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# Unit 4 2014 ppt wave characteristics

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### Unit 4 2014 ppt wave characteristics

1. 1. WAVE Physics Power Points CHARACTERISTICS Physics I Mr. Young www.pedagogics.ca
2. 2. What is a wave?
3. 3. What is a wave? a pattern of disturbances (oscillations, vibrations) caused by the movement of energy through matter or space
4. 4. What is a wave? a pattern of disturbances (oscillations, vibrations) caused by the movement of energy through matter or space
5. 5. What is a wave? a pattern of disturbances (oscillations, vibrations) caused by the movement of energy through matter or space
6. 6. Waves can be classified according to their nature. Mechanical waves Electromagnetic waves
7. 7. Waves can be classified according to their nature. Mechanical waves require a medium (matter) to travel through Sound waves, water waves, ripples in strings or springs are all examples of mechanical waves. Electromagnetic waves
8. 8. Waves can be classified according to their nature. Mechanical waves require a medium (matter) to travel through Sound waves, water waves, ripples in strings or springs are all examples of mechanical waves. Electromagnetic waves do not require a medium (matter) to travel through – they can travel through space (a vacuum) Radio waves, visible light, x rays
9. 9. Waves can be classified by direction of vibration Transverse waves Longitudinal waves
10. 10. A wave where the medium particles vibrate at right angles to the direction of energy transfer is called a transverse wave. Look carefully at the motion of ONE of the particles in this transverse wave!
11. 11. Waves can be classified by direction of vibration Tranverse waves direction of energy transfer direction of vibration Longitudinal waves
12. 12. Sound is an example of longitudinal wave A tuning fork causes surrounding air molecules to vibrate back and forth. Direction of sound travel Look carefully at the motion of ONE of the particles in this longitudinal wave!
13. 13. Waves can be classified by direction of vibration Transverse waves direction of energy transfer direction of vibration Longitudinal waves direction of vibration direction of energy transfer
14. 14. Wave pulses travel through a medium and therefore transfer energy. Particles in the medium are moved back and forth by each pulse but are not transferred from one place to another. direction of energy transfer Vibration of metal atoms in the spring (the medium) BIG IDEA! WAVES TRANSFER ENERGY NOT MATTER!
15. 15. Observing Waves Activity
16. 16. crest trough
17. 17. crest trough wave front
18. 18. crest trough wave front ray
19. 19. crest wave front trough ray
20. 20. Adjusting Frequency Water droplets hitting the surface create disturbances (ripples) which move outward from the source.
21. 21. What happens when frequency is increased? The rate of water droplets falling increases  creates more disturbances per unit time. Each disturbance has less time to travel before the next is created – so ripples are closer together
22. 22. On a plot:
23. 23. More drops per second, more waves per second, waves are closer together.
24. 24. What happens when the amplitude of the source (drops) is increased? This would be caused by the drops hitting the water with more force.
25. 25. Increasing the height or size of the falling water droplet is a BIGGER disturbance. What changes? The height of the ripples increases. What stays the same? The distance between the wavefronts
26. 26. A cork placed in the dish will bob up and down as the waves passed by. The relative position of the cork does not change.
27. 27. You can think of the cork as representing a molecule of water (the medium) So, the relative position of water molecules does not change (only motion is up and down)
28. 28. KEY CONCEPT Waves transfer ENERGY not matter!
29. 29. Describing Waves Waves are created by vibrating objects ONE vibration = ONE cycle = ONE oscillation (all describe one complete back and forth motion) FREQUENCY: the number of vibrations each second. Measured in s-1 (or Hertz). The symbol for frequency is f. A frequency of 12 Hz means that there are 12 complete waves generated each second! 풇 = 풏풖풎풃풆풓 풐풇 풗풊풃풓풂풕풊풐풏풔 풕풊풎풆 (풔)
30. 30. PERIOD: the time required for one vibration. Measured in seconds. The symbol for period is T. 푻 = 풕풊풎풆 (풔) 풏풖풎풃풆풓 풐풇 풗풊풃풓풂풕풊풐풏풔 푻 = ퟏ 풇 풇 = ퟏ 푻
31. 31. WAVELENGTH: How far the wave energy travels in one complete vibration. This is the distance traveled in an amount of time equal to the period of the wave. The symbol for wavelength is the Greek letter lambda (l).
