1.1.1

Use of SI Units

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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.
Jump to other topics
1

Physical Quantities & Units

1.1

Physical Quantities & Units

1.1.1Use of SI Units1.1.2SI Prefixes, Standard Form & Converting Units1.1.3End of Topic Test - Units & Prefixes1.1.4Avogadro's Constant1.1.5Scalars & Vectors
1.2

Errors & Uncertainty

1.2.1Measuring Length, Time & Volume1.2.2Measuring Temperature1.2.3Circuit Measurements1.2.4Limitation of Physical Measurements1.2.5Uncertainty1.2.6Estimation1.2.7Displaying Data1.2.8End of Topic Test - Measurements & Errors
2

Kinematics

2.1

Equations of Motion

2.1.1Motion in a Straight Line2.1.2Graphs of Motion2.1.3Bouncing Ball Example2.1.4End of Topic Test - Motion in a Straight Line2.1.5Acceleration Due to Gravity
3

Dynamics

3.1

Momentum & Newton's Laws of Motion

3.1.1Mass & Inertia3.1.2Newton's Laws3.1.3Momentum3.1.4End of Topic Test - Newton's Laws & Momentum3.1.5A-A* (AO3/4) - Newton's Third Law
3.2

Non-Uniform Motion

3.2.1Projectile Motion
3.3

Linear Momentum & Conservation

3.3.1Conservation of Momentum
4

Force, Density & Pressure

4.1

Force, Density & Pressure

4.1.1Fields4.1.2Force in Uniform Fields4.1.3Friction4.1.4Buoyancy4.1.5Terminal Speed4.1.6End of Topic Test - Acceleration Due to Gravity4.1.7Centre of Mass4.1.8Forces & Equilibrium4.1.9End of Topic Test - Scalars & Vectors4.1.10Moments4.1.11End of Topic Test - Moments & Centre of Mass4.1.12Density4.1.13Pressure
5

Work, Energy & Power

5.1

Work, Energy & Power

5.1.1Work & Energy5.1.2Power & Efficiency5.1.3Conservation of Energy5.1.4End of Topic Test - Work, Energy & Power
5.2

Gravitational Fields

5.2.1Newton's Law5.2.2Gravitational Field Strength5.2.3Gravitational Potential
6

Deformation of Solids

6.1

Materials

6.1.1Bulk Properties of Solids6.1.2Energy in Materials6.1.3Young Modulus6.1.4End of Topic Test - Materials
7

Waves

7.1

Simple Harmonic Motion

7.1.1Simple Harmonic Motion7.1.2Simple Harmonic Systems7.1.3Energy in Simple Harmonic Motion7.1.4Resonance7.1.5End of Topic Test - Simple Harmonic Motion7.1.6A-A* (AO3/4) - Simple Harmonic Motion7.1.7End of Topic Test - Gravitational Fields
7.2

Waves

7.2.1Progressive Waves7.2.2Intensity of Waves7.2.3Wave Speed & Phase Difference7.2.4Longitudinal & Transverse Waves7.2.5End of Topic Test - Progressive Waves7.2.6Electromagnetic Waves7.2.7Doppler Effect7.2.8Sound Waves7.2.9Measuring Sound Waves7.2.10End of Topic Test - Waves7.2.11Ultrasound Imaging7.2.12Ultrasound Imaging 2
8

Superposition

8.1

Superposition

8.1.1Stationary Waves8.1.2Stationary Waves 28.1.3End of Topic Test - Stationary Waves8.1.4A-A* (AO3/4) - Stationary Waves8.1.5Interference8.1.6Interference 28.1.7End of Topic Test - Interference8.1.8Diffraction8.1.9Diffraction Gratings8.1.10End of Topic Test - Diffraction
9

Thermal Physics

9.1

Circular Motion

9.1.1Circular Motion9.1.2Circular Motion 29.1.3End of Topic Test - Circular Motion
9.2

