Standard Grade Physics

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Standard Grade Physics
Summary Notes
Health Physics
Telecommunications
Electricity
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Health Physics
Thermometers
Sound
Light and Sight
Using EM Waves
Ionising Radiation
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Thermometers
You should be able to ? State thermometers
need a property that changes with temperature
and is easily measurable. ? Describe how a
liquid in glass thermometer works. ? Describe
the main differences between a clinical and
ordinary thermometer. ? Describe how body
temperature is measured using a clinical
thermometer. ? Explain how body temperature is
used in the diagnosis of illness.
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Types of Thermometer
Thermometers have a property which is easy to
measure and changes with temperature. Different
thermometers have different properties. Liquid
in Glass Volume of liquid changes Rotary
Contains a bimetallic strip which bends Digital
Some electrical property Crystal strip
Colour change Thermometers are carefully
designed to measure specific things,
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Clinical Thermometers
Clinical thermometers measure body temperature.
They are different from normal thermometers in a
number of ways Small Range The range only
goes from 32ºC - 42 ºC (outside this range a
person is dead) Small Divisions Allows more
accurate readings Kink in the glass Stops
mercury until it is reset Toughened Glass Safety
feature White background Make it easier to
read Lens-like glass Make it easier to read
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Measuring Body Temperature
Clinical thermometers can be used to measure body
temperature using the following steps. Disinfect
the thermometer Shake the thermometer Place the
thermometer under the patient's tongue Leave the
thermometer for a few minutes Remove the
thermometer and read the scale Normal Body
Temperature is 37 ºC Above this you are too hot
Fever Below this you are too cold - Hypothermia
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7
Sound
You should be able to ? State what sound energy
can and cannot travel through. ? Explain how a
stethoscope can aid to hearing. ? Give one
example of a use for ultrasound in medicine. ?
Name sounds too high for humans to hear. ? Give
examples of sound levels of some everyday
sounds. ? State that excessive noise can damage
hearing. ? Give two examples of noise
pollution. ? Explain a use for ultrasound in
medicine.
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Sound Waves
Sounds are vibrations the number of vibrations
every second is called the frequency. Sounds
need to travel through matter. Sounds can
travel through solids, liquids and gases Sounds
cant travel through a vacuum (like space).
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Stethoscope
Stethoscopes are used in medicine to listen to
sound made by the body. The stethoscope has
ear-pieces, tubing and two bells. The open bell
is for listening to low frequency sounds from the
heart The closed bell is for listening to high
frequency sounds from the lungs
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Ultrasound
Humans can hear sounds of certain
frequencies. The range of human hearing is
20-20,000 Hz. Sounds greater than this are called
ultrasounds Ultrasounds have many uses in
medicine, from shattering kidney stones to
scanning unborn babies. Ultrasound is very
safe and will not damage cells unlike X-rays.
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An Ultrasound Scan
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Sound Levels

• Sounds can be loud or quiet. Loudness of a sound
  is is measured in decibels (dB)
• Loud sounds (over 100dB) can damage hearing.
  People who work with loud noises wear protection
  to prevent damage.

Unwanted sounds are called noise pollution for
example traffic or loud music
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13
Light and Sight
You should be able to ? Explain what is meant
by refraction. ? Explain how an image is formed
in the eye ? Draw ray diagram of an object and
image ? Describe the affects of long and short
sightedness. ? State how lens can correct eye
defects. ? Describe how to measure the focal
length of a lens ? Carryout calculations with
power and focal length ? Describe how light
travels in a Fibre Optic ? Explain how fibre
optics are used in an endoscope ? Explain the
meaning of a cold-light source
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Refraction

• Refraction is the bending of light as it passes
  from one material to another.
• The normal is an imaginary line at 90 to the
  boundary.
• The angle between the ray and normal is small in
  denser materials.

