Physical Quantities (OCR A Level Physics): Revision Note
Exam code: H556
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
These letters, without any context, are meaningless
To represent a physical quantity, it must contain both a numerical value and the unit in which it was measured
The letter v is used to represent the physical quantity of velocity (or speed)
The letter V can be used to represent volume, potential or voltage
The units provide the context as to what v or 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 voltage, the unit would be V
All physical quantities must have a numerical magnitude and a unit
Examiner Tips and Tricks
The same letter in the Greek or English alphabet can often refer to a different quantity depending on its context in Physics. Make sure you understand what each variable in every equation is before you use it, in order to avoid confusion.
Estimating Physical Quantities
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)
Common estimations
Quantity | Size |
|---|---|
Mass of an adult person | 70 kg |
Mass of a car | 1000 kg |
Height of an adult person | 2 m |
Diameter of a hair | 10-4 m |
Diameter of an atom | 10−10 m |
Diameter of a nucleus | 10−15 m |
Wavelength of UV radiation | 10−7 m |
Distance between Earth and Sun (1 AU) | 1.5 × 1011 m |
Mass of a hydrogen atom | 10−27 kg |
Seconds in 1 day | 90 000 s |
Seconds in 1 year | 3 × 107 s |
Speed of sound in air | 300 m s−1 |
Power of a light bulb | 60 W |
Atmospheric pressure | 105 Pa |
Worked Example
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:
Here, g is approximately 10 N kg−1
Step 2: Estimate the mass and height
An adult person has a mass of approximately 70 kg
A flight of stairs gains around 3 m of height
Estimation of the adult man's mass and the height of the stairs
Step 3: Substitute these estimates into the equation:
The energy required for the man to walk up the stairs is approximately:
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