1. Waves
Randall Physics 2011

2. Waves
What is a wave?
Examples?

3. Wave Types Mechanical Wave Electromagnetic Wave
Disturbance that travels through a medium
Produced by vibration of particles
Transports energy
Electromagnetic Wave
No need for medium to travel
Produced by vibration of charged particles

4. Wave Properties

5. Vocabulary
Longitudinal Waves (long) – particles vibrate at a 90 degree angle to the direction the energy in the wave travels
Transverse Waves (compression) (trans = across) – particles vibrate in the same direction as the energy in the wave travels

6. Vocabulary Longitudinal Waves (long) (compression)
Transverse Waves (trans = across)
Amplitude - A
Wavelength - 
Frequency (Hertz = Hz) - f
Period (s) - T
Velocity - v

7. Wave Speed Velocity depends on the medium
So for a specific medium, there is a specific velocity
v =  f
Wavelength increases, what happens to the frequency? The velocity?

8. High Speed Physics
If a wave has a wavelength of 10 m, and a frequency of 2 Hz, what is its velocity?
If a wave takes 2 seconds to pass by (period) and is 4 meters long, what is its velocity?
If a wave has a velocity of 6 m/s and is 12 meters long, what is its frequency?

9. New Stuff How do waves propagate?
The energy of a wave is proportional to the amplitude squared
E  A2
What happens to the energy when the amplitude is tripled? Halved?

10. Questions Why doesn’t a floating buoy move with the waves?
Describe how the fans in a stadium must move in order to produce a longitudinal stadium wave.
Give evidence to support the claim that sound is energy transferred by a wave, not energy transferred by particles.

11. Questions
In Star Wars III, Luke and Han are escaping on the Millennium Falcon and being shot at by a Star Destroyer. Why are the sound effects incorrect?

12. Energy
Spring Lab – the move we moved our hands, the more energy we used.
Moving our hands more gave us more Amplitude
E  A2

13. Intensity
As you get farther from a source, the energy doesn’t disappear, BUT! It is spread out more.
We call this Intensity. I  1 / r2
At a Lady Gaga concert, Jose starts out close to the stage but moves 2x as far from the stage for the second half. The music is not ½ as loud, it is 1/4th as loud.

14. Velocity and Temperature
Temperature changes the velocity of a wave (because it changes the medium)
The warmer the air is, the faster sound will travel.
v = 330 m/s + (.6 * temperature in °C)

15. Characteristics of Waves
Interference
Reflection
Refraction
Diffraction

16. Interference What happens when waves pass each other?
What happens to the particle?
What happens to the energy of the disturbance?

17. Interference
Superposition - 2 or more waves travel through the same medium at the same time
These waves will interfere
Does A depend on v? f ? ?

18. Interference
If two speakers are sending out the exact same signal, what do you hear?
How might you move them to change the sound you hear?

19. Interference
If 2 waves are “in phase,” the crests line up with each other.
How do they interfere?
If 2 waves are “out of phase,” one crest lines up with one trough.
How will their wavelengths  compare in each case?

20. Interference Example
Two speakers generate the same signal at the same time with a frequency of 920 Hz. How far could you move them to completely cancel the intensity of the sound? (The speed of sound in air is 340 m/s)
Students complete this one

21. Interference
Constructive - wave amplitudes reinforce each other

22. Interference Destructive - wave amplitudes oppose each other

23. Interference Example

24. Interference
Things can get complicated, even with only two waves

25. Summary
Waves transmit energy
Amplitude  Energy
f = 1/T
v =  f