32. 32. Wavelength is measured as the distance between identical points on two successive waves.
33. 33. AMPLITUDE: a measure of the maximum distance the particles in the medium are displaced from their resting position as the wave passes. Direction of Energy transfer transverse vibration longitudinal vibration It is easier to visualize these characteristics by looking at plots
34. 34. AMPLITUDE: a measure of the maximum distance the particles in the medium are displaced from their resting position as the wave passes. Direction of Energy transfer transverse vibration longitudinal vibration It is easier to visualize these characteristics by looking at plots
35. 35. Activity 2 – Examining Wave Motion in a Slinky Spring
36. 36. A continuous wave is produced when a series of pulses are generated by a vibrating source. One shake produces a single vibration (a wave pulse). If the end of the spring is shaken back and forth, a continuous series of pulses is produced. This is a standing wave pattern.
37. 37. Longitudinal pulses can be produced by holding spring coils together at one end and then releasing. A continuous wave can be produced by sliding the end of the spring back and forth towards your partner (holding the fixed end)
38. 38. Longitudinal pulses can be produced by holding spring coils together at one end and then releasing. A continuous wave can be produced by sliding the end of the spring back and forth towards your partner (holding the fixed end)
39. 39. Using Graphs to Describe Waves
40. 40. Displacement – Position Graphs A displacement – position plot shows the different position of a particles in the medium at a given time for a section of the wave. Consider a rope that is being vibrated at one end. If we took a snapshot with a camera it might look like this.
41. 41. 1.5 1 0.5 0 -0.5 -1 -1.5 position displacement The graph shows the displacement of the rope at an instant in time.
42. 42. 1.5 1 0.5 0 -0.5 -1 -1.5 position displacement In transverse waves, the maximum and minimum displacements are called crests and troughs respectively. The distance between two successive crests OR troughs would be the wavelength crest l trough
43. 43. Longitudinal waves are a bit tricky to understand because the direction of vibration is not perpendicular to the direction of wave travel. Consider the following images compression rarefaction l
44. 44. 1.5 1 0.5 0 -0.5 -1 -1.5 position displacement In longitudinal waves, the maximum and minimum displacements are called compressions and rarefactions respectively. The distance between centers of two successive compressions OR rarefactions would be the wavelength center of compression l center of rarefaction
45. 45. Displacement-Time Graphs A displacement – time plot shows how the position of a particle in the medium changes as it vibrates back and forth. Transverse and longitudinal waves look the same on this type of plot. The only difference is the direction of the particle displacement. 1.5 1 0.5 0 -0.5 -1 -1.5 time displacement
46. 46. Amplitude can be measured on the y-axis. 1.5 1 0.5 0 -0.5 -1 -1.5 time displacement Period can be measured on the x-axis. The period would be the time for one complete wave.
47. 47. Wave Speed The speed of a wave is determined by the properties of the medium it travels through. Properties include:
48. 48. Wave Speed The speed of a wave is determined by the properties of the medium it travels through. Properties include: • type of material • elasticity • tension • density (solid, liquid or gas) • temperature Speed is calculated using the following equation. distance speed= time
49. 49. Recall: The definition of period The definition of wavelength
50. 50. Recall: The definition of period the time it takes to complete one vibration/oscillation/cycle The definition of wavelength the distance a wave travels in a complete vibration/oscillation/cycle
51. 51. So: wavelength wave speed= period
52. 52. So: wavelength wave speed= ?period
53. 53. wavelength wave speed= period Try this: Use frequency instead of period and derive the wave speed equation. Write the equation using appropriate symbols.
54. 54. Practice: 1. Find the speed of a wave in a metal spring if a pulse travels 7.5 m in 3.0 s. 2. What is the frequency of this wave if the distance between two successive wave crests is 0.25 m? 3. Challenge: A fisherman passes the time by counting waves passing under his boat. He notices that his boat rises and falls 8 times in 65 seconds. He counts 6 wave crests between his boat and a buoy located 54 m away. What is the speed of the water waves?