Units

  2. Basics
  3. Units
    4. 10'

We will distinguish between fundamental and derived units.

In IB Physics there are six fundamental units from which all others required can be derived.

Units can be used as a mechanism to check the possibility that an equation could exist.

Key Concepts

Fundamental units

These units are referred to as fundamental, which means they form the basis of all other derived units. These were agreed by scientists at a conference and, henceforth, known as the Systeme Internationale base units.

Quantity Unit Symbol
Length  meter  m 
Mass  kilogram  kg 
Time  second  s 
Electric current  ampere  A 
Temperature  kelvin  K 
Quantity of substance  mole  mol 
(Luminosity)  (candela)  (cd)

Note that the kilogram is the only fundamental unit with a unit prefix as standard.

 

Derived units

The units below are not base units; they can all be derived from the base units.

Quantity Unit Symbol
Plane angle  radian  rad 
Frequency  hertz  Hz 
Force  newton  N 
Pressure  pascal  Pa 
Energy  joule  J 
Power  watt  W 
Charge  coulomb  C 
Potential  volt  V 
Resistance  ohm  Ω 
Capacitance  farad  F 
Magnetic flux  weber  Wb 
Magnetic flux density  tesla  T 
Activity  becquerel  Bq 

 

Conversions

Area: Square the unit prefix conversion factor

1 m2 = 104 cm2 = 106 mm2

Volume: Cube the unit prefix conversion factor

1 m3 = 106 cm3 = 109 mm3 = 1000 litres

Temperature

273 K = 0 °C

Angle

2π radian = 360°

Energy: There are several units of energy, each with an appropriate use

1 kcal = 4.4 kJ

1 kWh = 3.6 MJ

1 kilo Watt hour is the energy required to power a 1 W device for 1 hour (NB: energy = power x time)

1 eV = 1.6x10-19 J

1 electron volt is the kinetic energy gained by 1 electron accelerated through a potential difference of 1 volt (NB: energy = charge x potential difference)

 

 

 

Essentials

Dimensional analysis

The units for any quantity in mechanics can be expressed in terms of mass (M) length (L) and time (T).

Quantity unit Dimensions
velocity m s-1 L T-1
acceleration m s-2 L T-2
Force N M L T-2
Energy J M L2 T-2
Power W M L2 T-3

This video gives an example - the dimensional analysis of power.

 

Dimensional consistency

In any formula, the dimensions on the right of the equals sign must be the same as the dimensions on the left. This is also true for quantities being added or subtracted.

 

 

 

Test Yourself

Use quizzes to practise application of theory.


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