Transcript Standard Grade Physics - Deans Community High School

Standard Grade Revision notes for
Telecoms, Electricity and Health
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Standard Grade
Physics
Summary Notes
Health Physics
Telecommunications
Electricity
Health Physics
Thermometers
Sound
Light and Sight
Using EM Waves
Ionising Radiation
Thermometers
You should be able to:
 State thermometers need a property that changes with
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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.
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,
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
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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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.
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 can’t travel through a vacuum (like
space).
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
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.
An Ultrasound Scan
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.
Sound
Level
(dB)
Whispering
30
Bird Song
40
Talking
50
Snoring
60
Rock Concert
110
Jet Engine
130
Unwanted sounds are called noise pollution – for example
traffic or loud music
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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”
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.
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.
Ray Diagrams
4 Steps to drawing a ray diagram:
1. Draw a line from the top of your object to the lens
2. Continue this line through the focal point.
3. Draw a line from the top of the object through the middle
of the lens.
4. Draw in your object to where the rays cross
object
f
lens
f
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
Lenses
Lens are transparent objects that bend light
–Convex Lens:
–Converges Light
–Corrects for Long-sightedness
Convex Lens:
Diverges Light
Corrects for Short-sightedness
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
Power and Focal Length
The power of a lens is measured in Dioptres (D)
Find it using the equation:
Power =
1
focal length
Example: A lens has a focal length of 20 cm
P=?
f = 0.2 m
Power = 1 = 1 = 5D
f 0.2
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
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 Doctor’s eye.
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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.
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
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.
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.
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.
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.
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 α, β and γ
 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”
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.
Ionising Radiation
There are three main types of nuclear radiation:
alpha (α) – a helium nucleus (2 protons, 2 neutrons)
beta (β) – a high energy electron
Gamma (γ) – part of the EM spectrum
Alpha is the
most ionising,
gamma is the
least ionising
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.
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.
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
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.
1000
Activity
(Bq)
500
0
Half life = 5 hours
5
10
15
Time (hours)
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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Telecommunications
Communication with Waves
Communication with Cables
Radio and Television
Transmission of Waves
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.
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)!
Measuring the Speed of Sound
Set up the equipment as shown
Make a sharp noise at X
As the sound passes mic 1 the
timer starts, as it passes mic 2
the timer stops.
Use the equation: Speed = distance/time
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.
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)
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)
Speed, frequency and wavelength
speed =frequency x wavelength
Example: Find the speed of a
20 m wave and a
frequency of 30 Hz.
λ = 20 m
v=?
f = 30 Hz
Speed
= f x λ = 20 x 30
= 600 m/s
λ = wavelength (m)
f = frequency (Hz)
v = speed (m/s)
Wave Equations
Speed = frequency x wavelength = distance
time
f = frequency (Hz)
n = no. of waves
t = time (s)
f = frequency (Hz)
v = speed (m/s)
t = time (s)
d = distance (m)
v = speed (m/s)
t = time (s)
Back to Telecoms Menu
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.
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
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 .
Sounds on an Oscilloscope
Low and Quiet
High and Quiet
Low and Loud
High and Loud
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.
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.
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
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)
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
The Radio
Aerial
Tuner
Decoder
Amplifier
Loudspeaker
Power supply
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
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.
1. A radio station makes a high
frequency carrier wave
2. Voices or music make
an audio wave
3. The two combine to make
an amplitude modulated wave
The Television
Aerial
Video
Decoder
Video
Amplifier
TV tube
Audio
Decoder
Audio
Amplifier
Loudspeaker
Tuner
The Television can be represented as a block
diagram – you must make sure you know the
function of each part.
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.
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’.
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
Back to Telecoms Menu
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
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)
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.
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:
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
Geostationary Satellites
Geostationary satellites orbit with a period of 24
hours. The stay above the same point on the Earth’s
equator.
Signals from Earth and
beamed up to a satellite
which then transmits
them back to Earth.
Back to Telecoms Menu
Electricity
From the Wall Socket
A.C and D.C
Resistance
Useful Circuits
Behind the Wall
Movement from Electricity
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
Energy Changes
Appliances in our home change the
electrical energy supplied by the
mains into another form.
Examples:
Radio: Electrical Energy to Sound Energy
Lamp: Electrical Energy to Light Energy
Whisk: Electrical Energy to Kinetic Energy
Heater: Electrical Energy to Heat Energy
E
P
t
Power = Energy/time
E = Energy (J, Joules)
P = Power (W, Watts)
t = time (s, seconds)
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.
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 > 700W = 3A fuse
Power < 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.
Plugs
Live (brown) – carries current into the appliance
Neutral (blue) – carries current out of the appliance
Earth (yellow/green) – safety devices
BLue wire in
BRown wire in
Bottom Left
Bottom Right
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
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.
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.
