Transcript waves and electricity 9scx

Waves and Electricity
Year 9 Extension Science
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1a
Waves transfer energy
Waves are a means of transferring energy from one place to another without
also transferring matter. Some waves need a medium (matter) to travel
through and are known as mechanical waves. Other waves can travel
through the vacuum of space where there is little or no atoms and are know
as electromagnetic waves. Examples of waves include ocean waves, sound
waves, light waves and earthquake waves.
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1a
Waves can be transverse or longitudinal
The two main types of wave form are transverse waves and longitudinal
waves.
All types of electromagnetic waves, including light, as well as water waves
travel as transverse waves. Sound waves travel as longitudinal waves.
3
1a
Waves can be transverse or longitudinal
The waves generated from an earthquake travel through the ground as both longitudinal
and transverse waves.
Primary (or P) waves are longitudinal waves and move the fastest. They are similar to
sound waves and can travel at 5000 metres per second through solid rock. Longitudinal
waves can also travel through liquid and gas, travelling at the speed of sound through air.
Secondary (or S) waves are transverse waves and can only travel through solids. They
travel at nearly half the speed of P waves.
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1a
Some objects,
such as the sun,
release large
amounts of
energy. The
energy can be
emitted from the
energy source in
the form of
electromagnetic
radiation and
travels in
electromagnetic
waves. Light,
radio waves and
x-rays are all
forms of
electromagnetic
radiation.
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Energy can be transferred as waves
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1a
Energy can be transferred as waves
Light and other types of electromagnetic radiation from the sun and even further
away stars travel through space in a vacuum – an area of very little or no atoms.
Light does not need matter or a substance through which to travel.
Each particular type of electromagnetic radiation, including each different colour
of light, has a unique fixed length of wave, called the wavelength (λ), that it
travels in.
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1b
Wavelength of a wave
Waves have troughs, the lowest point, and peaks, the highest point. A
wavelength is the distance between two closest peaks.
Wavelengths can be measured in metres (m) or nanometres (nm). The type of
electromagnetic radiation can be determined by its wavelength.
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1b
A nanometre is very small
extension
Small objects such as atoms,
viruses and light waves need to
be measured using very small
units called a nanometre.
A metre (m) can be divided into
one thousand equal parts called
millimetres (mm). If one
millimetre is divided into a
thousand equal parts then we
have a micrometre (µm). If one
micrometre is divided into a
thousand parts then we have a
nanometre (nm). A nanometre is
one-billionth of a metre.
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1b
Amplitude of a wave
The amplitude of a wave is a measure of its height. The height is taken from a
midpoint between a trough and a peak up to the top of a peak of a wave. A
higher amplitude wave indicates a wave has more strength and that a light
wave contains more photons, little packets of light energy.
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1b
Frequency of a wave
The frequency of a wave is calculated by the number of waves that past by a
fixed point in a given amount of time. The frequency is measured in hertz
(hz). Because all electromagnetic radiation travels at the same speed then
more waves of shorter wavelength will pass by a point over the same time as
waves of longer wavelength.
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1c
wave speed = wavelength x frequency
extension
Waves always travel at the same speed. A scientific value that always
remains the same is called a constant. The constant for the speed of light is c
= 3x108 m/sec or 300,000 kilometres per second.
Because we know the speed of light, if we know either the wavelength (λ) in
metres or the frequency ( f ) in hertz then we can calculate the other.
Wavelength = speed of light / frequency
Frequency = speed of light / wavelength
c
λ
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Light energy is carried by photons
extension
The amount of energy in a wave depends upon the frequency of the wave.
The energy of a photon can be calculated by multiplying the frequency by
another constant called Planck’s constant (h). This constant is named after a
famous German scientist called Max Planck who made many discoveries
about light and how it also travels as particles called photons. A photon does
not have mass like matter does, it only contains energy.
Planck’s constant is
6.626 x 10-34 joules per
second. This value is so
small because a photon
is so tiny but there are
so many of them within
a light wave.
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2a
Sound travels as a wave
extension
Sound waves are mechanical waves requiring particles. Air particles vibrate
back and forward creating repeating patterns of high (compressed particles)
and low (spaced apart particles) pressure. Sound travels in the form of
transverse waves. One wave stretches from one compressed area of particles
to the next.
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2a
Pitch and Loudness of sound
extension
Sound can be described by “characteristics” called pitch and loudness. Pitch is
related to frequency – the higher the frequency then the higher the pitch of
the note (a single sound at a particular level). Loudness is related to
amplitude – the higher the amplitude the louder the sound.
