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SI Units

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Units of length?

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Mile, furlong, fathom, yard, feet, inches, Angstroms, nautical miles, cubits

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The SI system of units There are seven fundamental base units which are clearly defined and on which all other derived units are based: You need to know these

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The metre This is the unit of distance. It is the distance traveled by light in a vacuum in a time of 1/299792458 seconds.

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The second This is the unit of time. A second is the duration of 9192631770 full oscillations of the electromagnetic radiation emitted in a transition between two hyperfine energy levels in the ground state of a caesium-133 atom.

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The ampere This 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.

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The kelvin This is the unit of temperature. It is 1/273.16 of the thermodynamic temperature of the triple point of water.

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The mole One 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 10 23.

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The candela This is the unit of luminous intensity. It is the intensity of a source of frequency 5.40 x 10 14 Hz emitting 1/683 W per steradian.

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The kilogram This 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!

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Derived units Other physical quantities have units that are combinations of the fundamental units. Speed = distance/time = m.s -1 Acceleration = m.s -2 Force = mass x acceleration = kg.m.s -2 (called a Newton) (sometimes we write m.s -1 rather than m/s)

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Some important derived units (learn these!) 1 N = kg.m.s -2 (F = ma) 1 J = kg.m 2.s -2 (W = Force x distance) 1 W = kg.m 2.s -3 (Power = energy/time)

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Prefixes It is sometimes useful to express units that are related to the basic ones by powers of ten

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Prefixes PowerPrefixSymbolPowerPrefixSymbol 10 -18 attoa10 1 dekada 10 -15 femtof10 2 hectoh 10 -12 picop10 3 kilok 10 -9 nanon10 6 megaM 10 -6 microμ10 9 gigaG 10 -3 millim10 12 teraT 10 -2 centic10 15 petaP 10 -1 decid10 18 exaE

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Prefixes PowerPrefixSymbolPowerPrefixSymbol 10 -18 attoa10 1 dekada 10 -15 femtof10 2 hectoh 10 -12 picop10 3 kilok 10 -9 nanon10 6 megaM 10 -6 microμ10 9 gigaG 10 -3 millim10 12 teraT 10 -2 centic10 15 petaP 10 -1 decid10 18 exaE

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Examples 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 10 6 W

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Checking equations If an equation is correct, the units on one side should equal the units on another. We can use base units to help us check.

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Checking equations For example, the period of a pendulum is given by T = 2π l where l is the length in metres g and g is the acceleration due to gravity. In units m= s 2 = s m.s -2

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You will also be required to convert units to the correct SI units before you commence calculations. Km/h to m/s Eg/ 140km/h to m/s

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Let’s try some questions for a change

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Let’s do some measuring!

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Errors/Uncertainties

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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.

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Individual 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

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Individual 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

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Individual measurements When using a digital scale, the uncertainty is plus or minus the smallest unit shown. 19.16 ± 0.01 V

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Repeated 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.

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Repeated 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 mm Average value = 1563 mm Uncertainty = 1563 – 1558 = 5 mm Length of table = 1563 ± 5 mm This means the actual length is anywhere between 1558 and 1568 mm

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Precision and Accuracy The same thing?

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Precision A man’s height was measured several times using a laser device. All the measurements were very similar and the height was found to be 184.34 ± 0.01 cm This is a precise result (high number of significant figures, small range of measurements)

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Accuracy Height of man = 184.34 ± 0.01cm This is a precise result, but not accurate (near the “real value”) because the man still had his shoes on.

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Accuracy The man then took his shoes off and his height was measured using a ruler to the nearest centimetre. Height = 182 ± 1 cm This is accurate (near the real value) but not precise (only 3 significant figures)

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Precise and accurate The man’s height was then measured without his socks on using the laser device. Height = 182.23 ± 0.01 cm This is precise (high number of significant figures) AND accurate (near the real value)

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Random 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.

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Systematic/zero errors Sometimes all measurements are bigger or smaller than they should be. This is called a systematic error/uncertainty.

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Systematic/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.

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Systematic/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

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Random Errors Errors 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.

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Uncertainties In the example with the table, we found the length of the table to be 1563 ± 5 mm We say the absolute uncertainty is 5 mm The fractional uncertainty is 5/1563 = 0.003 The percentage uncertainty is 5/1563 x 100 = 0.3%

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Uncertainties If the average height of students at BISAK is 1.23 ± 0.01 m We say the absolute uncertainty is 0.01 m The fractional uncertainty is 0.01/1.23 = 0.008 The percentage uncertainty is 0.01/1.23 x 100 = 0.8%

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Let’s try some questions.

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Combining 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.

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Combining uncertainties When multiplying (or dividing) quantities, to find the resultant uncertainty we have to add the percentage uncertainties of the quantities we are multiplying.

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Combining 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 cm 3 % 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 cm 3 This means the actual volume could be anywhere between 286 and 314 cm 3

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Combining uncertainties When adding (or subtracting) quantities, to find the resultant uncertainty we have to add the absolute uncertainties of the quantities we are multiplying.

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Combining 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

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Who’s going to win? New York Times Latest opinion poll Bush 48% Gore 52% Gore will win! Uncertainty = ± 5%

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Who’s going to win? New York Times Latest opinion poll Bush 48% Gore 52% Gore will win! Uncertainty = ± 5%

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Who’s going to win? New York Times Latest opinion poll Bush 48% Gore 52% Gore will win! Uncertainty = ± 5%

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Who’s going to win Bush = 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)

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Let’s try some more questions!

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