Transcript P1 – Energy for the home
P1 – Energy for the home
Revision lesson 2
The Electromagnetic Spectrum
Each type of radiation shown in the electromagnetic spectrum has a different wavelength and a different frequency: Gamma rays X-rays Ultra violet Visible light Infra red Microwaves Radio/TV γ High frequency, _____ wavelength All waves travel at 300 000 000m/s in a vacuum Low frequency, _____ (high) wavelength
Sending signals by light, electricity and radio and microwaves all have various advantages and disadvantages.
Light signals?
Can be easily sent with nothing more sophisticated than a torch, but the signal needs to be coded and is not secure.
Electrical signals? (eg house telephone)
Need equipment and wires linking the sender and receiver of the signal. Wires can be cut or damaged. Signal needs to be amplified at regular intervals because of loss of energy in form of heat due to the electric current.
Radio waves and microwaves? (eg satellite, mobile, radio)
These can travel very large distances and through the atmosphere – and even across space!
Comparing infra-red and microwave cookery
Higher Infra-red cooked food is usually browned, Microwave cooked food is not.
This is because of the way the energy is transferred.
Infra-red:
Energy is absorbed by the particles on the surface of the food. This increases the kinetic energy of the particles. The energy is then transferred to the rest of the food by conduction.
Microwaves:
the water molecules in the outer layers vibrate more and their kinetic energy increases. Which cooks quickest?
The energy is then transferred to the rest of the food by convection and conduction
Electromagnetic Spectrum - To do
1. Which parts of the spectrum can be used in 2. What are the concerns about using mobile phones?
3.
Which two waves in the spectrum can be used to cook food. Describe the differences between 1.
2.
3.
Radio, microwaves and light If microwaves are absorbed by water molecules in living tissue, cells may be burned/killed. Potential dangers are prolonged exposure but Microwaves and Infrared . Microwaves heat surface of the food
Reflection, Refraction and Diffraction
all electromagnetic waves can be reflected, refracted and diffracted.
Reflection: The wave changes direction at a boundary. It returns in the direction it originated.
Refraction: A change in direction due to the speed of the wave. (caused by entering a different medium; e.g. light from air into water) Diffraction: Bending of waves caused by obstacles. The degree and type of diffraction depends on the obstacle
Reflection
Angle of incidence = Angle of reflection
Normal
Reflected ray Incident ray
Angle of reflection Angle of incidence 26/04/2020 Mirror
Refraction through a glass block:
Wave slows down and bends towards the normal due to entering a more dense medium Wave slows down but is not bent, due to entering along the normal Wave speeds up and bends entering a less dense medium
Wave Properties
1) Reflection 2) Refraction 3) Refraction 4) Diffraction
Diffraction – Higher only
More diffraction if the size of the gap is similar to the wavelength More diffraction if wavelength is increased (or frequency decreased)
Diffraction depends on frequency…
A high frequency (short wavelength) microwaves -wave doesn’t get diffracted much – the house won’t be able to receive it…
Diffraction depends on frequency…
A low frequency (long wavelength) radio waves wave will get diffracted more, so the house can receive it…
Sending information - Finding the Critical Angle…
1) Ray gets refracted 2) Ray still gets refracted 3) Ray still gets refracted (just!)
THE CRITICAL ANGLE
4) Ray gets internally reflected
Uses of Total Internal Reflection
Optical fibres: Use total internal reflection to send information Uses – communication systems, endoscope, binoculars
L
ight
A
mplification by
S
timulated
E
mission of
R
adiation White light from a bulb is made up of many different colours. Each is of a different frequency and is out of PHASE.
The light from a LASER is at just one frequency and is in phase . This makes it very much brighter and able to travel further that ordinary white light.
Lasers produce narrow, intense beams of monochromatic light
Higher - Compact Discs
• • • • Plastic disc with a billions of pits on the bottom shiny surface (coated with metal film) The pits represent a digital signal The signal is read by a laser The shiny metal film reflects the laser light
Questions 1. Name 2 uses of lasers.
2. Why is it dangerous to look at a laser beam?
3. What is morse code and why is it important?
1. communication, dental treatment, surgery, light shows, bar code readers 2. Narrow intense beam of light capable of cutting, burning or vaporising materials 3. Digital signal - Series of dots and dashes representing letters of the alphabet. Used to send messages
6 mark question
•
The quality of written communication will be assessed in your answer to this question.