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2b
Function of the human ear
extension
Sound waves travel
through the ear canal and
cause the ear drum to
vibrate. The small bones
of the inner ear transfer
this vibration to the inner
ear cochlea.
The cochlea is fluid filled
and lined with many hairlike nerve cells. Different
length nerve cells detect
different wave
frequencies and transmit
this information to the
brain using electrical
impulses that move along
the nerves.
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3a
Light travels in straight lines called Rays
Light waves travel in straight lines called rays.
The rays will continue in a straight line until an
object stops, reflects or bends the light. An
object that stops direct light rays creates a
shadow. The shape of the shadow resembles
the shape of the object.
The shadow created when
the Moon blocks the light
from the Sun to the Earth
is called a solar eclipse.
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3b
The length of the shadow depends on the angle of the light source
The length of the shadow formed on the ground depends on the angle that
the light rays hit the object blocking the. If the light rays hit the object straight
on then this will create the smallest possible shadow. The greater the angle
the light rays hit the longer the resulting shadow.
The changing of length of shadow can be seen as the Sun moves across the
sky. In the morning and afternoon the shadows created are the longest as the
Sun is at the greatest angle. The shortest shadows are formed at midday
when the sun is directly over head (in Summer)
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4a
Light energy can travel as rays
Light travels fast and straight.
At the speed of light, which is 300,000 kilometers per
second, light from the sun takes about 8 minutes to
go 149 million kilometers to earth. Light can go
around the earth 7 times In one second.
Light travels perfectly straight, until something bends
it. The straight paths of light are called light rays.
SJ Gaze
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4a
Light energy can be reflected, refracted or dispersed
Light travels in a straight line until it strikes an object or a force. Light can be
1. Reflected by a mirror
2. Refracted by a lens
3. Absorbed by the object
Light interacts with matter by transmission (including refraction) which is
travelling through it, absorption where it enters but doesn’t leave again, or
scattering (including reflection) where it bounces off.
To see an object, light from that object— emitted by or scattered from it—must
enter the eye.
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4a
Light energy can be reflected by a mirror
Mirrors work because light is reflected from them
>the three types of mirror are
light
Plane
Concave
Convex
The images in the plane mirrors are the same size, the right way up but laterally
inverted (changed right to left)
The images in the concave mirror are
a) magnified and the right way up when you are near to the mirror
b) smaller and upside down when you are further away from the mirror
The images in the convex mirror are reduced and the right way up
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4a
Light energy can be reflected by a mirror
Plane
Object
Image
Concave
Object
Image
Convex
Object
Image21
4a
Ray diagrams in a plane mirror
Ray diagrams are used to
show an image of an object
reflected in a mirror. Straight
lines from the object are
drawn towards the mirror.
Using the rule from the angle
of incidence and reflection
the lines are then reflected
back. Arrows are used on the
lines to show the light rays
direction.
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4b
The angle of incidence and angle of reflection
The main rule for mirrors is that the angle of incidence equals the angle of
reflection.
This means that the
angle of the light
ray between where
it arrives and the
perpendicular line
to where it hits on
the surface of the
mirror is the same
angle it leaves and
the same
perpendicular line.
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4b
Light energy can be reflected by a mirror
extension
Images in plane
mirrors are:
the same size as the
object;
the same distance
behind the mirror as
the object is in front;
virtual (light does not
really go to them);
laterally inverted.
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4b
Light energy can be reflected by a mirror
extension
When you look directly at an object you can see where it is. But if you look at it in a
mirror, then you are looking at a reflection of the object – the image is behind the
mirror. An image is a view of an object at a place other than where the image is.
Images can either be real or virtual. A real image occurs when the light rays pass
through the place where the image is, for example, the image on a cinema screen
or the image that falls on film in a camera.
A virtual image occurs when the image
appears to be at a place where the light
rays do not pass
When you hold an object in front of a
mirror, the reflected image you see is
behind the mirror. Obviously no light can
come through the mirror, so the image
must be a virtual one.
All reflected images off flat surfaces are
virtual images.
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4c
Ray diagrams in a concave mirror
With concave mirrors, light being
reflected goes in an inward
direction towards the focal point
(x). From a distance, images
appear upside down but when
brought nearer, image become
larger in size and appears right
side up. Concave mirrors are
commonly found in the head
lights of vehicles making the light
more reflective and wider,
making it possible for the drivers
to have a better view at night.
