Key Equations
Physical Quantities & Units - OCR A-Level Physics
On Data Sheet
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Defining a derived unit from base units
$$[\text{derived}] = \text{combination of kg, m, s, A, K, mol}$$
Not a formula to memorise, but a technique. Replace each quantity in an equation with its SI base unit and simplify. This is how you check homogeneity or express derived units.
Newton in base units
$$1 \text{ N} = 1 \text{ kg m s}^{-2}$$
- Where:
- $N$ = kg m \(s^{-2}\)
From F = ma. The newton is the most commonly tested derived unit at A-Level.
Joule in base units
$$1 \text{ J} = 1 \text{ kg m}^{2} \text{ s}^{-2}$$
- Where:
- $J$ = kg \(m^{2}\) \(s^{-2}\)
From W = Fd = (kg m \(s^{-2}\))(m). Also the base units of all forms of energy.
Watt in base units
$$1 \text{ W} = 1 \text{ kg m}^{2} \text{ s}^{-3}$$
- Where:
- $W$ = kg \(m^{2}\) \(s^{-3}\)
From P = W/t = (kg \(m^{2}\) \(s^{-2}\)) / s.
Pascal in base units
$$1 \text{ Pa} = 1 \text{ kg m}^{-1} \text{ s}^{-2}$$
- Where:
- $Pa$ = kg \(m^{-1}\) \(s^{-2}\)
From p = F/A = (kg m \(s^{-2}\)) / \(m^{2}\). Commonly tested in thermal physics and materials.