Unit 1 Basic Physical Science

Unit 1 Basic physical science Physics is the science of matter and energy, and includes the principles that govern the motion of particles and waves, the interaction of particles, and the properties of molecules, atoms, nuclei.

Unit 1 Basic physical science • • • • •

1.1 Measurement and units 1.2 Nature of matter 1.3 Mass, weight and density 1.4 Force and energy 1.5 Wave and electromagnetic spectrum

1.1 Measurement and units –

SI (Système International) units. The metre (m) is the base unit of length. • • • • • •

The kilogram (kg) is the base unit of mass. The second (s) is the base unit of time. The ampere (A) is the base unit of electrical current. The kelvin (K) is the base unit of temperature. The mole (mol) is the base unit of the amount of a substance. The candela (cd) is the base unit of light intensity.

– Quantities such as speed (ms-1 ) and density (kgm-3 ) which are not expressed in a single base unit are expressed in derived units. – There are others system of unit, e.g. British system.

Quantity

Symbol

Name of unit

Symbol for unit

Base units

speed or velocity

v

m s-1

m s-1

acceleration

a

m s-2

m s-2

force

F

newton

N

kg m s-2

energy

E

joule

J

kg m2 s-2

power

P

watt

W

kg m2 s-3

pressure

p

pascal

Pa

kg m-l s-2

frequency

f

hertz

Hz

s-1

charge

Q

coulomb

C

As

Potential difference

V

volt

V

A-1 kg m2 s-3

resistance

R

ohm

Q

A-2 kg m2 s-3

capacitance

C

farad

F

A2 kg-1 m-2 s4

magnetic flux

B

tesla

T

A-1 kg s-2

• Quantities which have no units are described as being dimensionless - for example the refractive index of a material is a ratio of like quantities (light speeds) and therefore has no unit. • Workedexample Show that the base units of density are kgm-3.

mass density = volume

so the units are or kg/m3 This is more usually written as kgm-3 - division is indicated by a negative index.

Conversion of Units Units

can be treated algebraically in calculations

eg. :x=vt=80*3 (km/h)* h=240 km Conversion

of units: In a calculation all physical quantities should have consistent system of units. If not we use conversion factors to convert units between systems. 1mi eg.: 1 mi=1.61 km, the conversion factor is 1.61km = 1 1 but 1.61 ≠ 1 x=240 km=240km*1=240km* (1mi/1.61km)=149mi

• Powers of ten shorthand: e.g.

4000 = 4× 10× 10× 10 400 = 4× 10× 10 40 = 4× 10 4× 101 4 = 4× 1 0.4 = 4/10 = 4/101 0.04 = 4/100 = 4/102 0.004 = 4/1000 = 4/103

= 4× 103 = 4× 102 = = 4× 100 = 4× 10-1 = 4× 10-2 = 4× 10-3

• Prefixes Multiple

Prefix

Symbol

Example

10-12

pico

p

pF picofarad

10-9

nano

nm nanometre

10-6

micro

n µ

µ A microamp

10-3

milli

m

ms millisecond

103

kilo

k

kg kilogram

106

mega

M

MJ megajoule

109

giga

G

GW gigawatt

Number

Number in standard form 1 000

1.0 × 103

140 000 000

1.4 × 108

128 600

1.286 × 105

0.015

1.5 × 10-2

0.003 86

3.86 × 10-3

• Significant figures: 2 cm 2.0 cm 2.00 cm

1.5−2.5 1.95−2.05 1.995−2.005

cm cm cm

The greater the number of significant figures given, the greater the implied precision of the measurement. 0.0385 5000

(3 sig. fig.); 2.73× 103 (1, 2, 3 or 4 sig. fig.); 5.0× 103

(3 sig. fig.); (2 sig. fig.);

5× 103

(1 sig. fig.);

(3 sig. fig.);

5.00× 103

VERY IMPORTANT: Generally, in a physical question, the final solution should contains 2 or 3 sig. fig. Example: He is 180.123456789012…. m tall (what’s wrong with it?)

1.2 Nature of matter • Matter is made up of tiny particles or molecules which are too small for us to see directly. Molecules consist of even smaller particles called atoms.

• Brownian motion Qualitative evidence of the microscopic nature of gases is shown by an effect called Brownian motion. Robert Brown was a botanist studying how pollen fertilizes an egg (ovum) in a flowering plant. When observing grains of pollen under a microscope, he noticed tiny particles moving around.

