Physical quantities

  • Physical quantity is a property of an object or substance that can be measured using an appropriate measuring instrument.

SI units

  • All quantities in science consist of a number and a unit
  • There is a system of units used throughout the scientific world known as SI units. The SI units are based on the units of six base quantities:
    • Length          metre (m)
    • Mass          kilogram (kg)
    • Time          second (s)
    • Temperature     Kelvin (K)
    • Electric current      ampere (A)
    • Amount of substance mole (mol)


Derived Units

  • The units of all other quantities are derived from these base units. For example, speed is found by dividing the distance travelled by the time taken. Therefore the unit of speed is metres per second which can be written as m/s or ms-1
  • Each derived quantity has units which show how it is related to the base quantities.


  • Example The unit of force is the Newton. What is this in derived SI units?
    Force(N) = mass(kg) × acceleration(ms−2)

Unit of mass = kg

Unit of acceleration = ms-2

Therefore units of Newtons = kgms-2


Measuring length

  • Length is a straightline distance between two points along an object
  • SI unit for length is the metre (m)
  • Instruments used to measure length include
  • Ruler, measuring tape
  • Vernier callipers
  • Screw gauge micrometre



  • Have markings as small as a centimetre. Each centimetre is divided into 10 divisions which are millimetre.
  • When taking measurements your eye must be directly above the reading to avoid parallax error
  • Place the object to be measured in line with the zero mark to avoid zero error.
  • Rulers can measure to the nearest millimetre


Vernier callipers

  • Is used to accurately measure the thickness or internal diameter of small objects.
  • The callipers use a vernier scale and the simplest type enables a length to be measured to 0.01cm. it is a small sliding scale which is 9mm long but divided into 10 equal divisions so;

  • One end of length to be measured is made to coincide with the zero of the millimetre scale and the other end with the zero of the vernier scale
  • To measure internal diameter of a narrow tube, place the internal jaws inside the tube. Move the jaws apart until they touch the inner sides of the object. Once the jaws are in position, tighten the screw clamp to ensure the vernier scale does not move out of place while measurement is read.


How to read a vernier

  • To measure the internal or external diameter
    • Take the millimetres from the man scale marking before the zero on the vernier scale.
    • Take the next reading from the first vernier mark to coincide with a main scale mark
    • Add the two readings

Example :  What is the reading the instrument shown on the diagram above

Main scale reading = 11mm

Vernier reading = 0.4 mm

Final reading = 11.4mm



Read the following vernier scales

Screw gauge micrometer

  • Can measure smaller lengths than the vernier callipers.
  • It can measure very small objects to 0.001cm. one revolution of the thimble opens the accurately flat, parallel jaws by one division on the scale on the shaft of the gauge; this is usually , i.e. 0.05cm
  • If the thimble has a scale of 50 divisions round it, then rotation of the drum by one division opens the jaws by
  • A friction clutch ensures that the jaws exert the same force when the object is gripped.

To read the micrometer

  • Take the reading of millimetres and half millimetres from the sleeve 3.50mm
  • Take the reading from the thimble        0.11mm
  • Add the readings together            3.61mm



Read a micrometer screw gauge

Measuring current and voltage

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