Concave mirrors are also used in
microscopes and face mirrors to
enlarge the view.
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4c
Ray diagrams in a convex mirror
The image formed in a
convex mirror is always
upright and smaller in
size.
Convex traffic safety
mirrors are designed for
road safety to see better
at corner blind, concealed
entrances and exits.
Ceiling dome mirrors are
used in surveillance for
shops because they allow
someone to watch what
is going on in a wide area.
They are also used in car
side mirrors to see a wide
view from behind.
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4d
All light rays travel
through. Image is
not distorted when
looking from the
other side
Transparent
Transparent, Translucent and Opaque
Some light rays
travel through.
Image is distorted
when looking from
the other side
Translucent
No light rays travel
through no image
seen from the
other side
Opaque 28
4e
Refraction
A medium is any space or substance
which will allow light to travel through
it called transmission. Examples of
different media include air, water and
glass. Each medium has different
optical density. The optical density of
a medium affects the speed at which
light rays travel through. When a light
ray passes from one medium into
another (e.g. from air into water) it will
change direction where two media
meet. This ‘bending’ of light is called
refraction and it always occurs when
the two media have different optical
densities.
SJ
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4e
Light can be Refracted by a lens
A glass or plastic lens is
transparent. This means
that light is able to be
transmitted through the
object without the light
being absorbed.
The medium of a plastic or
glass lens has a different
optical density to air. Light
rays entering the lens are
refracted to a different
angle. If the lens is concave
or convex then the light rays
will leave the lens at a
different angle to which they
entered.
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4e
Concave lens
5b
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A concave lens will cause
parallel light rays entering
the lens to be spread
when leaving it. Concave
lenses are used in eye
glasses of near sighted
people who are able to
focus clearly on distant
objects.
4e
Convex lens
5b
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A convex lens will cause parallel
light rays entering the lens to
bend inwards when leaving.
Convex lenses are used in
refracting telescopes to enlarge
distant small objects. They are
also used in eye glasses of
farsighted people who have
trouble focussing on close
objects.
5a
Human eye
extension
The human eye is a “collecting” organ that allows light to reach sensory nerves
which then transmit electrical signals to the brain. The convex lens focuses the
images seen onto the retina of the eye. Various sensory cells called rods and
cones detect both amount of light and colour of light.
The brain
further
processes the
images in
various parts of
the brain
responsible for
language,
speech and
thinking
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5a
The Human Eye
extension
The lens of the eye is able to change shape to focus light clearly from different
distances.
Muscles surrounding the lens relax to produce a more rounded lens that is able
to focus on nearer objects.
Muscles surrounding the lens contract to produce a more flattened lens that is
able to focus on far away objects
While the eye
is focused to
see close up
images the
distance is
blurry and vice
versa.
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6a
White light is made from colours mixed together
White light is a combination of all of the other colours of light mixed together.
The main colours that make up white light can be seen in a rainbow. They are
red, orange, red, green, blue, indigo (a dark inky blue) and violet. They can be
remembered by the acronym ROY G BIV. A prism can be used to separate out
the different colours. This is called dispersal.
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6b
Prisms work by diffracting colours of different wavelengths
Light changes speed as it moves from one medium to another (for example, from air
into the glass of the prism). This speed change causes the light to be refracted and to
enter the new medium at a different angle The degree of bending of the light's path
varies with the wavelength or colour of the light used, called dispersion. This causes
light of different colours to be refracted differently and to leave the prism at different
angles, creating an effect similar to a rainbow. This can be used to separate a beam of
white light into its spectrum of colours.
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6c
Different colours have different wavelengths
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6d
The nature of colour vision - primary and secondary colours
extension
Combining light
colours is said to
be additive. Each
wavelength of
light adds to
another.
The three main
colours of light
are called
primary. Two
primary colours
together make a
secondary colour
and all three
primary colours
together make up
white light.
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6d
Paint pigments are subtractive colours.
extension
Paint absorbs light waves.
The paint is called
subtractive because with
each addition of colour more
of the wave lengths are
absorbed. The three primary
colours of paint are cyan,
yellow and magenta. When
these three are added
together the resulting colour
is black: all of the different
wavelengths of coloured light
are absorbed.
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6e
We see colours because of what wavelengths are reflected
extension
We see a tree as green
because the leaves absorb
the red and blue light
waves and the green light
only is reflected into our
eyes.