You must be able to draw this.
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The Eye
Light is focussed in the eye by the lens and
cornea. An image is produced on the retina
and information is carried to the brain by the
optic nerve. The blind spot is where the optic
nerve joins the retina.
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Ray Diagrams

• 4 Steps to drawing a ray diagram
• Draw a line from the top of your object to the
  lens
• Continue this line through the focal point.
• Draw a line from the top of the object through
  the middle of the lens.
• Draw in your object to where the rays cross

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Eye Defects

• Long sighted people can only
• see long distances clearly.
• Light focuses long of the retina

Short sighted people can only See short
distances clearly. Light focuses short of the
retina
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Lenses
Lens are transparent objects that bend light

• Convex Lens
• Converges Light
• Corrects for Long-sightedness

Convex Lens Diverges Light Corrects for
Short-sightedness
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Measuring Focal Length
Set up the equipment as shown and with light from
a distance source (e.g. the sun) focus an image
of a sheet of paper. Measure the distance form
the lens to the image in meters to find your
focal length
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Power and Focal Length
The power of a lens is measured in Dioptres
(D) Find it using the equation Example A
lens has a focal length of 20 cm P ? f 0.2 m
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Fibre Optics
Fibre optics are very thin strands of
glass. Light travels along them at 2x108 m/s by
total internal reflection. the normal
at 90º to the boundary
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Endoscopes
An endoscope is an instrument that Doctors can
use to look inside your body. It has two Optical
fibres. One fibre provides a cold light
source, allowing light (but not heat) to travel
down and light up the area, the other fibre
allows light to travel up to the Doctors eye.
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23
Using EM waves
You should be able to ? Describe one use of
the laser in medicine ? Describe one use of
X-rays in medicine ? State how X-rays can be
detected ? Describe the use of infrared and
ultraviolet radiation in medicine ? State that
too much ultraviolet radiation may cause skin
cancer ? Describe the advantages of computer
aided tomography
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Electromagnetic Waves
The electromagnetic spectrum is a collection of
light waves that have different frequencies.
Different frequencies of light have different
uses in medicine. Visible light Richard Of
York Gave Battle In Vain. Any light with a
frequency higher than violet light waves or lower
than red light waves are invisible to the naked
eye.
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Lasers

• A laser is a very concentrated form of light.
  Lasers can provide either heat or light to treat
  parents.
• Examples
• Laser Eye surgery
• Reattaching Retina
• Bloodless scalpel

