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HomeStudyPhysical Quantities - Physics As & A'levels | Chapter 1 | Part...

Physical Quantities – Physics As & A’levels | Chapter 1 | Part 1

A detailed Informative Account on SI Units

SI Base 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. SI units are based on the
units of six base quantities:

All quantities in Science consist of a magnitude and a Unit. The system for units used across the world is known as SI Units. SI units are based on seven base quantities.

  • Length – meter (m)
  • Time – second (s)
  • Amount of substance – mole (mole)
  • Electric current – ampere (A)
  • Temperature – kelvin (K)
  • Luminous intensity – candela (cd)
  • Mass – kilogram (kg)

Degree Celcius is a unit most commonly known and used but it’s not an SI unit.

In general physics, many units are used for measuring different quantities like Pressure, Velocity, Weight, acceleration, and joules these are Derived Units.

Derived Units

Derived units are results of mathematical calculations of SI units under the Laws and theorems of Physics. For example Velocity = Displacement/Time

V = displacement in meter/ time in seconds resulting in m/s. Acceleration is derived by the Formula Acceleration = Velocity/ time resulting in its unit m/s2

Here is a list of some physical quantities you should know:

Physical QuantityNameSymbolExpressed in SI Base Units
forcenewtonNm kg s-2
pressure, stresspascalPaN m-2 = m-1 kg s-2
energy, work, heatjouleJN m = m2 kg s-2
power, radiant fluxwattWJ s-1 = m2 kg s-3
electrical chargecoulombCA s
electrical potential, electromotive forcevoltVJ C-1 = m2 kg s-3 A-1
electrical resistanceohmΩV A-1 = m2 kg s-3 A-2
electric conductancesiemensSohm-1 = m-2 kg-1 s3 A2
electrical capacitancefaradFC V-1 = m-2 kg-1 s4 A2
magnetic flux densityteslaTV s m-2 = kg s-2 A-1
magnetic fluxweberWbV s = m2 kg s-2 A-1
inductancehenryHV A-1 s = m2 kg s-2 A-2
luminous fluxlumenlmcd sr
illuminanceluxlxcd sr m-2
radioactive activitybecquerelbqs-1
absorbed dose of radiationgrayGyJ kg-1 = m2 s-2
dose equivalent of radiationsievertSvJ kg-1 = m2 s-2
plane angleradianrad1 m m-1
solid anglesteradiansr1 m2 m-2
areasquare metreAm2
volumecubic metreVm3
speed, velocitymetre per secondvm/s
accelerationmetre per second squaredam/s2
wavenumberreciprocal metreσm–1
density, mass densitykilogram per cubic metreρkg/m3
surface densitykilogram per square metreρAkg/m2
specific volumecubic metre per kilogramvm3/kg
current densityampere per square metrejA/m2
magnetic field strengthampere per metreHA/m
concentrationmole per cubic metrecmol/m3
mass concentrationkilogram per cubic metreρ, γkg/m3
luminancecandela per square metreLvcd/m2
refractive indexonen1
relative permeabilityoneµr1
List for Derived Physical Units

Note: Always remember Units are important for your exam. In an examination any answer without an applicable unit is unacceptable and you should always recheck your paper for units.

Checking homogeneity of equations
If you are not sure if an equation is correct, you can use the units of the different
quantities to check it. The units on both sides of the equation must be the same.

Using Standard Forms

Always use Standard Forms to avoid confusion plus it’s a very efficient and easy way to cope up with large quantities and minute quantities. 15300000000000 can easily be written as 1.53 x 10^13.

Quick Question

Q1: Prove that the base Units for Potential Energy Ep = mgh and Kinetic Energy

Ek = 1/2mv2. Show your base units.

Q2: A Thermal Power Plant is providing 5600000000 watts of energy to an area and the Voltage supplied is 250000000000mV. What is the Current supplied in Amperes?

Multiples and submultiples of base units

Sometimes, the basic unit is either too large or too small. It would not be sensible to
use metres to measure the gap between the terminals of a spark plug in a car engine.
Instead, you would use millimetres (mm). The prefix ‘milli’ means divide by 1000 or
multiply by 10−3. Similarly, the metre would not be a sensible unit to measure the
distance between Cambridge and Hong Kong; you would use kilometres (km). Here,
the prefix is ‘kilo’, which means multiply by 1000 or 103.

Prefixes and Suffixes for Base Units

These are the recognised SI prefixes. The deci- (d) prefix is often used in measuring
volume, where the decimetre cubed (dm3), is particularly useful.
Remember that 1 dm3 is 1/1000 of 1 m3 as it is 1/10 m × 1/10 m × 1/10 m.

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