Scientific Work
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Transcript of Scientific Work
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Unit 1
THE SCIENTIFIC WORK
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Physics and Chemistry
What do they have in common? Physicists and Chemists study
the same: matter. Physicists, Chemists and other
scientists work in the same way: SCIENTIFIC METHOD
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Physics and Chemistry What makes them different?
Physics studies phenomena that don't change the composition of matter.
Chemistry studies phenomena that change the composition of matter.
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SCIENTIFIC METHOD
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SCIENTIFIC METHOD
The observation of a phenomenon and curiosity make scientists ask questions.
Before doing anything else, it's necessary to look for the previous knowledge about the phenomenon.
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SCIENTIFIC METHOD
Hypotheses are possible answers to the questions we asked.
They are only testable predictions about the phenomenon.
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SCIENTIFIC METHOD We use experiments for
checking hypotheses. We reproduce a
phenomenon in controlled conditions.
We need measure and collecting data in tables or graphics
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SCIENTIFIC METHOD We study the relationships
between different variables.
In an experiment there are three kinds of variables
Independent variables: they can be changed.
Dependent variables: they are measured.
Controlled variables: they don't change.
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SCIENTIFIC METHOD
After the experiment, we analyse its results and draw a conclusion.
If the hypothesis is true, we have learnt something new and it becomes in a law
If the hypothesis is false. We must look for a new hypothesis and continue the research.
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Magnitudes, measurements and units
Physical Magnitude: It refers to every property of matter that can be measured.
Length, mass, surface, volume, density, velocity, force, temperature,...
Measure: It compares a quantity of a magnitude with other that we use as a reference (unit).
Unit: It is a quantity of a magnitude used to measure other quantities of the same magnitude. It's only useful if every people uses the same unit.
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Magnitudes, measurements and units
Length of the classroom = 10 m
means
The length of the classroom is 10 times the length of 1 metre.
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The International Systemof Units
The SI has: a small group of magnitudes whose units
are fixed directly: the fundamental magnitudes.
E.g.: Length → meter (m); Time → second (s)
The units for the other magnitudes are defined in relationship with the fundamental units: the derivative magnitudes.
E. g.: speed → meter/second (m/s)
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The International Systemof Units
The fundamental magnitudes and their units
Length meter m
Mass kilogram kg
Time second s
Amount of substance mole mol
Temperature Kelvin K
Electric current amperes A
Luminous intensity candela cd
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The International Systemof Units
Some examples of how to build the units of derivative magnitudes:
Area = Length · width → m·m = m2
Volume = Length · width · height → m·m·m = m3
Speed = distance / time → m/s Acceleration = change of speed / time →
(m/s)/s = m/s2
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The International Systemof Units
Some examples of how to build the units of derivative magnitudes:
Area = Length · width → m·m = m2
Volume = Length · width · height → m·m·m = m3
Speed = distance / time → m/s Acceleration = change of speed / time →
(m/s)/s = m/s2
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The International Systemof Units
More derivative units.
Area square meter m2
Volume cubic meter m3
Force Newton N
Pressure Pascal Pa
Energy Joule J
Power Watt W
Voltage volt V
Frequency Hertz Hz
Electric charge Coulomb C
Quantity Name Symbol
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The International Systemof Units
Prefixes: we used them when we need express quantities much bigger or smaller than basic unit.
Power of 10 for Prefix Symbol Meaning Scientific Notation_______________________________________________________________________
mega- M 1,000,000 106
kilo- k 1,000 103
deci- d 0.1 10-1
centi- c 0.01 10-2
milli- m 0.001 10-3
micro- 0.000001 10-6
nano- n 0.000000001 10-9
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The International Systemof Units
Prefixes: the whole list Factor Name Symbol Factor Name Symbol
10-1 decimeter dm 101 decameter dam
10-2 centimeter cm 102 hectometer hm
10-3 millimeter mm 103 kilometer km
10-6 micrometer m 106 megameter Mm
10-9 nanometer nm 109 gigameter Gm
10-12 picometer pm 1012 terameter Tm
10-15 femtometer fm 1015 petameter Pm
10-18 attometer am 1018 exameter Em
10-21 zeptometer zm 1021 zettameter Zm
10-24 yoctometer ym 1024 yottameter Ym
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Changing units
We can change a quantity into another unit. Conversion factors help us to do it.
