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# Waves Basics

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GCSE Physics double award notes

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• Mexican wave – people are the medium and the disturbance is their standing up and down Air-zooka – Make your own wave generator, plastic bottle and balloon. Feel the wave pass you by! The air particles bounce off each other and push the energy along from the balloon.
• Wave motion is from left to right, particle motion is up and down
• Moves up and down, while the wave passes from left to right.
• Undisturbed position – equilibrium point Right angles is sometimes called perpendicular One quarter of a wavelength
• Bit more difficult to notice, next slide illustrates this better! Particles move on the same ‘plane’ as the wave’s motion. A ‘plane’ is a flat or level surface, which may be imaginary
• Particles move back and forth on the same plane as the movement of the wave
• Frequency = 2 Hz (2 Waves per second) http://id.mind.net/~zona/mstm/physics/waves/partsOfAWave/waveParts.htm#frequency
• Set up a table, two heading; longitudinal and transverse http://hyperphysics.phy-astr.gsu.edu/hbase/sound/wavplt.html#c3
• http://surendranath.tripod.com/Applets/Waves/Twave01/Twave01Applet.html What happens to the speed of the wave as the frequency is altered? If the speed stays the same then what other feature of the wave changes as well as the frequency? Page 61 (handout from CGP) answer question 10, A wave example
• Derivation using speed = distance / time Frequency is the number per second (inverse of time for one wave, one over time), Wavelength is the distance of one complete wave. Stick those values into speed equation and bing bang bong – wave equation!
• Try question 9, 11 and 12 on page 61
• One of the common uses of waves – we’re still getting to grips with the basics though that even x-ray waves would display
• Waves can be represented using wave-front diagrams. These show the crests of the waves passing through the medium. The point half way between two crests can be regarded as the trough.
• NB. The normal line is a reference line. It is set at a right angle (perpendicular) to the surface of the barrier. All angles are measured from this line e.g. the angle of incidence.
• Do Now – Use your notes from last day to complete the question…
• Analogy of car brakes… running on the beach… soldiers in line
• White rocks - Portrush How does the wavelength change as the waves come closer to shore? It becomes smaller, the waves are closer together – from deep to shallow water, change in medium (actually slows down due to energy lost through friction with the seabed).
• Surfing in Indonesia… check out the different waves (video) Some of the best swells out there!
• Surf Science Chapter 6 – Refraction (pics of defocusing and focusing page 53) Surf-able waves require a change in depth from deep to shallow to cause the wave to grow in size and peel. A reef, sand-bar or point will usually be the reason for the waves changing and bending. The wave will allows bend towards the slower (shallower) part of the coast. Think of a car moving which then can apply its brakes to only the left hand-side, it will veer off to that side. Point Break – headland sticking out with deep water in front and shallow water on either side. As the swell comes in one end will slow down as it hits the shallow water and bend in towards the headland. The rest of the wave will ‘fan out’ (defocus) and the energy will be spread out along the wave. The wave will reduce in size and power but the wall will be longer and not diminish much as you ride along. Reef Break- waves are concentrated or focused onto a slab of rocking sticking out from a stretch of open coast. The shallow reef would have a deep channel of water beside it and then a steep beach on the coastline. The swell coming in hits the reef first and slows down with the rest of the swell travelling in the deep water focusing its energy and bending in towards the reef. This makes a bigger more powerful wave with a high peak, ‘swell magnet’.
• Point break
• Reef break
• Try Question 1 page 105
• ### Waves Basics

