How long left ? Until my exams.. Until the weekend 🙂 We are always racing off into time (t), that intangible, unknowable quantity. There is an awful lot of it but it seems to be passing all the same. Another week has passed since the last. So one week less.

**Non renewable and finite**. The time we waste will not be replaced.

Time is one of the **seven SI base units** you will come across in your Physics course. **SI** refers to Le **S**ystème** I**nternational *d’Unités *which were established in 1960 so that scientists across the world would have a common language in which to discuss and interchange information regarding **quantities and their units**.

The others are

- The
**ampere (A)**The ampere is the SI base unit of electrical current. … - The
**candela (cd)**The candela is the SI base unit of luminous intensity. … - The
**kelvin (K)**The kelvin is the SI base unit of thermodynamic temperature. … - The
**kilogram (kg)**The kilogram is the SI base unit of mass. … - The
**metre (m)**The metre is the SI base unit of distance.. - The
**mole (mol)**The mole is the SI base unit for quantity..

As far as** IGCSE and A-level Physics** are concerned the only base unit that causes some confusion is the **ampere (A)** because it is defined as the rate of flow of charge (Q/t) which might make you think that charge (measured in coulombs (C) ) would be the base unit. The story of electric current **(A)** is however a story for another day. As for the **mole** **(mol)** please **don´t use the word amount** when describing quantities in Physics or Chemistry. It is just too imprecise.

**Precision and accuracy** also cause confusion sometimes. When talking about **accuracy** we refer to the fact that the measurement is close to a true underlying value, some truth. **Precision** really relates to our instrumentation and in simple terms refers to how many significant figures we can make our measurement to. **Be careful though**, although a reading may give a value of high precision eg 8.1417… it may be completely wrong ie not accurate !! You need to be clear about these definitions.

**All other units in Physics are derived** ie they come from these 7 base units. So the unit for force which we all know is the Newton (N) can be expressed in its base units by going back a little. Using a few common equations (F=ma, W=Fd etc) we can work out the base units for any derived unit. It may look difficult initially but with practice it gets easier. Until in the end you will just think of the Newton as a Kgm/s^2 and a **Joule** as a **Kgm^2/s^2.**

**Standard form** to clear things up, when we talk about powers for example two squared, it will be expressed as 2^2. But one divided by two squared has two ways in which it can be expressed; the first is 1/2^2 and the second is 2^-2. Get used to both notations.

Some things to think about as we end this post;

When using the** kelvin (K)** scale we do not use º. just kelvin as in -273.15K (absolute zero of course)

You can see the** kilogram (Kg)** prototype, a platinum-iridium alloy stored where ?

and just what is meant by a **candela (Cd)** ?

see you next week when we will be looking at **scalars and vectors** and why you need to know about them !!