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Infra Red
Infra-red (I.R.) Radiation is low frequency light
waves. Tumours tend to give off heat
(infra-red) , which can be detected by an I.R.
camera. Infra-red radiation is used to treat
muscle strains.
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UV
Ultra Violet radiation is a high frequency light
wave. It can be used to treat skin problems and
jaundice. UV light is also used to sterilise
medical instruments. Overexposure has been shown
to increase the risk of skin cancer.
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X-rays
X-Rays are very high frequency light waves which
pass through soft tissue but not bone. This means
that if X-rays are sent in to the body, they will
pass through skin and muscle but will be
reflected by bone.
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CAT Scan
C.A.T. (Computer Assisted Tomography) scans make
use of X-Rays. The patient is placed in to a tube
and X-Rays are emitted in to the patient from
many different positions and at many different
trajectories. This results in a 3D image of the
patient which also shows tissue.
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Ionising Radiation
You should be able to ? State the effect of
nuclear radiation on living cells ? Explain how
radiation can be used in medicine ? Describe
the range and absorption of a, ß and ? ?
Describe a model of the atom and ionisation ?
Give one effect of radiation on non-living
things ? State the unit of radiation activity ?
Describe how to measure activity ? Describe
the activity of a source over time and calculate
half-life ? State the unit used to measure dose
equivalent
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Radiation and Cells
Nuclear radiation can mutate and kill living
cells. Because of this they are used in medicine
to Sterilise instruments (by killing
bacteria) Kill cancer cells Radiation can also
be used as a tracer a picture of the body can
be taken with a gamma camera to show if an organ
is working correctly.
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Ionising Radiation
There are three main types of nuclear
radiation alpha (a) a helium nucleus (2
protons, 2 neutrons) beta (ß) a high energy
electron Gamma (?) part of the EM spectrum
Alpha is the most ionising, gamma is the least
ionising
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Atoms
Atoms are made up of Protons (ve) Neutrons
Electrons (-ve) Atoms have no overall charge
as the no. of protons is cancelled out by the
equal number of neutrons. Ionisation is when an
electron is lost of gained by an atom and it
becomes charged. Alpha radiation causes the most
ionisation.
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Detecting Radiation
Ionisations caused by radiation can be measured
using a Geiger- Müller tube. The tube contains a
gas which conducts a pulse of electricity every
time an atom is ionised. Radiation also turns
photographic film white. Radiation badges
are worn by people who have to work with
radiation the amount that a piece of film has
fogged shows the exposure to radiation.
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Measuring Radioactivity
The activity of a radioactive source is the
number of nuclear decays per second measured in
Becquerels, Bq. Activity (Bq) Number of decays
time (s) To calculate the activity of a
source Find the background activity Find the
activity next to the source. Subtract the
background activity form your results
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Half-life
Over time the activity of a source decreases.
The half-life of a source is the time taken for
the activity to decrease to half its original
value. You should be able to calculate
half-life from a graph and from information
about the Source.
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Equivalent Dose
The biological effect of radiation is called the
equivalent dose it has the units Sieverts (Sv).
It depends on The type of tissue exposed The
type of ionising Radiation The energy of ionising
radiation Alpha has greater effect than beta or
gamma. The longer you are near radiation the
greater the risk.
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38
Telecommunications
Communication with Waves
Communication with Cables
Radio and Television
Transmission of Waves
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Communication with Waves
You should be able to ? Compare the speed of
sound and light with examples. ? Describe to
measure the speed of sound in a lab. ? Use the
following terms correctly wave, frequency,
wavelength, speed, energy transfer and
amplitude ? Use the equation speed
distance/time. ? Use the equation speed
frequency x wavelength. ? Explain the
equivalence of frequency x wavelength and
distance / time.
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Speed of Sound and Light
The speed of light is about a million times
faster than the speed of sound The speed of
sound is about 340 m/s The speed of light is 300
000 000 m/s This is obvious during a lightning
storm. You see the lightning then you hear the
