04 Oscillations, Waves After Class

Today: Notes about quiz; Oscillations, Waves, Doppler effect
Quiz #2 due Thursday
Exercises due Tuesday (To be assigned for this week’s material)
Waves by hb19 on flickr

Results of first quiz THANK YOU to those students who took the quiz!

Brownian motion clicker question

Scanning Tunneling Microscopy -- pseudocolor 6. Scanning Tunneling Microscopy, color Scanning Tunneling Microscopy: Think of those pretty pictures we looked at in class of iron atoms on an atomically flat copper surface. Q: Why are different parts of the image different colors than others? a. Different elements emit different colors of light. b. The color of the light detected by the microscope depends on the distance between the tip and the surface. c. The colors were invented by the IBM scientists to make the easier to make sense of (pseudocolor).

Self-sustaining oscillations are usually energy flowing from one form to another, back and forth
What are some common oscillations we can think of?
Students 2008: Electromagnetic; vibration of vocal cords or speaker; water surface; pendulum; mass on a spring; motion of a piston in an engine; atom oscillating
Are there two forms of energy that you can identify?

Mass on a spring…oscillation has a frequency and an amplitude
Frequency (hertz) = number of cycles per second
Period (seconds) = time per cycle (inverse of frequency)
Amplitude (often height) = magnitude of oscillation
(Tricky!)
Oleg Alexandrov wikipedia Amplitude is displacement from equilibrium

Oscillations are a foundation of waves they can easily be visualized as waves User:Evil_saltine wikipedia Oleg Alexandrov wikipedia

A wave is a disturbance that propagates through space…it carries energy from one place to another.
The author of the book says:
“ The source of all waves—sound, light or whatever—is something that is vibrating.” (“Oscillating”)
This is a pretty good definition, but sometimes it’s not obvious or evident what source is “oscillating”
For our purposes, the waves we care about must propagate through space and carry energy
We also think about waves that don’t carry energy…examples?

Let’s brainstorm on waves that carry energy
… Microwaves, sound waves, light waves, radio waves, ocean waves, heat waves (infrared), gamma rays, x-rays, earthquake waves (seismic), gravitational waves,

Kinds of waves in physics
Sound waves
Seismic waves (infrasound); audible sound ; ultrasonic waves
Electromagnetic waves
Radio waves; microwaves; infrared; Visible light; ultraviolet; x-rays; gamma rays
“ Other” waves
Wind-driven ocean waves; ripples in a pond; waves in the demos today; waves in other fluids (see picture)
Matter waves
Gravitational Waves
http://www.3sigma.ca/gravityWaves.html

Wave has properties of an oscillation ( frequency and an amplitude ) plus speed and wavelength
Frequency (hertz) = number of cycles per second
Period (seconds) = time per cycle (inverse of frequency)
Amplitude (often height) = magnitude of oscillation
Speed (meters per second) = how quickly energy propagates through space
Wavelength (meters) = distance between repeated features.
http://www.3sigma.ca/gravityWaves.html

Frequency and speed are the fundamental properties of a wave
The frequency of the wave is determined by the frequency of the driving oscillation.
The speed of the wave is determined by the physics.
The wavelength (e.g. distance between crests) is determined by the frequency and speed.
(Wave speed = wavelength x frequency) Wave table demo Frequency is fundamental Energy transfer Wavelength = Wave speed frequency

Clicker Question—Properties of waves
What is described incorrectly in this picture?
Wavelength
Amplitude
Frequency
period

Clicker Question—Properties of waves
What is described incorrectly in this picture?
Wavelength ( this is really common…see figure 19.3 in textbook )
Amplitude
Frequency
period

Ripple tank experiments with speed, frequency, wavelength
http://www.falstad.com/ripple/ (linked on WebCT)
This is really worth trying out!
Google search for: ripple applet
This simulation is based on physics

Clicker question I am going to decrease the frequency of the oscillating source. What will happen to the wavelength? A. Nothing B. Wavelength will increase C. Wavelength will decrease D. Impossible to determine http://www.falstad.com/ripple/

Clicker Question—Wavelength, frequency, speed
Suppose the wave speed is fixed for all frequencies and wavelengths. Which has a longer wavelength?
Frequency = 770,000 Hertz
Frequency = 97,300,000 Hertz
Both are the same, since speed is fixed

Clicker Question—Wavelength, frequency, speed
Suppose the wave speed is fixed for all frequencies and wavelengths. Which has a longer wavelength?
Frequency = 770,000 Hertz
Frequency = 97,300,000 Hertz
Both are the same, since speed is fixed
Wave speed = wavelength x frequency Wavelength = Wave speed frequency

Transverse Waves and Longitudinal Waves
Transverse waves :
“ Motion of the medium” is perpendicular to the direction of wave propagation E.g. Ocean waves
Longitudinal waves:
“ Motion of the medium” is in same direction as direction of wave propagation
E.g. compression waves in slinky; SOUND
Transverse wave demo From Zhang Jiang (our TA): The speed of longitudinal waves and transversal waves differ If there is an earthquake you will feel the vertical shake before the horizontal shake

Key concepts:
1. Waves transmit energy, not the material that is waving
If a transmitter and receiver are not moving relative to each other then:
Frequency observed = frequency transmitted

Doppler effect
Frequency observed NOT equal to frequency transmitted!
Because source and receiver are moving relative to each other
Ripple tank Doppler
Doppler demo

Clicker Question to think about: Wind and Doppler Effect

04 Oscillations, Waves After Class

by Steve Koch

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