QUANTITIES AND UNITS

Introduction

This module should be read in conjunction with the Programme of Study.

The Programme of Study lists the topics that are in the module and gives you the relevant page references for your textbook. This module aims to give you more direction as you work through the various topics. It will aim to highlight the most important items in the topic, as well as areas that need particular care. It will also point you towards the resources you should be using.

Remember to double-check the syllabus to make sure you cover all the items that you need to.

Module Overview

The first module introduces you to the international system of units, dealing with errors, uncertainty and accuracy, planning and conducting investigations, and analysing results. These topics are fundamentally important for the rest of your course. Physics is all about finding out truth about the universe through experiments, and obtaining clear, trustworthy answers from these is a very important skill.

A quantity in Physics is simply anything a physicist might measure. A Physics quantity is not just a number; it is a number together with a unit. So, in a consistent system of units, it is not just the numbers that must work out the same on both sides of the equation, the units on both sides of the equation must also be equivalent.

The international system of units used by scientists is not just a collection of units, it is indeed a system where all the units fit together. Confusion over units led to a Mars probe crashing into the surface instead of landing in 1999! If you put S.I. units into an equation, you know the answer will come out in S.I. units. All the units we use can be defined in terms of just seven base units. These are very precisely and cleverly defined. You do not actually need to remember the definitions of the units.

Accuracy, uncertainty and precision of measurements have special meanings in Science and Maths. Accuracy and uncertainty refer to the fact that all measurements will differ slightly from the actual ‘true’ value, so we can never be certain what the exact value is. The accuracy is the difference between the observed measurement and the true value, and precision means how fine the divisions are on a measuring instrument such as a ruler, or for a set of measurements, how close they are to each other (though not necessarily to the true value).

There are many other skills involved in planning and conducting investigations, and what you learn in this module will be an important foundation for all the exam questions which cover aspects of experimental investigations. If you are doing the full A Level in Physics, it will also help you to prepare for the Practical Endorsement. So, study this module carefully, make good summary notes, and prepare to refer often to this material as you progress through the course.

Maths required for this module

You will need to:

Round numbers to given significant figures or decimal places.
Use the logic of upper and lower bounds of individual numbers and calculated quantities.
Use standard form for representing very large or very small numbers.
Use appropriate units in calculations.
Estimate results, for example by rounding values to 1 significant figure and using these in mental calculations.
Use algebra, including rearranging equations and substituting values into equations.

These skills will also be needed for all subsequent modules (so they will not be mentioned again in these later modules).

The textbook gives a useful review of Maths skills needed for this course, so it may help to work through Pages 206 to 211 first.

Topics

Topic One (1.1): The International System of Units

Topic Two (1.2): Estimation

Topic Three (1.3): Errors and Accuracy

Topic Four (1.4): Planning and Conducting Investigations