thunder even though they are produced
simultaneously (at the same time)!
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Measuring the Speed of Sound
Set up the equipment as shown Make a sharp
noise at X As the sound passes mic 1 thetimer
starts, as it passes mic 2the timer stops. Use
the equation Speed distance/time
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Wave Properties
You should know the following terms Amplitude
(A) - height of wave Wavelength (?) - length of
wave Wave-speed (v) - speed of wave Frequency
(f) - waves per second Waves transfer energy.
The greater the energy the greater the amplitude.
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Frequency of Waves
Frequency no. of waves
time Example 240 waves pass a point in 1
minute. f ? n 240 t 1 minute 60 s f n
240 4 Hz t 60
f frequency (Hz)v speed (m/s)t
time (s)
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Speed, distance and time
speed distance time Example A
wave travels 120 m in 1 minute. v ? d
120 t 1 minute 60 s v d 120 2 m/s t
60
d distance (m)v speed (m/s)t time
(s)
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Speed, frequency and wavelength
speed frequency x wavelength Example Find the
speed of a 20 m wave and a frequency of 30
Hz. ? 20 m v ? f 30 Hz Speed f x ? 20
x 30 600 m/s
? wavelength (m)f frequency (Hz)v
speed (m/s)
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Wave Equations
Speed frequency x wavelength distance
time
f frequency (Hz) n no. of wavest
time (s)
f frequency (Hz)v speed (m/s)t time
(s)
d distance (m)v speed (m/s)t time
(s)
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Communication with Cables
You should be able to ? Describe a method of
communication using wires. ? Explain how a
telephone sends and receives signals. ? State
that electrical signals travel along wires ?
Describe how signal patterns change with
volume/freq. ? Explain the term reflection ?
Explain the term total internal reflection ?
State what is meant by an optical fibre. ?
Describe how signals are transmitted in a fibre
optic ? State advantages/disadvantages of a
fibre optics ? Carryout calculations involving v
d/t for fibre optics.
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Communicating with cables
Messages can travel through air or through
cables. Messages which travel through cables are
usually more private and faster than messages
which travel through air. Examples include The
telephone (landline) Broadband
Internet Morse code
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The Telephone
Telegraphs and telephones use wires to send
messages. Telephones have a receiver and
transmitter. The earpiece contains a
loudspeaker. (electrical energy ? sound
energy) The mouthpiece contains a
microphone. (sound energy ? electrical
energy) Telephones transmit electrical signal .
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Sounds on an Oscilloscope
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Reflection
Reflection is when light bounces of the surface
of an object . The angle of incidence is equal to
the angle of reflection. The principle of
reversibility is that if the direction of a ray
of light is reversed it will follow same path,
but in the opposite direction.
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Total Internal Reflection
Total Internal Refraction occurs when the angle
of incidence is greater than the critical
angle. Angle of incidence i is greater than the
critical angle.
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TIR in Fibre Optics
Fibre optics are very thin strands of
glass. Light travels along them at 2x108 m/s by
total internal reflection. the normal
at 90º to the boundary
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Fibre Optics
Advantages are They are lighter and cheaper They
carry more information for the same
thickness They are less likely to experience
interference The signals travel faster and there
is less energy loss Disadvantages are They are
slightly slower (only 200 000 000 m/s)
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Fibre Optics (d vt)
speed distance time Example
Light travels 200 km along a fibre optic. t
? v 200 000 000 m/s d 200 000 t d 200
000 0.001 s v 200 000 000
d distance (m)v speed (m/s)t time
(s)
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Radio and Television
You should be able to ? Draw a block diagram of
a radio receiver ? Describe function of each part
of the radio ? Describe radio transmission ? Draw
a block diagram of a TV receiver ? Describe
function of each part of the TV ? Explain how a
picture is produced on a TV ? Describe how a
moving picture is seen on a TV screen ? Describe
how to make different colours using RGB light
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The Radio
Aerial picks up all available signals Tuner
selects one frequency Decoder removes the carrier
frequency Amplifier increases the energy of the
wave Power Supply provides this extra
energy Loudspeaker changes electrical to sound
energy
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Radio Transmission

