4The SI system of unitsThere are seven fundamental base units which are clearly defined and on which all other derived units are based:You need to know these
5The metreThis is the unit of distance. It is the distance traveled by light in a vacuum in a time of 1/ seconds.
6The secondThis is the unit of time. A second is the duration of full oscillations of the electromagnetic radiation emitted in a transition between two hyperfine energy levels in the ground state of a caesium-133 atom.
7The ampereThis is the unit of electrical current. It is defined as that current which, when flowing in two parallel conductors 1 m apart, produces a force of 2 x 10-7 N on a length of 1 m of the conductors.
8The kelvinThis is the unit of temperature. It is 1/ of the thermodynamic temperature of the triple point of water.
9The moleOne mole of a substance contains as many molecules as there are atoms in 12 g of carbon-12. This special number of molecules is called Avogadro’s number and equals 6.02 x 1023.
10The candelaThis is the unit of luminous intensity. It is the intensity of a source of frequency 5.40 x 1014 Hz emitting 1/683 W per steradian.
11The kilogramThis is the unit of mass. It is the mass of a certain quantity of a platinum-iridium alloy kept at the Bureau International des Poids et Mesures in France.THE kilogram!
12Derived unitsOther physical quantities have units that are combinations of the fundamental units.Speed = distance/time = m.s-1Acceleration = m.s-2Force = mass x acceleration = kg.m.s-2 (called a Newton)(sometimes we write m.s-1 rather than m/s)
13Some important derived units (learn these!) 1 N = kg.m.s-2 (F = ma)1 J = kg.m2.s-2 (W = Force x distance)1 W = kg.m2.s-3 (Power = energy/time)
14PrefixesIt is sometimes useful to express units that are related to the basic ones by powers of ten
15Prefixes Power Prefix Symbol Power Prefix Symbol 10-18 atto a 101 deka da10-15 femto f 102 hecto h10-12 pico p 103 kilo k10-9 nano n 106 mega M10-6 micro μ 109 giga G10-3 milli m tera T10-2 centi c peta P10-1 deci d exa E
16Prefixes Power Prefix Symbol Power Prefix Symbol 10-18 atto a 101 deka da10-15 femto f 102 hecto h10-12 pico p 103 kilo k10-9 nano n 106 mega M10-6 micro μ 109 giga G10-3 milli m tera T10-2 centi c peta P10-1 deci d exa E
17Examples 3.3 mA = 3.3 x 10-3 A 545 nm = 545 x 10-9 m = 5.45 x 10-7 m 2.34 MW = 2.34 x 106 W
18Checking equationsIf an equation is correct, the units on one side should equal the units on another. We can use base units to help us check.
19Checking equations For example, the period of a pendulum is given by T = 2π l where l is the length in metresg and g is the acceleration due to gravity.In units m = s2 = sm.s-2
20You will also be required to convert units to the correct SI units before you commence calculations. Km/h to m/sEg/ 140km/h to m/s
24Errors/Uncertainties In EVERY measurement (as opposed to simply counting) there is an uncertainty in the measurement.This is sometimes determined by the apparatus you're using, sometimes by the nature of the measurement itself.
25Individual measurements When using an analogue scale, the uncertainty is plus or minus half the smallest scale division. (in a best case scenario!)4.20 ± 0.05 cm
26Individual measurements When using an analogue scale, the uncertainty is plus or minus half the smallest scale division. (in a best case scenario!)22.0 ± 0.5 V
27Individual measurements When using a digital scale, the uncertainty is plus or minus the smallest unit shown.19.16 ± 0.01 V
28Repeated measurements When we take repeated measurements and find an average, we can find the uncertainty by finding the difference between the average and the measurement that is furthest from the average.
29Repeated measurements - Example Iker measured the length of 5 supposedly identical tables. He got the following results; 1560 mm, 1565 mm, 1558 mm, 1567 mm , 1558 mmAverage value = 1563 mmUncertainty = 1563 – 1558 = 5 mmLength of table = 1563 ± 5 mmThis means the actual length is anywhere between 1558 and 1568 mm
31PrecisionA man’s height was measured several times using a laser device. All the measurements were very similar and the height was found to be± 0.01 cmThis is a precise result (high number of significant figures, small range of measurements)
32AccuracyHeight of man = ± 0.01cmThis is a precise result, but not accurate (near the “real value”) because the man still had his shoes on.
