Scientific Work

Unit 1

THE SCIENTIFIC WORK

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

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.

SCIENTIFIC METHOD

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.

SCIENTIFIC METHOD

Hypotheses are possible answers to the questions we asked.

They are only testable predictions about the phenomenon.

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

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.

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.

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.

Magnitudes, measurements and units

Length of the classroom = 10 m

means

The length of the classroom is 10 times the length of 1 metre.

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)

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

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

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

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

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

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

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

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

Significant figures

They indicate precision of a measurement. Sig Figs in a measurement are the really

known digits.

2.3 cm

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

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)

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.

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.

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.

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.

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 %

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 %