WJEC Physics for AS Level Student Book 2nd Edition

9 Physics is an experimental science. It involves making measurements of quantities, such as pressure, speed, electric current and temperature, and discovering laws, which concern relationships between quantities, and formulating theories, to explain why natural phenomena occur. This topic includes some details of how to handle physical quantities, which are covered in Level 2 courses such as GCSE Physics. Some of its contents will therefore be familiar but they are taken to a higher level. 1.1.1 Quantities and units In physics, a quantity is a physical property of an object or material which can be measured. An example of a quantity is density . The value of the density of air at room temperature and pressure is 1.28 kg m – 3 . Notice that the unit of density ( kg m – 3 ) is expressed in terms of two other units, kg and m (kilograms and metres). This is explored in the next section. (a) Base quantities and units In order to measure a quantity, such as length, we need to have a defined standard to compare the length with. In our system of units, Le système international d’unités (abbreviated to SI), the defined unit of length is the metre with the abbreviation m . What does a reported length of, say, 53.7 m mean? 53.7 m = 53.7 × the defined unit of length; in other words the distance light can travel in 53.7/ 299792458 of a second! Table 1.1.1 shows the 7 base quantities with their SI units. Quantity Unit Name of quantity Symbol Name of unit abbreviation mass m kilogram kg length Ɛ metre m time t second s electric current I ampère A temperature T kelvin K amount of substance n mole mol luminous intensity L candela cd Table 1.1.1 SI quantities and units Notice that the definition of the metre depends upon another definition (the second) as well as a physical property (the speed of light). The table also has common symbols for the quantities, e.g. t for time and Ɛ for length. Other symbols can be used, e.g. x and r for lengths and M for mass. Study point Don’t think of a theory as just a hunch or a guess. In order to be called a theory an explanation needs a lot of experimental evidence to support it and it has to explain a range of phenomena. The statement, ‘It’s just a theory!’ misses the point. Study point Since 1983, the metre has been defined as ‘the length of the path travelled by light in a vacuum during a time interval of 1/299792458 of a second’. Key term Quantity: A quantity is represented by a number multiplied by a unit. Study point The symbols for quantities are printed in italics , e.g. m , T . The symbols for units are printed in plain print, e.g. kg , K . Top tip Luminous intensity is not used in AS or A level Physics. 1.1 Basic physics