Thermal Physics

9.2.1Temperature9.2.2Measuring Temperature9.2.3Ideal Gas Law9.2.4Ideal Gases9.2.5Boyle's Law & Charles' Law9.2.6Molecular Kinetic Theory Model9.2.7Molecular Kinetic Theory Model 29.2.8Thermal Energy Transfer9.2.9Thermal Energy Transfer Experiments9.2.10End of Topic Test - Thermal Energy & Ideal Gases9.2.11First Law of Thermodynamics
10

Communication

10.1

Communication Channels

10.1.1Communication Channels10.1.2Modulation10.1.3Attenuation
10.2

Digital Communication

10.2.1Digital & Analogue Signals10.2.2Representing Sound
11

Electric Fields

11.1

Electric Fields

11.1.1Coulomb's Law11.1.2Electric Field Strength11.1.3Electric Field Strength 211.1.4Electric Potential11.1.5End of Topic Test - Electric Fields11.1.6A-A* (AO3/4) - Electric and Gravitational Field
11.2

Capacitance

11.2.1Capacitance11.2.2Parallel Plate Capacitor11.2.3Energy Stored by a Capacitor11.2.4Capacitor Discharge11.2.5Capacitor Charge
12

Current Electricity

12.1

Current Electricity

12.1.1Basics of Electricity12.1.2Mean Drift Velocity12.1.3Current-Voltage Characteristics12.1.4End of Topic Test - Basics of Electricity12.1.5Resistivity12.1.6End of Topic Test - Resistivity & Superconductors12.1.7Power and Conservation12.1.8Microphones12.1.9Components12.1.10Relays12.1.11Strain Gauges
12.2

DC Circuits

12.2.1Circuits12.2.2Potential Divider12.2.3Emf & Internal Resistance12.2.4End of Topic Test - Power & Potential
13

Magnetic Fields

13.1

Magnetic Fields

13.1.1Magnetic Flux Density13.1.2End of Topic Test - Capacitance & Flux Density13.1.3Moving Charges in a Magnetic Field13.1.4Magnetic Flux & Flux Linkage13.1.5Electromagnetic Induction13.1.6Electromagnetic Induction 213.1.7Magnetic Flux Density13.1.8Magnetic Resonance Scanning
13.2

Alternating Currents

13.2.1Alternating Currents13.2.2Operation of a Transformer13.2.3End of Topic Test - Electromagnetic Induction & AC13.2.4Rectification
14

Modern Physics

14.1

Quantum Physics

14.1.1The Photoelectric Effect14.1.2The Photoelectric Effect Explanation14.1.3End of Topic Test - The Photoelectric Effect14.1.4Collisions of Electrons with Atoms14.1.5Energy Levels & Photon Emission14.1.6Wave-Particle Duality14.1.7End of Topic Test - Absorption & Emission14.1.8Band Theory14.1.9Diagnostic X-Rays14.1.10X-Ray Image Processing14.1.11Absorption of X-Rays14.1.12CT Scanners
14.2

Nuclear Physics

14.2.1Rutherford Scattering14.2.2Atomic Model14.2.3Isotopes14.2.4Stable & Unstable Nuclei14.2.5A-A* (AO3/4) - Stable & Unstable Nuclei14.2.6Alpha & Beta Radiation14.2.7Gamma Radiation14.2.8Particles, Antiparticles & Photons14.2.9Quarks & Antiquarks14.2.10Particle Interactions14.2.11Radioactive Decay14.2.12Half Life14.2.13End of Topic Test - Radioactivity14.2.14Nuclear Instability14.2.15Mass & Energy14.2.16Binding Energy14.2.17A-A* (AO3/4) - Nuclear Fusion

Practice questions on Use of SI Units

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    The base SI units are:Fill in the list
  2. 2
    Which of these is the SI unit for time?Multiple choice
  3. 3
    Which of these are SI base units?True / false
  4. 4
    Which of the following are SI base units?True / false
  5. 5
    What is the SI unit for frequency?Multiple choice
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SI Prefixes, Standard Form & Converting Units