• Radio waves are sent out by a transmitter and
  picked up by a receiver in the aerial.
• Radio transmission occurs by Amplitude
  Modulation.
• A radio station makes a high
• frequency carrier wave
• Voices or music make
• an audio wave
• The two combine to make
• an amplitude modulated wave

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The Television
The Television can be represented as a block
diagram you must make sure you know the
function of each part.
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Television Pictures
A television picture is made up of many pixels.
An electron gun fires electrons at the
phosphor screen. Where the electron hit, the
screen glows. Brighter images are made when
more electrons are fired at the screen. The
electron gun scans the screen. There are 625
lines on a television.
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Moving Pictures
On a TV screen, there are 25 still pictures
created per second (an image every 0.04s). It
takes our eye about 0.1s to become aware of a
picture and this vision persists for 0.1s after
the object has disappeared. On TV, a still image
changes to another still image before our eyes
have time to become aware of it. Our brain puts
these images together and we see a moving
picture. This is called image retention.
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Mixing colours
Colour television has three electron guns. These
pass through a shadow mask to make sure they hit
the right coloured pixel

• Colour television has three primary colours
• Red, Blue and Green
• These colours mix to make all the other colours

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Transmission of Waves
You should be able to ? State that radio and TV
transfer energy at 3x108 m/s ? Explain how
wavelength affects diffraction ? State that
curved reflectors on certain aerials or
receivers make the received signal stronger ?
Explain why curved reflectors boost a signal ?
State that the period of satellite orbit depends
on its height above the earth ? State that a
geostationary satellite stays above the same
point o the Earth's surface ? Describe how
signals are transmitted from dish aerials
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TV and Radio waves
TV and Radio waves travel at 3 x 108 m/s They do
not need cable to carry the signals as the
electro-magnetic waves travel through the
air Each radio and television station broadcasts
using EM waves with a unique frequency and
wavelength. BBC Radio 1 broadcasts using EM
waves with a frequency of 97-99 MHz. (97-99
million Hertz)
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Diffraction
Diffraction is the bending of waves around
objects. Waves with a low frequency diffract
more than waves with a high frequency As radio
waves have a lower frequency than TV waves, they
diffract more easily. Therefore, in mountainous
areas, you are more likely to pick up radio waves
than TV waves.
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Curved Reflectors
Curved reflectors (such as satellite dishes) are
used to boost these weak signals. The bigger the
diameter of a curved reflector, the better it
works. This is because a larger curved reflector
will focus more waves to a point than a smaller
curved reflector
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Satellites

• Satellites are objects that orbit our planet.
  Many satellites are used for telecommunications.
• The time taken for a satellite to orbit the earth
  is called the period.
• The higher the satellite,
• the longer the Period

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Geostationary Satellites
Geostationary satellites orbit with a period of
24 hours. The stay above the same point on the
Earths equator. Signals from Earth andbeamed up
to a satellitewhich then transmits them back to
Earth.
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69
Electricity
From the Wall Socket
A.C and D.C
Resistance
Useful Circuits
Behind the Wall
Movement from Electricity
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From the Wall Socket
You should be able to ? Describe the energy
changes in household appliances ? Choose the
correct flex based on the power rating ? Explain
why there is a fuse in a plug ? Choose the
correct fuse for an appliance ? State the
colours of live, neutral and earth wires?
Explain why switches/fuses must be in the live
wire ? Explain how the earth wire works as
safety device ? Explain the term double
insulation and draw the symbol ? State that the
human body conducts electricity ? Give some
examples of dangerous electrical situations
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Energy Changes
Appliances in our home change the electrical
energy supplied by the mains into another form.
Examples Radio Electrical Energy to Sound
EnergyLamp Electrical Energy to Light
EnergyWhisk Electrical Energy to Kinetic
EnergyHeater Electrical Energy to Heat Energy
Power Energy/time E Energy (J,
Joules) P Power (W, Watts) t time (s, seconds)
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Flex
Appliances must have a flex thick enough to carry
electric current without overheating. Appliances
that have higher power ratings and draw more
current from the mains need thicker flexes.
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Fuses
The flex and appliance are protected by a
fuse. Depending on the power of the appliance a
plug will have a 3A or a 13A fuse. Power gt
700W 3A fuse Power lt 700W 13A fuse The fuse
is a thin piece of wire that completes an
electrical circuit. When the current becomes too
large, the fuse wire overheats and breaks. This,
breaks the circuit.
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Plugs
Live (brown) carries current into the
applianceNeutral (blue) carries current out of
the applianceEarth (yellow/green) safety
devices
BRown wire in Bottom Right
BLue wire in Bottom Left
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The Live Wire
The live wire allows current to enter an
appliance. Switches and fuses are connected to
the live wire so if there is a fault and the
circuit becomes live the circuit will break
before current flows into the appliance
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The Earth Wire
The Earth wire acts as a safety device and it
connects the casing of an appliance to ground.
If a fault causes the live wire to touch the
casing, the current will follow the path of least
resistance through the Earth wire to ground.
Without the Earth wire, the casing would remain
live and cause electric shocks to anybody who
touched it.
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Double Insulation
Some appliances have no Earth wire are double
insulated. Double insulated appliances only have
live and neutral wires. They are marked with this
symbol.
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Human Conductivity
Humans conduct electricity Humans conduct
better when wet increasing the chance of
electrocution
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Dangerous Electrical Situations
Dangerous situations with electricity include the
following Proximity of water Wrong fuses
Frayed flexes Wrongly connected flexes
Badly connected flexes Short circuit Misuse
of multiway adapters
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A.C and D.C
You should be able to ? Explain the difference
between voltage and current ? State the units of
current and voltage ? Describe how the supply
voltage affects the amount of energy which is
given to the charges flowing in a circuit ?
Describe what an electric current is ? Explain
the terms a.c. and d.c. ? State the frequency
and voltage of the mains supply ? Draw symbols
for common components ? Carry out calculations
involving charge, current and time ? Compare the
peak voltage with the value usually given
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Current
Current is a flow of electrons. The more
electrons flowing through a component, the higher
the current reading. Current (I) is measured in
Amperes or Amps (A). Current is measured with
an ammeter
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Voltage
Voltage is the energy given to electrons, by a
power supply, to travel around a circuit. The
more energy given to the electrons, the higher
the voltage reading. Voltage is measured with a
voltmeter across the component
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Current

