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ASPhysicsCIERevision Notes1. Physical Quantities & Units1.1 Physical QuantitiesPhysical Quantities

Physical Quantities (CIE AS Physics)

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Dan MG

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Dan MG

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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

 

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)

Estimating physical quantities table

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 10−27 kg
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

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:

increment E subscript p space equals space m g increment h

    • 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

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:

  • The energy required for the man to walk up the stairs is approximately:

increment E subscript p space equals space 70 space cross times space 10 space cross times space 3 space equals space 2100 space straight J

Examiner Tip

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! 

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    Dan MG

    Author: Dan MG

    Expertise: Physics

    Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.