•
Describe how light signals can travel through optical fibres and why they are important
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off the sides of narrow core, they enter the fibre so it hits the boundary between the core and the outer cladding at an angle greater than the critical angle, causing total internal reflection. Pulse of light enters at one end and is reflected again and again until it emerges at the other end .Uses – telephone conversations and computer data All information in answer is relevant, clear, organised and presented in a structured and coherent format. Specialist terms are used appropriately. Few, if any, errors in grammar, punctuation and spelling. (5 –6 marks)
Level 2
Answer applies knowledge of how light signals can pass through an optical fibre and why they are important. For the most part the information is relevant and presented in a structured and coherent format. Specialist terms are used for the most part appropriately. There are occasional errors in grammar, punctuation and spelling. (3 –4 marks)
Level 1
An incomplete answer, states that light signals are totally internally reflected and mentions a use. Answer may be simplistic. There may be limited use of specialist terms. Errors of grammar, punctuation and spelling prevent communication of the science. (1 –2 marks)
Level 0
Insufficient or irrelevant science. Answer not worthy of credit. (0 marks
)
Analogue vs. Digital
Analogue signals (like talking or music) continually vary in amplitude and/or frequency + 1 0 Digital signals, however, are either off or on, and the information is sent in a series of pulses There are two main advantages of digital: 1) More information can be sent down the same cable 2) Better quality, because a digital signal can be amplified without amplifying the extra noise:
Transmitting information – Higher
As we said before, different types of electromagnetic radiation can be used to send different types of information, e.g. an optical fibre: Optical fibres have two main advantages: they can send more information compared to electrical cables of the same diameter and with less signal weakening.
Another example is radio waves: Ionosphere The short wave radio (TV and FM) and medium radio waves are transmitted by being reflected off the ionosphere (an electrically charged layer in the Earth’s atmosphere).
Long wave radio waves diffract (bend) around the Earths surface
Uses of radio waves
• terrestrial frequency Ultra waves long high
Very curvature distant ionosphere Longer Earth transmit
• Radio waves are used to __________ radio and television programmes between different points on the __________’s surface. __________ wavelength radio waves are reflected from an electrically charged layer in the Earth’s upper atmosphere, which is called the __________. This enables them to be sent between __________ points despite the __________ of the Earth’s surface. __________ __________ frequency (VHF) radio waves allow higher quality audio transmission than __________ waves or medium __________ __________ high __________ (UHF) radio waves are used for television and by the police.
Multiplexing: This allows many different signals to be transmitted simultaneously, with less interference
higher Each digital signal is divided into segments of very short duration. A combined signal takes each segment in turn and transmits it. A multiplexer combines the individual signals to be sent and a demultiplexer separates them at the receiving end of the transmission.
demultiplexer Signal A aaaaaaaaaaaaaaaaa Signal B bbbbbbbbbbbbbbb Signal C cccccccccccccccc multiplexer Transmitted as abcabcabcabcabcabcabcabc Signal A aaaaaaaaaaaaaaaaa Signal B bbbbbbbbbbbbbbb Signal C cccccccccccccccc Used in telecommunications and computer networks
Multiplexing and the digital switchover
Due to the large amount of data that can be sent due to multiplexing, the switch from analogue to digital has been made possible.
The main benefits of digital television and radio are: Improved signal quality (sound and image) Improved choice of programmes Interaction with programmes (e. g. red button Programme guides
6 mark question - Higher
The quality of written communication will be assessed in your answer to this question.
Lisa lives in a house in the valley. He can listen to long-wave and medium wave stations on his radio. He can’ t get any signal on his mobile phone and VHF (very high frequency used for FM) reception is very poor. Explain these differences
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Answer correctly applies knowledge of how long and medium wave 26/04/2020 signals diffracted around the obstacle to his house; some diffraction with VHF; however microwaves have an even shorter wavelength so do not diffract as much. Microwave transmitter needs to be in the line of sight. All information in answer is relevant, clear, organised and presented in a structured and coherent format. Specialist terms are used appropriately. Few, if any, errors in grammar, punctuation and spelling. (5 –6 marks)
Level 2
Answer applies knowledge of how microwaves do not diffract as much as radio waves therefore it can’t be received at the house behind the hill, and how the transmitter needs to be in the line of sight. For the most part the information is relevant and presented in a structured and coherent format. Specialist terms are used for the most part appropriately. There are occasional errors in grammar, punctuation and spelling. (3 –4 marks)
Level 1
An incomplete answer, states that radio waves diffract more than microwaves . Answer may be simplistic. There may be limited use of specialist terms. Errors of grammar, punctuation and spelling prevent communication of the science. (1 –2 marks)
Level 0
Insufficient or irrelevant science. Answer not worthy of credit. (0 marks
)
Questions
1.
2.
What type of EM wave is used in satellite communication?
Give 2 disadvantages of the use of microwaves in mobile phones 3. What are digital and analogue signals. What is the advantage of sending information digitally?
4.
Give 3 uses of IR.
5. Why is it said that texting is a safer way to communicate than making a call from a mobile phone?
1.
2.
3.
4.
5.