Human Conductivity
Humans conduct electricity
Humans conduct better when wet – increasing the
chance of electrocution
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
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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
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
A
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
V
Current
Current is the flow of electrons around a circuit.
There are 2 different types of current:
d.c.
1. 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.
2. Alternating Current (a.c.)- Current travels
around a circuit and is continually changing
direction, it is known as Alternating Current.
a.c.
Mains electricity is a.c. 230V and 50 Hz.
Circuit Symbols
Some common circuit symbols
Battery
Resistor
Lamp
V
Fuse
Voltmeter
Capacitor
A
Diode
Variable
Resistor
Ammeter
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
Q
I
t
Charge = Current x time
Q = Charge (C, Coulombs)
I = Current (A, Amps)
t = time (s, seconds)
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).
Back to Electricity Menu
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 P=IV or P=I2R
Measuring Current and Voltage
An ammeter measures the current through the
circuit
A
A voltmeter measures the voltage across the
component
V
Resistance
Resistance (R) is measured in Ohms (Ω) 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.
Voltage Current and Resistance
Voltage = current x resistance
Example:
V=?
I = 10 A
R = 10 Ω
Find the voltage across
a 10 Ω resistor when
10 A flows through it
Voltage = V = I x R = 10 x 10
= 100 V
V
I
R
V=IxR
V = Voltage (V, Volts)
I = Current (A, Amps)
R = resistance (Ω, ohms)
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.
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.
Power, Energy and time
Power = Energy / time
Example:
E=?
P = 50 W
t = 120 s
Energy = E
Find the energy used
by a 50 W lamp in
2 minutes
E
P
t
Power = Energy/time
= P x t = 50 x 120
= 6000 J or 6 KJ
E = Energy (J, Joules)
P = Power (W, Watts)
t = time (s, seconds)
Power, Current and Voltage
Power = Current x Voltage
Example:
P=?
I = 50 A
V=2V
Energy = P
Find the Power when
50 A is supplied with
2V
P
I
V
P=IxV
= I x V = 50 x 2
= 100 W
P = Power (W, Watts)
I = Current (A, Amps)
V = Voltage (V, Volts)
Power, Voltage and Resistance
Voltage = current x resistance
Example:
V=?
I = 10 A
R = 10 Ω
Find the voltage across
a 10 Ω resistor when
10 A flows through it
Voltage = V = I x R = 10 x 10
= 100 V
V
I
R
V=IxR
V = Voltage (V, Volts)
I = Current (A, Amps)
R = resistance (Ω, ohms)
P, V, I and R
Power (P), Resistance (R) and Current (I) are
related by this equation:
P = I2R
P = VI
P = (IR) x I
P = I2R
(but V = IR so IR can replace V)
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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
Current in Series and Parallel
In a series circuit the
current is the same at
all points.
I1 = I2 = I3 …
In a parallel circuit the
current is split between
the branches.
Is = I1 + I2 + I3 ….
2A
2A
6A
2A
2A
6A
2A
1A
3A
Voltage in Series and Parallel
In a series circuit the
voltage across each
component adds to make
the supply Voltage.
Vs = V1 + V2 …
In a parallel circuit the
voltage is the same across
each branch.
Vs = V1 = V2 = V3 ….
12V
3V
6V
3V
4V
4V
4V
4V
Resistance in Series and Parallel
In a series circuit the total
resistance is the sum of the
resistance of each component.
RT = 36Ω
3Ω
RT = R1 + R2 …
9Ω
24Ω
RT = 2Ω
In a parallel circuit the
resistance can be found
using the following equation:
1/RT = 1/R1 + 1/R2 + 1/R3 ….
4Ω
8Ω
8Ω
Continuity Tester
When checking for faults in a circuit you can use:
An Ohmmeter
or
Ω
A Continuity tester (ct)
A continuity tester can be made using a battery and a bulb.
Fault Finding
Short circuit: ohmmeter = 0 Ω & ct. = very bright
Ω
Open circuit: ohmmeter = ∞ Ω & ct. = not lit
Ω
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.
Ignition Switch
Sidelights
Headlights
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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
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)
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.
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.
Ring Circuit
Advantages:
•
•
Uses less cable
Uses Thinner cable (as there are two paths for the electricity
to flow along)
Electricity Bills
To calculate Energy used in the
home use: Power = Energy / time
E
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
P
t
Power = Energy/time
E = Energy (kWh)
P = Power (kW)
t = time (h)
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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.
Current in a Wire
When electric current passes through a wire, a
magnetic field is produced around the wire.
Magnetic Field
Current
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.
Electromagnets are far stronger than permanent magnets.
They have many uses (see next two slides)
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.
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.
.
4.5V
12V
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).
The Electric Motor
Here are the main components of the simple
electric motor:
Rotating Coil
Commutator
Magnets
Brushes
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.
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 it’s 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.
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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