33AccuracyThe man then took his shoes off and his height was measured using a ruler to the nearest centimetre.Height = 182 ± 1 cmThis is accurate (near the real value) but not precise (only 3 significant figures)
34Precise and accurateThe man’s height was then measured without his socks on using the laser device.Height = ± 0.01 cmThis is precise (high number of significant figures) AND accurate (near the real value)
35Random errors/uncertainties Some measurements do vary randomly. Some are bigger than the actual/real value, some are smaller. This is called a random uncertainty. Finding an average can produce a more reliable result in this case.
36Systematic/zero errors Sometimes all measurements are bigger or smaller than they should be. This is called a systematic error/uncertainty.
37Systematic/zero errors This is normally caused by not measuring from zero. For example when you measured height without taking shoes off!For this reason they are also known as zero errors/uncertainties. Finding an average doesn’t help.
38Systematic/zero errors Systematic errors are sometimes hard to identify and eradicate.Normally occurs because of the equipment or method.Can be minimised by taking sensible precautions such as checking for zero errors and avoiding parallax errors and by drawing a suitable graph
39Random ErrorsErrors that occur when the readings are too big or too small.Can be minimised by taking averages for a number of repeated measurements and by drawing a graph that in effect averages t range of values.
40UncertaintiesIn the example with the table, we found the length of the table to be 1563 ± 5 mmWe say the absolute uncertainty is 5 mmThe fractional uncertainty is 5/1563 = 0.003The percentage uncertainty is 5/1563 x 100 = 0.3%
41Uncertainties If the average height of students at BISAK is 1.23 ± 0.01 mWe say the absolute uncertainty is 0.01 mThe fractional uncertainty is 0.01/1.23 = 0.008The percentage uncertainty is 0.01/1.23 x 100 = 0.8%
43Combining uncertainties When we find the volume of a block, we have to multiply the length by the width by the height.Because each measurement has an uncertainty, the uncertainty increases when we multiply the measurements together.
44Combining uncertainties When multiplying (or dividing) quantities, to find the resultant uncertainty we have to add the percentage uncertainties of the quantities we are multiplying.
45Combining uncertainties Example: A block has a length of 10.0 ± 0.1 cm, width 5.0 ± 0.1 cm and height 6.0 ± 0.1 cm.Volume = 10.0 x 5.0 x 6.0 = 300 cm3% uncertainty in length = 0.1/10 x 100 = 1%% uncertainty in width = 0.1/5 x 100 = 2 %% uncertainty in height = 0.1/6 x 100 = 1.7 %Uncertainty in volume = 1% + 2% + 1.7% = 4.7%(4.7% of 300 = 14)Volume = 300 ± 14 cm3This means the actual volume could be anywhere between 286 and 314 cm3
46Combining uncertainties When adding (or subtracting) quantities, to find the resultant uncertainty we have to add the absolute uncertainties of the quantities we are multiplying.
47Combining uncertainties One basketball player has a height of 196 ± 1 cm and the other has a height of 152 ± 1 cm. What is the difference in their heights?Difference = 44 ± 2 cm
48Who’s going to win? New York Times Bush 48% Gore 52% Gore will win! Latest opinion pollBush 48%Gore 52%Gore will win!Uncertainty = ± 5%
49Who’s going to win? New York Times Bush 48% Gore 52% Gore will win! Latest opinion pollBush 48%Gore 52%Gore will win!Uncertainty = ± 5%
50Who’s going to win? New York Times Bush 48% Gore 52% Gore will win! Latest opinion pollBush 48%Gore 52%Gore will win!Uncertainty = ± 5%Uncertainty = ± 5%
51(If the uncertainty is greater than the difference) Who’s going to winBush = 48 ± 5 % = between 43 and 53 %Gore = 52 ± 5 % = between 47 and 57 %We can’t say!(If the uncertainty is greater than the difference)