Seneca
Sign up

Use of SI Units

Physics: Cambridge International A Level, topic 1.1.1

SI Base Units

SI base units are the standard units used to describe physical quantities in physics. They are:

Metre (m)

Metre (m)

  • The SI unit for distance (length).
Kilogram (kg)

Kilogram (kg)

  • The SI unit for mass.
Second (s)

Second (s)

  • The SI unit for time.
Ampere (A)

Ampere (A)

  • The SI unit for electric current.
Kelvin (K)

Kelvin (K)

  • The SI unit for temperature.
Mole (mol)

Mole (mol)

  • The SI unit for chemical mass.
Candela (cd)

Candela (cd)

  • The SI unit for luminous intensity.

Combining SI Units

SI base units can be combined to form lots of other useful SI units:

Hertz (Hz)

Hertz (Hz)

  • The SI unit for frequency.
Newton (N)

Newton (N)

  • The SI unit for force.
Joule (J)

Joule (J)

  • The SI unit for energy.
Watt (W)

Watt (W)

  • The SI unit for power.
Pascal (Pa)

Pascal (Pa)

  • The SI unit for pressure.
Coulomb (C)

Coulomb (C)

  • The SI unit for electric charge.
Volt (V)

Volt (V)

  • The SI unit for electric potential difference.
Ohm (Ω.)

Ohm (Ω.)

  • The SI unit for electrical resistance.
Tesla (T)

Tesla (T)

  • The SI unit for magnetic flux density.

Celsius (°C)

  • Degrees Celsius are derived from the base SI unit for temperature (Kelvin).
  • To convert between Kelvin and °C use the following equation:
  • °C = K - 273.

Homogeneous Equations

We can use the SI base units to check the equations that relate different physical quantities.

Homogeneous equations

Homogeneous equations

  • In maths, we are used to the left-hand side of equations equalling the right-hand side.
  • When the numbers involved have units, the units of the left-hand side must be of the same type as the units of the right-hand side.
  • This is what is meant by a homogeneous equation.
Homogeneous equations

Homogeneous equations

  • For example, 5 m + 5 m = 10 g doesn't make any physical sense, though the numbers add up correctly.
  • However, 10 km = 10,000 m does make sense, as both kilometres and meters are units of distance.
Equation for speed

Equation for speed

  • We know that the SI base unit for time is the second and for distance is the metre.
  • So the SI derived unit for speed is metres per second.
  • Writing this in equation form:
  • speed=distancetime→metres per second=metresseconds\text{speed} = \frac{\text{distance}}{\text{time}}\rightarrow \text{metres per second} = \frac{\text{metres}}{\text{seconds}}
Derived units

Derived units

  • We can use the fact that equations are homogeneous to work out units for physical quantities in terms of the SI base units.
  • For example, Newton's second law or F=maF=ma has dimensions:
  • Newtons=kilograms×metres per second squared\text{Newtons} = \text{kilograms}\times \text{metres per second squared}
  • So a force of 10 N is equivalent to 10 kgms-2.

Test yourself on Use of SI Units

  • Which of these is the SI unit for time?

  • Which of these are SI base units?

  • Which of the following are SI base units?

  • What is the SI unit for frequency?

Practise this topic for free