Joules, kilojoules and megajoules

What you will learn

Energy units J, kJ and MJ; prefixes, standard form and coherent quantities.

Specification mapping: C4.1

Prerequisites: C2.1.3

Physical principles

The SI unit of energy is the joule. One kilojoule is 10³ J and one megajoule is 10⁶ J, so 1 MJ = 1000 kJ. Prefixes change the scale of a unit, not the kind of quantity being measured. Both electrical and thermal energy can be expressed in these units.

Convert all energy values to compatible units before adding, comparing or substituting. In ΔQ = mcΔT with c in J kg⁻¹ K⁻¹ and mass in kilograms, ΔQ must be in joules. Using a value quoted in kJ without multiplying by 1000 creates a thousandfold error.

Standard form helps compare large or small values. Retain the number of significant figures justified by the data: 2.50 kJ is 2.50 × 10³ J, not an instruction to round to one significant figure. Unit conversion factors such as 1000 are exact and do not themselves limit measurement precision.

Scientific diagram and interpretation

Equations and relationships

1 kJ = 10³ J; 1 MJ = 10⁶ J

Worked examples and applications

Common misconceptions

An energy in MJ is not a power in MW; identical prefixes do not make the quantities interchangeable.

Examination technique

Check energy prefixes before rearranging a thermal equation.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.