter.
Power is measured in watts (W). This can be conceived as rate of energy use or production.
Kilowatt-hours (kWh) is an alternative unit often used for substantial amounts of energy, particularly in the context of electricity usage.
The specific heat capacity, defining the amount of energy needed to raise the temperature of 1kg of a substance by 1°C, is measured in joules per kilogram per degree Celsius (J/kg°C).
The efficiency of energy transfer is expressed as a percentage (%). It’s a ratio of useful energy output to total energy input.

The Energy transfer can be calculated by multiplying power (P) by time (t). Hence, its units become a multiplication of the units of power and time, i.e., watt-hours (Wh) or joules (J).
The Heat gained or lost is calculated by the product of mass (m), specific heat capacity (c), and change in temperature (ΔT). Therefore, heat is measured in joules (J).

Remember, 1 watt-hour equals 3600 joules. Hence, when converting from watt-hours to joules, multiply by 3600.
Conversely, when converting from joules to watt-hours, divide by 3600.
To convert from kilowatt-hours to joules, multiply by 3.6 million (3.6 × 10⁶ J), and vice versa.

It’s key to ensure that all numbers used in a calculation are in the same type of unit. If they’re not, convert them before starting the calculation.
Keep an eye on the unit the question wants the answer in, especially when dealing with derived units.
Often, efficiency is a ratio of two energy or power values, so has no units but is expressed as a percentage.
Always check your workings, and make sure that your final answer is reasonable and has the correct units. Errors in unit conversion are a common source of mistakes.