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7a
Visible light belongs to the electromagnetic spectrum
extension
Light is a type of energy called electromagnetic (EM) radiation. There are
other kinds of EM radiation such radio waves, microwaves, x-rays, etc., but
light is the only part we can see with our eyes. All of the types of EM radiation
together are known as the electromagnetic spectrum.
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7a
The electromagnetic spectrum
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7a
The sun is an incandescent light source
extension
Virtually all of the electromagnetic radiation that arrives on Earth is from the Sun. The Sun
is an incandescent light source because the light energy is generated from heat. The
nuclear reactions that occur within the high pressure centre of the Sun release large
amounts of heat. The heat causes the atoms that make up the Sun to move around fast .
As the atoms collide together the electrons move up and down orbits around the nucleus
and release photons of light each time.
Each different type of element releases combinations of light in different wave lengths or
colours called its spectra. We can “read” what type of elements make up the Sun, and
other stars as well, by looking at what spectrum of light is emitted.
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7a
Spectroscopy is the study of spectra
extension
Our Sun is mainly made up of Hydrogen and Helium. Helium is very rare and unreactive
on Earth. It was not discovered until scientists using spectroscopy on the Sun found an
unknown element emitting a spectra of light that did not match to any known
elements. They named this element Helium after Helios, the Latin name for Sun. We
now know many stars contain Helium along with assorted other elements rare and
common on earth.
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7a
Luminescent light is produced from chemical or electrical energy
extension
Some animals can produce
their own light through
chemical reactions in their
bodies. This light is known as
luminescence. It is much
cooler than incandescent as it
does not require heat energy
to produce it. Glow sticks and
florescent lamps also emit
this type of light without
producing heat so are far
more efficient to use than a
incandescent filament light
bulb.
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7b
Short wave length EM radiation can be dangerous
Forms of EM radiation that have a
shorter wavelength than visible light
such as ultra-violet radiation (UV) and
X-rays can be dangerous to us in high
quantities.
The sun emits EM radiation of all
different wavelengths but a magnetic
layer, produced by the magnetised iron
inside earth, shield us from the most
harmful radiation. The ozone layer
(oxygen molecules made of 3 oxygen
atoms) shield us from a large amount of
UV-B radiation but if we remain out in
the Sun for to long we can be sunburnt.
Sunblock provides a layer on our skin,
often containing chemicals like zinc,
that stop UV radiation.
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7b
long wave length EM radiation can provide us with information
EM Radiation with wave lengths longer than visible light can be detected with special
types of receptors. Infra-red radiation is emitted from warm objects. Night vision
goggles pick up this type of EM radiation from living bodies when no visible light is
present.
Microwaves are longer again and
can be used to heat food. The
waves cause the water molecules
in food to move back and forward
rapidly and produce heat. Objects
such as plastic without water, do
not heat up directly.
Radio waves are the longest type of
EM radiation. Radio receivers are
used to pick up these waves that
can be generated by radio stations
or even from objects in space. Cell
phones work by receiving radio
waves reflected from satellites
orbiting above earth.
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8a
Electricity is a form of Energy
Electricity is a type of
energy. It can be
transformed from many
other types of energy;
kinetic, chemical, nuclear
etc.
We make use of electricity
by transforming it into
other types of energy;
light, heat, sound, kinetic
etc., to run many
appliances and machines.
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8a
Electricity is a form of Energy
Electricity is all about electrons and their movement. Electrical energy is carried
by electrons, and isn’t the electrons themselves. Electrons can carry varying
amounts of energy. The more energy, the faster they move about.
All matter is made up
of atoms. Atoms
consist of protons,
neutrons and
electrons. Protons
have a positive
charge, neutrons
have no charge and
electrons have a
negative charge. The
charges of protons
and electrons are
equal and opposite.
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8a
Electric charge produced by friction is the same charge which, moving
around a circuit, produces an electric current
There are two types of electricity. Static electricity involves electrons that are
moved from one place to another, usually by friction and it is stationary.
Current electricity involves the movement of electrons through a conductor
and it flows.
Static electricity
Current electricity
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8a
Static Electricity
Usually, two materials are involved in static
electricity, with one having an excess of
electrons or negative (−) charges on its surface
and the other material having an excess of
positive (+) electrical charges. Atoms near the
surface of a material that have lost one or
more electrons will have a positive (+)
electrical charge.
Static electricity is the build up of
electrical charges on the surface of a
material, usually an insulator (nonconductor of electricity). It is called
"static" because there is no current
flowing, as there is in alternating current
(AC) or direct current (DC) electricity.
