S.I. units: (c) units listed in this specification

The idea

You should be able to state the physical quantity for each unit and convert between units where necessary. For example, the pascal (Pa) is the unit of pressure and equals N m⁻², which is kg m⁻¹ s⁻². The coulomb (C) is the unit of charge and equals A s. The volt (V) is the unit of potential difference and equals J C⁻¹, or kg m² s⁻³ A⁻¹. The ohm (Ω) is the unit of resistance and equals V A⁻¹. Knowing these relationships helps you check equations and solve problems accurately.

What earns the marks

• Recognise and use the S.I. base units: m, kg, s, A, K, mol and cd.
• Recognise derived units with special names, such as N, J, W, Pa, C, V, Ω, F, T and H.
• State the physical quantity measured by each unit.
• Express derived units in terms of base units, for example Pa = kg m⁻¹ s⁻².
• Convert between units, such as cm³ to m³ by multiplying by 10⁻⁶.
• Use the correct unit symbols in answers.

Common mistakes and how to put them right

• Confusing the pascal with the newton.

  Put it right: Pressure is N m⁻², so Pa = kg m⁻¹ s⁻², not kg m s⁻².

• Writing the coulomb as A s⁻¹.

  Put it right: Charge is current multiplied by time, so C = A s.

• Forgetting that the volt can be written as kg m² s⁻³ A⁻¹; omitting the ampere leads to an incorrect combination.

• Using the ohm as V A instead of V A⁻¹.

  Put it right: Resistance is potential difference divided by current.

Exam tips

• In multiple-choice questions, match each unit to its physical quantity and eliminate options that mix them up.
• Memorise the base-unit forms of common derived units to speed up dimensional checks.
• When converting volumes, remember that 1 cm³ = 1 × 10⁻⁶ m³, so multiply by 10⁻⁶.