• Current is the flow of electrons around a
  circuit.
• There are 2 different types of current
• Direct Current (d.c.)- Current travels in only
  one direction Batteries supply direct current.
  The electrons flow from the negative terminal to
  the positive terminal.
• Alternating Current (a.c.)- Current travels
  around a circuit and is continually changing
  direction, it is known as Alternating Current.
• Mains electricity is a.c. 230V and 50 Hz.

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Circuit Symbols
Some common circuit symbols
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Charge Current and Time
Charge current x time Example Find the
charge when 10 A flows for 3 minutes. Q ? I
10 A t 3 minutes 180 s Charge Q I x t
10 x 180 1800 C
Charge Current x time Q Charge (C,
Coulombs) I Current (A, Amps) t time (s,
seconds)
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Peak Voltage
Mains supply is a.c. The peak value of an
alternating voltage is greater than the declared
value. This can be seen on the sketch graph.
The frequency of the mains supply is 50
hertz (50 Hz). The declared value of the mains
supply in Britain is 230 volts (230 V).
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87
Resistance
You should be able to ? Draw circuit diagrams
to show ammeters voltmeters ? State how
resistance in a circuit affect the current ?
Carry out calculations involving V, I and R ?
Give two uses for variable resistors ?
Describe devices which turn electrical energy
into heat ? Carry out calculations involving P,
E and t ? Carry out calculations involving P, I
and V ? Carry out calculations involving P, I
and R ? Explain why electrical power can be
calculated using either PIV or PI2R
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Measuring Current and Voltage
An ammeter measures the current through the
circuit A voltmeter measures the voltage
across the component
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Resistance

• Resistance (R) is measured in Ohms (O) and can be
  measured using an ohmmeter.
• A resistor is a device that is placed in to a
  circuit to reduce the current flowing through a
  component. In this instance, the resistor is
  protecting the bulb from blowing as a result of
  too much current flowing through it.

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Voltage Current and Resistance
Voltage current x resistance Example Find
the voltage across a 10 O resistor when 10 A
flows through it V ? I 10 A R 10 O
Voltage V I x R 10 x 10 100 V
V I x R V Voltage (V, Volts) I
Current (A, Amps) R resistance (O, ohms)
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Variable Resistors
A variable resistor (or a rheostat) is a
resistor that can change its value this allows
you to control the amount of current flowing
through a circuit. Variable resistors can be
used as dimmer switch by changing the quantity of
current flowing through the bulb.
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Current in a wire

• When current flows through a wire, the wire will
  heat up and give out energy (heat and light) such
  as in a light bulb.
• In filament bulbs and gas discharge tubes, the
  energy change is from
• Electrical Energy ? Light Energy Heat
• Energy Gas discharge tubes are more
• efficient than filament bulbs because
• Less heat is produced.
• More light is produced.