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Attraction or repulsion
There are only two types of charge, which we call positive and negative. Like
charges repel, unlike charges attract, and the force between charges decreases
with the square of the distance. Both positive and negative charges exist in
neutral objects and can be separated by rubbing one object with another. For
objects (large enough to be visible),negatively charged means an excess of
electrons and positively charged means a depletion of electrons.
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8b
Law of Conservation of charge
extension
No charge is actually created or destroyed when charges are separated.
Instead, existing charges are moved about. In all situations the total amount of
charge is always constant. This universally obeyed law of nature is called the
law of conservation of charge.
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8b
Charging by contact
extension
Static electricity involves a build up of charge when two different objects are
rubbed together and electrons from one jump across to another. This is called
charging by contact. Some materials, such as plastic, hold onto electrons
better than others and they will become negatively charged. The other object,
due to electrons being lost, will become positively charged. The two objects
will be attracted to each other due to their positive and negative charges.
Materials that hold
electrons well include
plastic, silk and glass –
these become negative.
Objects that lose
electrons include metals
which become positive.
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8b
Charging by induction
extension
Objects can also be charged by induction. When a negatively charged object
is held close to another object but not touching then the negative electrons
are repelled and move away (if a path is created which “earths” the object)
and the non moving protons cause the object to be positively charged.
If the object being
charged is not earthed
then as soon as the
negatively charged
object is moved away
then then electrons will
just shift back again and
neutralise it once more.
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8c
Electrical discharge in air
Electric discharge describes any flow of electric charge through a gas, liquid or solid. If there
are enough positive (+) electrical charges on one object or material and enough negative (−)
charges on the surface of the other object the attraction between the charges may be great
enough to cause electrons to jump the air gap between the objects.
Once a few electrons start to move
across the gap, they heat up the
air, encouraging more electrons to
jump across the gap. This heats the
air even more. It happens rapidly ,
and the air gets so hot that it glows
for a short time. That is a spark.
The same thing happens with
lightning, except on a much larger
scale, with higher voltages and
current.
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8c
Lightning is a form of Static Electricity discharge
The build up of charge can be
released when the electrons move
through the air and make contact with
an earthed object. This discharge can
be seen as a bright spark. On a larger
scale during a storm when particles in
clouds rub together the discharge is
seen as lightning. The lightning will
usually make contact with the closest
object (the tallest) that is conductive.
Some tall buildings have lightning rods
on them. These give a path for the
lightning to travel down to the ground
and prevent the energy of the
lightning from damaging and burning
the building. Animals and people can
be harmed if they are struck by
lightning because of the huge
amounts of energy being released.
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8c
Earthing: how earthing removes excess charges
Electrical earthing (or grounding) diverts potentially dangerous electrical curernt by
providing a conductive path between the area where static charge has built up
and the earth where the charge can spread out. Lightning can be a source of
dangerous or damaging charges that can be dissipated through a earthing system.
Many tall buildings that attract lightning strikes have earthing electrodes (also called
lightning rods) connected to the building that are sunk into the ground and disperse
the excess charge.
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8c
The dangers and uses of static charges
A build up of electrostatic charge can result in sparks or flashes of light. If this spark
occurs near any combustible material then it may cause it to ignite. Fuel trucks often use
earthing cables when refuelling tanks to discharge any build up of static electricity that
the truck may have gained when travelling.
Air rushing past a moving
vehicle drags electrons off
the car and leaves it
positive causing a build up
of charge. This can cause
travel sickness for some
people. Conducting tails
allow the car to pick up
electrons from the road
surface and lose its
charge.
Electrostatic charges are
important and useful in
photocopying machines
and in removing
(extracting) dust
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9a
An electric current is a flow of charge
Electric current is the rate of flow of electric charge. Particles called electrons
carry the electric charge. While some substances called conductors conduct
very well, e.g. metals, other substances are not able to conduct or nearly
conduct no electric current, e.g. glass. Electric current is nearly as fast as the
speed of light.
NOTE:
The charge of an electron is
negative. Previously people
thought that positive particles
serve as charge carriers. Due
to this error the current flow
is moving in the opposite
direction of the electrons by
convention from the positive
terminal to the negative
extension
terminal.
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9a
The ‘voltage’ of an electrical supply is a measure of the energy it can
transfer from an electrical supply elsewhere
An electric current won't flow
through a circuit unless there's a
source of energy like a battery or
mains electricity to push the electric
charges along through the wire.
'Voltage' is a measure of how much
energy the electric charges have
between two points in a circuit.