A conversion factor is a fraction with the same quantity in its denominator and in its numerator but expressed in different units.
1h60min
=1
60min1h
=1
1 km1000m
=1
1000m1 km
=1
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Changing units
Let's see a few examples of how to use them
30ms=30
ms·
1 km1000m
·3600 s
1h=30 ·3600 km
1000h=108
kmh
500 cm² · 1m100 cm
2
=500 cm² · 1m²10000 cm² =500m²
10000=0,05m²
3500 s ·1h
60min·1min60 s
= 3500h3600
=0,972h
2570m·1 km
1000m= 2570 km ·1
1000=2,570 km
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Significant figures
They indicate precision of a measurement. Sig Figs in a measurement are the really
known digits.
2.3 cm
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Significant figures Counting Sig Figs:
Which are sig figs? All nonzero digits. Zeros between nonzero digits
Which aren't sig figs? Leading zeros – 0,0025 Final zeros without
a decimal point – 250 Examples:
0,00120 → 3 sig figs; 15000 → 2 sig figs 15000, → 5 sig figs; 13,04 → 4 sig
figs
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Significant figures
Calculating with sig figs Multiplicate or divide: the factor with the
fewer number of sig figs determines the number of sig figs of the result:
2,345 m · 4,55 m = 10,66975 m2 = 10,7 m2
(4 sig figs) (3 sig figs) → (3 sig figs)
Add or substract: the number with the fewer number of decimal places determines the number of decimal places of the result:
3,456 m + 2,35 m = 5,806 m = 5,81 m (3 decimal places) (2 decimal places) → (2 decimal places)
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Significant figures
Calculating with sig figs Exact number have no limit of sig fig:
Example: Area = ½ · Base · height. ½ isn't taken into account to round the
result. Rounding the result:
If the first figure is 5, 6, 7, 8 or 9, the last figure taken into account is increased in 1
If not, it doesn't change.
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Scientific notation
Is used to write very large or very small quantities: 385 000 000 Km = 3.85·108 Km 0,000 000 000 157 m = 1,57·10-10 m
Changing a number to scientific notation: We move the decimal point until there is an only
number in its left side. The exponent of 10 is the number of places we
moved the decimal point: The exponent is positive if we move it to the left side It's negative if we move it to the right side.
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Measurement errors
It's impossible to measure a quantity with total precision.
When we measure, we'll never know the real value of the quantity.
Every measurement has an error because: The measurement instrument can only see
a few sig figs. It may not be well built or calibrated. We are using it in the wrong way.
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Measurement errors
There are two ways for expressing the error of a measurement:
Absolute error: it is the difference between the value of the measurement and the value accepted as exact.
Relative error: it is the absolute error in relationship with the quantity.
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Measurement errors
How to calculate the error. EXAMPLE 1: We have measured several times the mass of a ball:
20,17 g, 20,21 g, 20,25 g, 20,15 g, 20,28 g It's supposed that the real value of the ball of the
mass is the average value of all the measurements: Vr = (20,17 g + 20,21 g + 20,25 g + 20,15 g + 20,27 g )/5 = 20,21 g
The absolute error of the first measurement is: Er = |20,17 g – 20,21 g| = 0,04 g
The relative error is calculate dividing the absolute error by the value of quantity.
Ea = (0,04 g / 20,21 g) = 0,002 = 0,2 %
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Measurement error
How to calculate the error. EXAMPLE 2: We have measured once the length of a
piece of paper using a ruler that is graduated in millimetres: 29,7 cm
We suppose that the real value is the measured value.
The absolute error is the precision of the rule:
Ea = 0,1 cm
Relative error: Er = 0,1 cm / 29,7 cm = 0,0034 = 0,34 %