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Power, Energy and time
Power Energy / time Example Find the energy
used by a 50 W lamp in 2 minutes E ? P
50 W t 120 s Energy E P x t 50 x 120
6000 J or 6 KJ
Power Energy/time E Energy (J,
Joules) P Power (W, Watts) t time (s, seconds)
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Power, Current and Voltage
Power Current x Voltage Example Find the
Power when 50 A is supplied with 2 V P ? I
50 A V 2 V Energy P I x V 50 x 2
100 W
P I x V P Power (W, Watts) I
Current (A, Amps) V Voltage (V, Volts)
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Power, Voltage and Resistance
Voltage current x resistance Example Find
the voltage across a 10 O resistor when 10 A
flows through it V ? I 10 A R 10 O
Voltage V I x R 10 x 10 100 V
V I x R V Voltage (V, Volts) I
Current (A, Amps) R resistance (O, ohms)
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P, V, I and R

• Power (P), Resistance (R) and Current (I) are
  related by this equation
• P I2R
• P VI (but V IR so IR can replace V)
• P (IR) x I
• P I2R

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97
Useful Circuits
You should be able to ? Describe current in
series and parallel circuits ? Describe
voltage in series and parallel circuits ?
Calculate the total resistance of a number of
resistors connected in series or parallel ?
Describe how to make a simple continuity
tester ? Describe how a continuity tester may be
used to test for open and short circuits ? Draw
circuit diagrams for car lighting
98
Current in Series and Parallel
In a series circuit the current is the same at
all points. In a parallel circuit the current
is split between the branches.
I1 I2 I3
Is I1 I2 I3 .
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Voltage in Series and Parallel
In a series circuit the voltage across each
component adds to makethe supply Voltage. In a
parallel circuit the voltage is the same across
each branch.
Vs V1 V2
Vs V1 V2 V3 .
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Resistance in Series and Parallel
In a series circuit the total resistance is the
sum of the resistance of each component. In a
parallel circuit the resistance can be found
using the following equation
RT R1 R2
1/RT 1/R1 1/R2 1/R3 .
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Continuity Tester
When checking for faults in a circuit you can
use An Ohmmeter or A Continuity tester
(ct) A continuity tester can be made using a
battery and a bulb.
102
Fault Finding
Short circuit ohmmeter 0 O ct. very
bright Open circuit ohmmeter 8 O ct.
not lit
103
Car Lighting Circuit
If the ignition switch is open, no lights turn
on. If one bulb blows, all other bulbs remain
on. The headlights and sidelights operate
independently from each other.
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104
Behind the Wall
You should be able to ? State household wiring
connects appliances in parallel ? Explain the
purpose of mains fuses ? State that a circuit
breaker is an automatic switch which can be used
instead of a fuse ? State why a circuit breaker
is better than a fuse ? Use a circuit diagram to
describe a ring main circuit ? State advantages
of using a ring circuit ? Give two differences
between lighting and ring circuits ? Say that
the kilowatt hour (kWh) is a unit of energy?
Explain the relationship between kWh and joules
105
Household Electrics

• Household appliances are connected in parallel
  across the mains supply. This means that if one
  appliance breaks, all of the other appliances
  connected to the mains will stay on.
• Houses have two types of circuit
• Lighting Just for lights, low current (5A)
• Ring Main For plug sockets (30 A)

106
House Fuses
House wiring is protected by fuses or by circuit
breakers (automatic switches). A circuit
breaker is an automatic switch that can be used
in place of a fuse. Advantages of circuit
breakers over fuses Circuit breakers can be
reset whereas fuses have to be replaced Circuit
breakers operate faster than fuses.
107
Ring Circuit