Voltage is also known as potential
difference. The more potential
difference the more energy is
available to be transferred into
components attached to a circuit.
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9b
The properties of simple electric circuits
Electrical current occurs when electrons flow through a conductor from an
area which is negatively charged to an area which is positively charged.
A circuit is a continuous pathway
around which electrons can flow.
The movement of electric current
can be compared with a pipe full
of water: If water is put in the
pipe on the one end, water will
drip out on the other side
immediately.
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9b
There is a need for a complete circuit when making use of electricity
A circuit is made
up of electrical
components
connected
together so
electrons move
through the
components.
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9b
There is a need for a complete circuit when making use of electricity
A circuit must be closed for the electrons to flow and produce a current.
A switch
breaks the
circuit when it
is opened and
the flow of
electrons
stops, resulting
in no current.
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9c
Draw Circuit diagrams using symbols
These symbols can
be used universally
by electricians and
scientists
regardless of their
different languages
to show how
different circuits
are arranged.
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9d
Circuit diagrams use symbols to represent components in a circuit
All circuits will need: a power source such as a battery or cell, A complete
circuit travelling from the positive (larger line) terminal to the negative
(smaller line) terminal and one or more components (power users) such as a
bulb.
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9e
There are two types of circuits; Series and Parallel
In a Series circuit there is only one pathway for the electricity to flow, and in a
Parallel circuit there is more than one pathway for the electricity to flow.
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One pathway
More than one pathway
9e
In a series circuit, the electrons move along one path
The electrical current flows through one component then the next – more
lamps added in series cause their brightness to decrease.
Series Circuit
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Circuit drawing
68
9e
In a parallel circuit, electrons have a choice of two or more pathways.
More lights added in parallel do not effect the brightness.
Parallel Circuit
GZ Science Resources
Circuit drawing
69
10a
The effects and uses of conductors and insulators
Electrons can travel freely in conductors such as metal.
Electrons can’t travel through insulators such as plastic.
insulators
Conductors
e-
electrons
e-
e-
No
flow
electrons
e-
e-
Direction
of flow
good conductors have very low
resistance
e-
Insulators have high resistance.
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GZ Science Resources
10a
Conductors allow the flow of current through them and insulators
prevent the flow of current through them
Conductors
Copper is considered to be a
conductor because it “conducts” the
electron current or flow of electrons
fairly easily. Most metals are
considered to be good conductors of
electrical current. Copper is just one
of the more popular materials that is
used for conductors.
Other materials that are sometimes
used as conductors are silver, gold,
and aluminium.
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10a
Conductors allow the flow of current through them and insulators
prevent the flow of current through them
Insulators
Insulators are materials that have
just the opposite effect on the flow
of electrons. They do not let
electrons flow very easily from one
atom to another. Insulators are
materials whose atoms have tightly
bound electrons. These electrons are
not free to roam around and be
shared by neighbouring atoms.
Some common insulator materials
are glass, plastic, rubber, air, and
wood
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10b
We make use of the conducting and insulating properties of
materials in technological applications
Every day we use materials because of their ability to conduct electricity. The most
common product around the home is cables and wiring. Our houses are wired with
metals such as copper which carry charge around. The wires are coated in plastic which
acts as an insulator to prevent electricity flowing away from the wire.
Wires are also used to transport
electricity around from where it is
generated such as at a hydropower
station to towns where it is used. The
insulating material around the wire
needs to be much thicker and the
wires are suspended from pylons by
other insulators made from glass or
ceramic materials.
GZ Science Resources
73
10b
The dangers of electricity and the hazards of poor insulation,
extension
overloading and damp conditions
If electricity flows through your body with enough
voltage or current it could kill you or cause damage.
If your body is earthed, that is touching the ground
or another conductive object that is touching the
ground, and then you come in contact with an
electrical source you will form a circuit for a current
to flow. The electrical current will follow a path of
least resistance which may be across your skin but it
also may enter the body at one point and exit
through another. The electrical current may cause
bad burns as it converts some of its electrical energy
to heat energy. It may also stop your heart as the
electricity interferes with the pacemaker of your
heart that sends out small pulses of electricity
to keep the heart muscles all contracting
and relaxing in rhythm.
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74
10b
The hazards of poor insulation
extension
Machines, wires and appliances that use
electricity or have electricity flowing through
them must have a insulating material such as
plastic surrounding parts that we may come in
contact with.
Power lines are usually held off the ground by
wooden or concrete posts and suspended by
glass or ceramic (material that coffee cups are
made of) insulators.
