Physical Quantities | CIE A Level Physics Revision Notes 2025

What is a physical quantity?

Speed and velocity are examples of physical quantities; both can be measured
All physical quantities consist of a numerical magnitude and a unit
In physics, every letter of the alphabet (and most of the Greek alphabet) is used to represent these physical quantities
To represent a physical quantity, it must contain both a numerical value and the unit in which it was measured
For example, the letter v can be used to represent the physical quantities of velocity, volume or potential difference (voltage)
The units provide the context as to what v refers to:
- If v represents velocity, the unit would be m s^–1
- If v represents volume, the unit would be m³
- If V represents potential difference, the unit would be V

What is a Physical Quantity, downloadable AS & A Level Physics revision notes

All physical quantities must have a numerical magnitude and a unit

There are important physical quantities to learn in physics
It is useful to know these physical quantities, they are particularly useful when making estimates
A few examples of useful quantities to memorise are given in the table below (this is by no means an exhaustive list)

Quantity	Size
Diameter of an atom	10⁻¹⁰ m
Wavelength of UV radiation	10 nm
Height of an adult human	2 m
Distance between Earth and Sun (1 AU)	1.5 × 10¹¹ m
Mass of a hydrogen atom	10⁻²⁷kg
Mass of an adult human	70 kg
Mass of a car	1000 kg
Seconds in a day	90 000 s
Seconds in a year	3 × 10⁷ s
Speed of sound in air	300 m s⁻¹
Power of a light bulb	60 W
Atmospheric pressure	1 × 10⁵ Pa

Estimate the energy required for an adult man to walk up a flight of stairs.

Answer:

Step 1: Recall the equation for energy for gain in gravitational potential energy:

For a man of mass m to gain height h in a gravitational field of strength g, the energy E required to do so is:

$∆ E_{p} = m g ∆ h$

Step 2: Estimate the mass and height

1-1-estimating-physical-quantities-cie-new

Estimation of the adult man's mass and the height of the stairs

Step 3: Substitute these estimates into the equation:

$∆ E_{p} = 70 \times 10 \times 3 = 2100 J$

The mark scheme for calculations involving estimates are normally quite generous and offer a range of values as the final answer.

Many values are already given in your data booklet that therefore may not be given in the question, so make sure to check there too!