Page 554 - Physics Coursebook 2015 (A level)

Cambridge International AS and A Level Physics
Prefix
Symbol
Value
542
Physical quantity
Unit
Appendix 1:
Physical quantities and units
Physical quantities have a numerical value and a unit. In physics, it is essential to give the units of physical quantities. For example, mass can be measured in kilograms. Hence you might write the mass of the trolley as:
mass of trolley = 0.76 kg
It would be a serious error to omit the unit kg at the end of the numerical value.
The scientific system of units is called the Système Internationale d’Unités (or SI system). The seven base units of this system are listed in Table 1. Each of the units is carefully defined, but the definitions need not concern us here.
All other units can be derived from the seven base units. For example:
■■ volume is measured in cubic metres (m3)
■■ velocity is measured in metres per second (m s−1)
■■ density is measured in kilograms per cubic metre (kg m−3).
Prefixes
In physics, you will have to cope with very small and very large numbers. Numbers are written using powers of 10
to make them less awkward. This is known as scientific notation. Prefixes are used as an abbreviation for some of the powers of 10. For example, the height of a 5400 m high mountain may be written as either 5.4 × 103 m or 5.4 km. The prefixes you will need most often are shown in Table 2.
mass
length
time
temperature
electric current
amount of substance
luminous intensity
kilogram, kg
metre, m
second, s
kelvin, K
ampere, A
mole, mol
candela, cd
pico p 10−12
nano n 10−9
micro 10−6
milli m 10−3
centi c 10−2
deci d 10−1
kilo k 103
mega M 106
giga G 109
tera T 1012
Estimation
Table 2 Some of the prefixes used in the SI system.
Table 1 The seven base units of the SI system. (Note that you are not required to use the candela in this book.)
When you carry out an experiment or a calculation, it
is sensible to look at the answer that you get (and the results of intermediate calculations) to see if they seem reasonable. The only way you can know if an answer is absurd is if you are aware of some benchmarks. Some suggestions are given below. Try to add to this list as you go through your physics course.
mass of a person
height of a person
walking speed
speed of a car on the motorway volume of a can of drink density of water
weight of an apple
typical current in domestic appliance e.m.f. of a car battery
70 kg
1.5 m 1 m s−1
30 m s−1 300 cm3 1000 kg m−3 1 N
13 A 12 V