Standard Grade Revision notes for Telecoms, Electricity and Health Galshiels Academy’s web-site
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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 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 Small Divisions Kink in the glass
The range only goes from 32ºC - 42 ºC (outside this range a person is dead) Allows more accurate readings
Toughened Glass White background Lens-like glass
Stops mercury until it is reset Safety feature Make it easier to read 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
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
Whispering Bird Song Talking Snoring Rock Concert Jet Engine
Level (dB)
30 40 50 60 110 130 Unwanted sounds are called noise pollution – for example traffic or loud music
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 2x10 8 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.
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
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.
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 v = speed (m/s) t = time (s) (Hz)
d = distance (m) v = speed (m/s) t = time (s)
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 2x10 8 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)
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
Power supply
Loudspeaker
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
Aerial Tuner
The Television
Video Decoder Video Amplifier TV tube Audio Decoder Audio Amplifier Loudspeaker
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
Transmission of Waves
You should be able to: State that radio and TV transfer energy at 3x10 8 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 10 8 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.
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 P E 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) Neutral (blue) Earth – carries current into the appliance – carries current out of the appliance (yellow/green) – safety devices BL ue wire in B ottom L eft BR own wire in B ottom R ight
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
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
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 Fuse Diode Resistor V Voltmeter Variable Resistor Lamp Capacitor A 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 I Q 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).
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=I 2 R
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: Find the voltage across a 10 Ω resistor when 10 A flows through it V = ?
I = 10 A R = 10 Ω V = I x R I V Voltage = V = I x R = 10 x 10 = 100 V R 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: 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 P E t Power = Energy/time E = Energy (J, Joules) P = Power (W, Watts) t = time (s, seconds)
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 I P V P = I x V P = Power (W, Watts) I = Current (A, Amps) V = Voltage (V, Volts)
Power, Voltage and Resistance
Voltage = current x resistance
Example: Find the voltage across a 10 Ω resistor when 10 A flows through it V = ?
I = 10 A R = 10 Ω V = I x R I V Voltage = V = I x R = 10 x 10 = 100 V R 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 = I 2 R P = VI P = (IR) x I P = I 2 R (but V = IR so IR can replace V)
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 … 2A 2A 2A 2A In a parallel circuit the current is split between the branches.
Is = I1 + I2 + I3 ….
6A 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 ….
6V 12V 3V 4V 4V 4V 4V 3V
Resistance in Series and Parallel
In a series circuit the total resistance is the sum of the resistance of each component.
3Ω R T = R 1 + R 2 … R T = 36Ω 9Ω 24Ω R T = 2Ω In a parallel circuit the resistance can be found using the following equation: 1/R T = 1/R 1 + 1/R 2 + 1/R 3 ….
4Ω
8Ω 8Ω
Continuity Tester
When checking for faults in a circuit you can use: An Ohmmeter Ω or 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
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 connected to the mains will stay on.
parallel across the mains supply. This means that if one appliance breaks, all of the other appliances 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 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 E t Power = Energy/time E = Energy (kWh) P = Power (kW) t = time (h)
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.