Topic One (1.1): The International System of Units

 Introduction

Measurements only have meaning if they are based on a coherent system of units. So proper units underpin all the rest of Physics, and it is vital to keep a good understanding and awareness of our internationally agreed system of base and derived units (SI Units).

You may have noticed that people from older generations may say “It’s really hot today, it’s 90 degrees outside!”. To younger people that sounds extreme as it is nearly the boiling point of water. But of course, they mean 90 degrees Farenheit, where we would say 32 degrees Celsius. Scientists quickly realised the importance of having everyone use the same units so that they could share data to improve reproducibility of experiments.

The SI units began with the metre in 1793, defined as one ten millionth of the distance between the north pole and the equator. Nearly two centuries of work followed until the SI unit system became official in 1960. It includes the units you will be familiar with from (I)GCSE Physics, such as metres, seconds, Amperes, Newtons, etc. You will need to know each quantity symbol and its unit, for example “mass (m) measured in kilograms (kg)”. Seven of the SI units are base units. You will need to be able to express the other units in terms of the base units. For example, force can be expressed as $kg\, m\,s^{-2}$. This is because $F=ma$, so Newtons = kilograms x metres per second squared.

 Reading

Edexcel AS/A Level Physics Student Book 1 pages 10 to 11 

Tasks

Complete the following:

• Read pages 10 to 11 of the textbook.
• Copy down and learn tables A, B and C – you will use these a lot during your course!

READ - SI Units and Prefixes Download READ - SI Units and Prefixes

WATCH – SI Units and Derived Units (YouTube) Links to an external site.

WATCH – SI Base Units and Derived Units | Cowan Physics (YouTube) Links to an external site.

WATCH – Units of Measurement in Physics (YouTube) Links to an external site.

WATCH – Making Measurements in Science (YouTube) Links to an external site.

WATCH – The kg is Dead, Long Live the kg (YouTube) Links to an external site.

WATCH - World’s Roundest Object (YouTube) Links to an external site.

COMPLETE – Gameboard: ‘A2 Derived and Base SI Units’ (Isaac Physics) Links to an external site.

• Complete the questions on page 11. You will find the answers in General Resources under the heading Textbook Answers.

 Top Tips

Make sure that you understand the following key points:

• Learn all the quantity symbols and their units from the textbook.
• Learn all the prefixes (e.g. kilo, mega, milli, etc.) and their matching power of ten (e.g. kilo is 10³ , mega is 10⁶ , milli is 10^-3).
• Use your list of Physics equations to practice deriving these quantities: force, energy, power, frequency, charge, potential difference, resistance.
• Pick simple equations to derive force, frequency and charge, such as $F=ma$, $f=1/T$ and $Q=It$.
• Several of the derived quantities (energy, power, potential difference and resistance) need combined equations to eliminate derived units. For example, energy is $W=Fd$ (work = force x distance), but force is a derived unit from $F=ma$. So put these together and you get  $W=mad$ (work = mass x acceleration x distance). Change these quantities to base units and you get $E = kg\,ms^{-2} m$. Which simplifies down to $E=kg\,m^{2}\,s^{-2}$.
• Always check you give appropriate units with every answer!

Key Terms

Add the following key terms with definitions to your word list:

• Base units
• Derived units
• SI prefixes