• an animation of Brownian motion http://www.phy.ntnu.edu.tw/~hwang/gas2D/gas2D.html

• States of matter

Particles have a regular pattern, are close together, they move slightly and have high density

Particles are more widely spaced, move more freely and have a medium density

Particles are widely spaced, move randomly at high speed and have a low density

The electric forces between atoms in a solid can be represented by springs

• Diffusion Smells, pleasant of otherwise, travel quickly and are caused by rapidly moving molecules. The spreading of a substance is called diffusion and is due to molecular motion.

1.3 Mass, weight and density • Density is the mass per unit volume of a substance mass density = volume

m d= V

• SI unit of density is the kilogram per cubic meter. • To convert a density from g/cm3 to kg/m3, we multiply by 103. For example the density of water is 1.0g/cm3 or 1.0× 103kg/m3.

•Worked example Taking the density of copper as 9g/cm3, find a. the mass of 5cm3 copper b. the volume of 63g copper.

3 3 m = V × d = 5 cm × 9 g / cm = 45 g a.

b.

m 63 g 3 V= = = 7cm 3 d 9 g / cm

• Simple density measurements a. Regularly shaped solid The mass is found on a balance and the volume by measuring its dimensions with a ruler.

b. Irregularly shaped solid, e.g. pebble The solid is weighed and its volume measured by one of the methods shown in the following figures.

c. Liquid – A known volume is transferred from a measuring cylinder into a weighed beaker which is reweighed to give the mass of liquid.

d. Air – A flask is weighed full of air and then after removing the air with a vacuum pump; the difference gives the mass of air in the flask. The volume of air is found by filling the flask with water and pouring it into a measuring cylinder.

• Floating and sinking An object sinks in a liquid of smaller density than its own; – otherwise it floats, partly or wholly submerged. –

For example, a piece of glass of density 2.5g/cm3 sinks in water (density 1.0 g/cm3) but floats in mercury (density 13.6 g/cm3). An iron nail sinks in water but an iron ship floats because its average density is less than that of water.

Buoyant force Any body completely or partially submerged in a fluid is lifted up by the buoyant force. According to the Archimedes’ principle, the magnitude of the buoyant force always equals to the weight of the fluid displaced by the body.

1.4 Force and energy • Force A force is a push or a pull. It can cause a body at rest to move, or if the body is already moving it can change its speed or direction of motion. It can also change its shape or size.

• Weight The weight of a body is the force of gravity of the Earth on the body. – gravity is a attractive force between masses Weight has to be distinguished from mass, which is a measure of a resistance for an object to move under a force

• The Newton The unit of force is Newton (N). The weight of a body can be measured by hanging it on a spring balance marked in newton. The greater the pull the more the spring stretches. On most of the Earth’s surface: The weight of a mass of 1kg is 9.8N. (Often this taken as 10N) or 1 kg = 10 N

• Hooke’s Law extension ∝ stretching force It is true only if the elastic limit of the spring is not exceeded.

The force constant k of a spring is the force needed to cause unit extension, i.e. 1 m. F k= e

( N / m)

• Worked example A spring is stretched 10mm (0.01m) by a weight of 2.0N. Calculate a. the force constant k b. the weight W of an object which causes an extension of 80mm (0.08m). a.

F 2.0 N k= = 200 N / m e 0.01m

b. W = stretching force F = k × e = 200 N / m × 0.08m = 16 N

• Energy When an apple of mass m falls freely, its altitude z and speed v change together in such a way that the 1 2 E = mgz + mv stays constant, where g is the quantity 2 acceleration of gravity at Earth’s surface.

v

mg z

mg

mgz, 1 2 mv 2

the potential energy the kinetic energy.

1 E = mgz + mv 2 2

the mechanical energy.

E stays constant, conservation of mechanical energy

• kinetic energy is the energy possessed by an object due to its motion. A fast moving object will have more kinetic energy than an identical slow moving object. • Potential energy is the energy possessed by an object due to its position relative to some others objects. For example, an object will store up potential energy when it is lifted up, or when an elastic object (such as a spring) is stretched or compressed.

Energy has the unit of Joule (J): 1 J = 1 N x 1 m= kg m2 s-2

• Worked example A rock is thrown vertically 30 m into the air. Calculate the speed with which the stone is thrown into the air. (Take the gravitational acceleration g=10 m s-2). By conservation of mechanical energy:

1 2 mv = mgz 2

1 2 ∴ v = gz 2

∴ v = 2 gz = 2 × 10 × 30 2

∴ v = 24.5m / s

• Conservation of Energy Energy can not be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.