Appliances including the cords and plugs usually
have plastic or rubber coverings.
If the coverings become cracked or worn and
expose the metal that conducts the electricity
then we are in danger of an electric shock if we
touch that part.
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11a
Ammeters are used in circuits to measure Amps
We can measure the amount of electric current
flowing in a circuit with a device called an
ammeter. The unit of electric current is the Amp
- which is often abbreviated to the letter A,
especially if it comes after a number. So, for
example, 3 Amps can also be written 3A.
To measure the current flowing in a circuit you
must connect the ammeter in series with the
other components
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11a
Voltage can be measured with a voltmeter
A voltmeter is used to measure voltage or
potential difference and is placed in
parallel to an appliance. We can measure
the energy of electric charges in a circuit
before they enter a bulb and after they
leave it by putting a voltmeter in parallel
across the bulb like this:
GZ Science Resources
77
11b
In Series circuits, the current is the same at any point on the circuit
Current =
1 truck
(amps)
A
Current
A
In series circuits, components
are connected on after the
other. All of the current must
travel through each of the
components in turn.
Current =
1 truck
(amp)
A
GZ Science Resources
78
11b
In Series circuits, the voltage is shared out around the circuit
Voltage
Current =
1 truck
(amps)
A
Current
A
The current is the same at all points
around a series circuit.
The total voltage = sum of voltage
across all components i.e. voltage is
shared out
V
Voltage = 1/2
load (volts)
Current =
1 truck
(amp)
V
A
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11b
Total
Current =
2 trucks
(amps)
In parallel circuits, the current is shared out between branches
A
Current
A
Branch
Current = 1
truck (amp)
A
GZ Science Resources
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11b
In parallel circuits, the voltage is the same across all branches
Voltage
Current =
2 trucks
(amps)
Current
A
V
A
Voltage =
whole load
(volts)
Current = 1
truck (amp)
V
GZ Science Resources
A
The total
current in the
circuit = sum
of the
currents i.e.
current is
shared.
The voltage is
the same
across all
branches
around a
parallel
circuit.
81
11b
Current and Voltage in Parallel and Series circuits
Current
Voltage
Series
>Same everywhere in the
circuit
>Doesn’t increase as more
bulbs added
>total voltage coming out of
battery is all used up by
components (i.e. bulb)
>total voltage loss is shared
between components
Parallel
>total current coming out of >total voltage loss is the same
battery is shared amongst
across all components
branches
>increases as more bulbs
added
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11c
Advantages and Disadvantages of Parallel and Series circuits
Advantage
extension
Disadvantage
Wiring
done in
parallel
Other bulbs remain working if More current is needed when
one bulb is blown or removes extra bulbs added
All bulbs glow brightly
The battery runs out quicker
Wiring
done in
series
You can turn off all of the
appliances / lights with one
switch
The wiring is simpler
GZ Science Resources
If one bulb is disconnected the
circuit is not complete and all the
bulbs will go out
Resistance of the circuit is greater
if more than one bulb – the other
bulbs don’t glow as brightly
Hard to find the blown bulb
83
11d
Predictions of Ammeter and voltmeter readings
Predictions can be made about the current (amps) in both the series and parallel
circuits using the rules. In a series circuit if one component reads 2A then all
components will read the same. In a Parallel circuit the current reading leaving the
power supply must be divided between branches.
Predictions can also be made about voltage readings with the total voltage across the
power supply shared out to components in a series circuit and equal to the voltage in
each branch of a parallel circuit.
Predictions can be tested by setting up each circuit and taking multiple voltage and
current readings
Series
GZ Science Resources
Parallel
84
11e
Investigating the brightness of adding bulbs in series and
parallel circuits
Investigation will show that the more bulbs that are added to a series
circuit the dimmer they will collectively be.
In a parallel circuit if each bulb has its own circuit then the brightness of
the bulbs will not be affected.
Series
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85
11f
Electrical resistance
extension
Resistance (symbol R) measures how difficult it is for current to move through a
component. Resistance is measured in ohms (symbol Ω)
Resisters will reduce the
current that flows through
a circuit. Components that
add resistance to a circuit
can often transform
electrical energy in light,
sound or heat energy.
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11f
Electrical resistance
extension
Parts of a circuit which offer high resistance
transform a greater amount of electrical
energy into light and heat energy. This is
why the high resistance, very thin wire of a
filament light bulb glows hot and bright
while the lower resistance thicker wire
providing the current to the bulb stays
cooler.