1. 1. Waves GCSE Physics
2. 2. Objectives of the Lesson <ul><li>Understand the concept of wave motion in the physical world </li></ul><ul><li>Recall that there are two types of wave motion- Longitudinal and Transverse </li></ul><ul><li>Explain the difference between the types of waves and use the correct terms for each </li></ul><ul><li>State some examples of each of the types of wave motion </li></ul>
3. 3. Waves Introduction <ul><li>A wave can be described as a disturbance that travels through a medium from one place to another. </li></ul><ul><li>Medium – a substance which something is transmitted or carried through e.g.. air </li></ul>
4. 4. Examples of waves <ul><li>Sea wave (medium- liquid, water) </li></ul><ul><li>Shotgun blast (medium- gas, air) </li></ul>
5. 5. Types of Waves- Transverse Wave motion vs Particle motion
6. 6. Watch one particle Describe its motion
7. 7. Transverse wave… In terms of the wavelength, what is the length of the red line on the graph? Wavelength λ – distance from one point on one wave to the same point on the next wave (for example- from crest to crest) Crest Crest Trough Trough Wavelength λ Wave spreads Wave movement Light travels as a transverse wave Wave movement is at right angles to the direction that the wave is travelling Amplitude – maximum displacement from its undisturbed position Amplitude
8. 8. Types of Waves- Longitudinal Wave motion vs Particle motion
9. 9. Keep your eye on 1 particle Describe its motion
10. 10. Longitudinal wave If the diagram shows the motion of the wave in 1 second then what is the frequency of the wave? Frequency – the number of waves passing a point every second (Hz) Rarefaction Compression Sound waves travel as longitudinal waves Wave movement is parallel to the direction the wave is travelling
11. 12. Water Waves <ul><li>Water waves are an example of waves that involve a combination of both longitudinal and transverse motions. As a wave travels through the water, the particles travel in clockwise circles . </li></ul>
12. 13. Longitudinal Vs Transverse <ul><li>Create a table with two column headings, Longitudinal and Transverse </li></ul>Wave motion is at right angles to the direction of the wave Wave motion is parallel to the direction of the wave Amplitude is maximum displacement from the wave’s rest position Frequency is the number of waves passing a point in one second Wavelength is the distance from one crest to the next crest Wavelength is the distance from one compression to the next compression This is how sound energy moves This is how light travels L L or T L L T T T or L T L T
13. 14. Graph action… http://surendranath.tripod.com/Applets/Waves/Twave01/Twave01Applet.html
14. 15. Objectives of the Lesson <ul><li>Recall the wave equation and use it to solve simple problems </li></ul><ul><li>Describe, using simple wavefront diagrams, how plane waves are reflected at a plane barrier, refracted at a plane boundary </li></ul>
15. 16. The Wave Equation <ul><li>Speed ( v ) = Frequency ( f ) x Wavelength ( λ ) </li></ul><ul><li>m/s Hz m </li></ul>
16. 17. Example <ul><li>What is the speed of a water wave of frequency 4 Hz and wavelength 3 cm? </li></ul><ul><li>v = f x λ </li></ul><ul><li>= 4 x (3 / 100) </li></ul><ul><li>= 0.12 m/s </li></ul>
17. 18. Try this one… <ul><li>What is the wavelength of a sound wave of frequency 264 Hz and speed 330 m/s? </li></ul><ul><li>v = f x λ </li></ul><ul><li>330 = 264 x λ </li></ul><ul><li>λ = 330 / 264 = 1.25 m </li></ul><ul><li>What is the distance between one compression and the next rarefraction on this wave? </li></ul>1.25 / 2 = 0.625 m
18. 19. Question Time <ul><li>Pg 105 Question 1 a) parts i) – iv) </li></ul>
19. 20. Objectives of the Lesson <ul><li>Describe, using simple wavefront diagrams, how plane waves are reflected at a plane barrier, refracted at a plane boundary </li></ul>
20. 22. Ripple Tank Wave motion Crest Trough
21. 24. Wave Diagrams <ul><li>Law of Reflection- The angle of incidence is always equal to the angle of reflection </li></ul>Wave front (crest)
22. 25. The angle the wave hits the barrier at is 23 degrees. 1. What is the angle of incidence ? 2. Draw the three waves after they have hit the barrier and label the angle of reflection and include the normal line . Example 23 º Normal line Angle of incidence 67 º Angle of reflection 67 º
23. 26. Objectives of the Lesson <ul><li>Describe, using simple wavefront diagrams, how plane waves are reflected at a plane barrier, refracted at a plane boundary </li></ul>
24. 27. Shallow water Deep water
25. 29. <ul><li>Refraction - For a wave moving from deep to shallow, the wavelength will shorten because the wave slows down. </li></ul><ul><li>The frequency of the wave stays the same </li></ul>Refraction Shorter Wavelength- waves speed slower Shallow water Deep water λ λ wavefront
26. 34. Refraction <ul><li>If the incident wave travels at an angle and meets shallow water, then the wave will be refracted (shifted) towards the normal </li></ul>Angle of refraction Angle of incidence Normal line Shallow water Deep water Angle of incidence Angle of refraction >
27. 35. Refraction <ul><li>What do you think will happen when a wave travels into deeper water? Draw a diagram with labels to represent the situation. </li></ul>
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