• Household appliances are usually connected across
  the mains supply in a special kind of parallel
  circuit. This circuit is called a ring circuit.
• Advantages
• Uses less cable
• Uses Thinner cable (as there are two paths for
  the electricity to flow along)

108
Electricity Bills

• To calculate Energy used in the home use Power
  Energy / time
• However because the you use so much the units are
  kWh
• 1 kWh 1000 Wh
• 1000 x 60 x 60 Ws
• 3600000 Ws
• 3,600,000 J

Power Energy/time E Energy (kWh) P
Power (kW) t time (h)
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109
Movement from Electricity
? State a mag. field exists around a current
carrying wire? Give two examples of practical
applications which make use of the magnetic
effect of a current ? State that a current
carrying wire experiences a force when the is in
a magnetic field ? State the direction of the
force on a wire depends upon the direction of
current and of the field ? Identify the parts
of an electric motor ? Explain the operation of
a motor in terms of forces ? Explain the need
for parts in an industrial motor.
110
Current in a Wire
When electric current passes through a wire, a
magnetic field is produced around the wire.
111
Electromagnets
If a wire is coiled around a metal bar and a
current is passed through it, the resulting
magnetic field is like that of a bar magnet This
is how an electromagnet is created. Electromag
nets are far stronger than permanent magnets.
They have many uses (see next two slides)
112
Electromagnets - Bell

• When a doorbell is pressed, the switch closes and
  the circuit is completed. Current in the coils
  create a magnetic field and an electromagnet is
  created that attracts the armature. The armature
  is connected to the hammer, which strikes the
  bell. When the hammer hits the bell, the circuit
  is no longer complete because the contact has
  been broken.
• The electromagnet turns off, causing the hammer
  to return to its original position because it
  has a spring attached to it.
• When this happens, the circuit is again
  complete, the electromagnet turns on and the
  hammer again hits the bell. This means that
  continual ringing will be heard as long as the
  switch remains closed.

113
Electromagnets - Relay
A relay is an electrically operated switch. When
the switch is closed, a current flows through the
coils, which creates an electromagnet, which
attracts the other switch, which causes the
contacts to meet and this completes the other
circuit. .
114
Force in a magnetic field

• When a current-carrying wire is placed in a
  magnetic field, a force acts on it.
• This is caused by the magnetic field
• around the wire being repelled or
• attracted to the magnetic field that
• the wire is placed in to.
• The direction of this force can be altered by
• Changing the direction of the current through
  the wire.
• Reversing the polarity of the magnets (switch N
  and S).

115
The Electric Motor

• Here are the main components of the simple
  electric motor

116
Parts of a Motor
Magnets Create a strong magnetic
field. Brushes Allow current to pass from the
supply to the commutator Commutator Changes
the direction of the current through the
rotating coil every half turn. This keeps the
coil continually rotating. Rotating Coil When
current passes through the coil, the fact that
it is in a magnetic field means that it rotates.
117
Commercial Motors

• The commercial motor differs from a simple
  electric motor in the following ways
• A commercial motor uses an electromagnet rather
  than a permanent magnet because its stronger.
• Instead of metal brushes, a commercial motor uses
  carbon brushes because this reduces wear on the
  commutator.
• A Multi-segmented commutator is used in a
  commercial motor rather than a split-ring
  commutator. This makes for a smoother movement of
  the rotating coil.
• Commercial motors are used in washing machines,
  drills, etc.

118
How a motor works

• A coil of wire with a lot of turns is used to
  increase the effect of the magnetic field. The
  brushes and the commutator make sure that one
  side of the coil always carries the current into
  the screen, and out again on the other side. This
  means that one side of the coil always
  experiences a force in the downward direction and
  the other side always experiences a force in the
  upwards direction
• To make any motor spin faster, we can
• Increase the number of coils.
• Increase the magnetic field around the coils.
• Increase the current passing through the coils.

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