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The resistance of a component (in ohms) = voltage across
extension
component / current through component
11g
Resistance is calculated using R = V/I
V
I R
The higher the resistance the less the
current.
GZ Science Resources
The resistance of an object determines
the amount of current through the
object for a given voltage across the
object.
where
R is the resistance of the object,
usually measured in ohms
V is the voltage across the object,
usually measured in volts
I is the current through the object,
usually measured in amperes
88
12a
Magnetic materials have the ability to attract some materials but to
extension
attract and repel each other
Some objects attract iron and steel. They are called magnets.
A magnet has a magnetic force field around it. When another magnet or an
iron object enters the field it experiences a force as either a push or a pull.
The force field of a magnet can be reveled by sprinkling iron fillings around it,
or by moving a small compass around the magnet and marking the needle
direction.
Magnets have a variety
of uses. Examples of
uses of permanent
magnets in the home
include: fridge
magnets, cupboard
door latch, magnetic
knife holder, magnetic
screwdriver etc.
GZ Science Resources
89
12a
Magnetic materials have the ability to attract some materials but to
extension
attract and repel each other
A magnet will have a positive and negative end, sometimes called North and
South.
Like charges will repel each other. e.g. positive and positive.
Unlike charges will attract each other. e.g. positive and negative
Repulsion
+ve
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Attraction
+ve
+ve
-ve
90
12a
Magnets attract some metals but not others
extension
Only iron, cobalt and nickel and some iron alloys like steel are able to act as
magnets. The particles that they consist of are able to align themselves so that all
their negative ends are facing the same direction.
Aluminium cans are not magnetic whereas ‘tins’ are largely made of iron and are
magnetic.
It is sometimes difficult to
distinguish between a magnet
and a magnetic material. When
two magnets are put together
there is either attraction or
repulsion, but when a magnet
and a magnetic material are put
together there is just attraction.
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12b
Magnetic fields are arranged in fixed patterns
extension
Field patterns produced by bar magnets can be visualized using iron
filings. This is the magnetic field. The field lines move out from the N end
of a magnet and into the S end.
Compasses, which contain a movable magnet, can also be used to show
magnetic fields. The needles will align in the direction of the field.
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92
12b
Magnetic field interactions
extension
A strong field is produced between unlike
poles.
Between the middle of like poles the net
magnetic force is zero due to the fields
cancelling out. This is shown by a blank
space between.
The field lines move out from a North pole
(and into a South pole).
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12c
The Earth is surrounded by a magnetic field
extension
The Earth has a magnetic field. The outer core of the Earth is liquid iron and as heat from
the very hot solid iron inner core moves through it then electrical currents are produced.
Current Scientific theory suggests that this in turn produces an electric field that stretches
far beyond Earth.
This magnetic field
produces a North and
South Pole, although
they are not exactly in
the same place as the
geographical North
and South Pole.
The North of a needle
compass is attracted to
the South, so the
North pole is actually
the South Pole!
GZ Science Resources
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12c
The Earth is surrounded by a magnetic field
extension
Harmful radiation (which would kill living organisms) emitted from the Sun is deflected by
our magnetic field. Small amounts of radiation which enter through the small gaps in the
magnetic field lines over the poles interact with the ionosphere layer around Earth and
cause beautiful coloured lights in the sky called the Aurora borealis (Northern lights) and
Aurora Australis (Southern lights)
The moving inner solid
core maybe the cause of
the shifting magnetic
field. Evidence in ancient
rocks shows us that in the
past the magnetic field
around Earth has
switched direction
reasonably quickly many
times. During these
switch overs the Earth
may have been left
defenceless from
dangerous radiation.
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12d
An electric current itself has a magnetic field
extension
When electrical charges are
moving they create or induce
magnetic fields.
A changing magnetic field will
create an electric current and an
electric current will induce a
magnetic field.
This is called electromagnetic
induction, it is the principle used
to drive generators, motors,
transformers, amplifiers and many
more electrical devices.
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12d
Electrical currents moving around a magnet can produce an
extension
electromagnet
A magnetic field can be made stronger with a coil of conductive wire
wrapped around it and an electric current flowing through the wire. This
is called an electromagnet. An electromagnet can be made stronger by:
increasing the number of turns (how many times the wire is wound) and
by increasing the current. A coil of wire is called a solenoid.
Electromagnets are used
when a stronger magnet is
required such as for
picking up cars at a
wreckers and has the
advantage of being
“switched off” when the
current is stopped.
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