Answers
Microwaves Don’t diffract as much due to short wavelength so transmitters need to be in line of sight, interference between signals (affected by weather) Digital – on/off, analogue can take any value. Advantage – better quality, send several signals at the same time (multiplexing) Security systems, remote controls, mobile phones and computers (only short distances though and must point directly) There has been reports that mobile phones may be dangerous if absorbed by water molecules in living tissue, cells may be burned. However there is no conclusive evidence, if there was surely mobile phones would be designed to give out less microwaves and there would be a legal age limit for use or use for emergency calls only
Term
A radio wave B refraction C frequency
Meaning
1 distance occupied by one complete cycle of a wave 2 the shortest wavelength radio waves in the electromagnetic spectrum 3 the complete range of observed frequencies of electromagnetic waves D microwaves E aerial F ionosphere G interference H wavelength 4 the change in direction of a wave when it passes from one medium to another of different density 5 poor radio reception caused by overlapping waves 6 part of the electromagnetic spectrum used mainly for communication 7 a layer of the upper atmosphere that reflects some radio waves 8 the number of complete waves passing a point in 1 second I electromagnetic spectrum 9 effect of two signals from the same source which have travelled different distances to reach a television aerial J ghosting 10 device for receiving or transmitting radio signals
Some definitions…
Transverse waves
are when the displacement is at right angles to the direction of the wave…
Longitudinal waves
are when the displacement is parallel to the direction of the wave…
Some definitions…
1) Amplitude – this is “how high” the wave is:
2) Wavelength (
) – this is the distance between two corresponding points on the wave and is measured in metres: 3) Frequency – this is how many waves pass by every second and is measured in Hertz (Hz)
The Wave Equation
The wave equation relates the speed of the wave to its frequency and wavelength: Wave speed (v) = frequency (f) x wavelength (
)
in m/s in Hz in m V f
Some example wave equation questions
1) A water wave travels through a pond with a speed of 1m/s and a frequency of 5Hz. How far apart are the waves?
2) The speed of sound is 330m/s (in air). When Ricky hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
3) Purple light has a wavelength of around 6x10 -7 m. If its frequency is 5x10 14 Hz what is the speed of light?
4) Red light travels at the same speed. Work out its frequency if its wavelength is about 4x10 -7 m.
The Structure of the Earth
A thin crust 10-100km thick A mantle – has the properties of a solid but it can also flow A core – made of molten nickel and iron. Outer part is liquid and inner part is solid
How do we know this? These facts have all been discovered by examining seismic waves (earthquakes)
Seismic waves
Earthquakes travel as waves through the Earth – we call them SEISMIC WAVES. There are two types:
P waves:
1) They are longitudinal so they cause the ground to move up and down 2) They can pass through solids and liquids 3) They go faster through more dense material
S waves:
1) They are transverse so they cause the ground to move from right to left 2) They ONLY pass through solids 3) They are slower than P waves 4) They go faster through more dense material
Waves through the Earth - Questions
1.
How do we know the structure of the Earth?
2. Describe two differences between p and s waves 3. Name one advantage and disadvantage of seismometer.
1. How Seismic waves, such as P and S waves travel through the Earth 2. S waves can only travel through solids and are slower than p waves. P waves can travel through solids and liquids. S waves are transverse and P waves are longitudinal 3. Advantage e.g. seismometers detect p waves before more damaging s waves arrive so people can get to a safe place. Disadvantage – they record information after an earthquake has happened so may not give much warning
Human effect on the Earth
Use of CFC’s in aerosols has meant that the ozone layer has become damaged. This means that more of the Sun’s harmful U.V. is reaching us Burning fossil fuels is producing carbon dioxide (a greenhouse gas). This is trapping in the heat and contributing to global warming
Avoiding Sunburn
Sun index gives information about the strength of the sun and how long you can stay in the sun without burning if you are not wearing sunscreen.
Sun protection factor (SPF) is an indication of how much longer you can stay in the Sun when using sunscreen; SPF 10 means 10 times longer, etc.
SPF safe time given by sun index = time you can stay in the Sun with that SPF sunscreen.
Quick question
What is the maximum amount of time you could spend in the sun if the sun index is 8 and you put on sun cream with a SPF factor of 8?
Safe time = SPF no. X time given in sun index 8 x 20-30 min = 160 min – 240 min
Questions
1.
2.
3.
4.
What can happen when a person is exposed to too much UV radiation?
Why are people with darker skin less likely to suffer from skin cancer?
What does a SPF 15 mean?
Why are scientists concerned about the thinning of the ozone layer?
5.
UV radiation can damage the DNA in your cells, which may lead to skin cancer, it can cause eye problems such as cataracts, as well as premature skin aging 2. The skin absorbs more of the UV so less reaches the cells that may become cancerous 3. You can stay out in the sun 15 times as long 4. Ozone layer has got thinner due to the overuse of CFCs, this allows more UV rays to reach us at the surface of the Earth
Plenary - To Do
Produce a summary sheet / mind map for the P1 – Energy for the Home topic