Bike Generator Project• Each lab section will build 1 bike generator•Each lab group will

•build 1 energy board•Connect and test energy board and bike generator•Create curriculum materials and demos to teach a class

•How current creates a magnetic field•How a motor works•How a generator works•Your hypothesis that students to test while riding bike•Handouts that explain activities (electronic versions to me)•VISIT A CLASS and show demos and do activities

•Each person will produce a formal lab write-up for this lab project

Transverse and Longitudinal Waves

Transverse and Longitudinal Waves

Wave Speed

The speed of transverse waves on a string stretched with tension Ts is

where μ is the string’s mass-to-length ratio, also called the linear density.

EXAMPLE 20.1 The speed of a wave pulse

QUESTION:

EXAMPLE 20.1 The speed of a wave pulse

One-Dimensional Waves

• To understand waves we must deal with   functions of two variables, position and time.• A graph that shows the wave’s displacement as a   function of position at a single instant of time is   called a snapshot graph. For a wave on a string,   a snapshot graph is literally a picture of the wave   at this instant.• A graph that shows the wave’s displacement as a   function of time at a single position in space is   called a history graph. It tells the history of that   particular point in the medium.

EXAMPLE 20.2 Finding a history graph from a snapshot graph

QUESTION:

EXAMPLE 20.2 Finding a history graph from a snapshot graph

Sinusoidal Waves• A wave source that oscillates with simple harmonic   motion (SHM) generates a sinusoidal wave.• The frequency f of the wave is the frequency of the   oscillating source.• The period T is related to the wave frequency f by

• The amplitude A of the wave is the maximum value of   the displacement. The crests of the wave have   displacement Dcrest = A and the troughs have displacement   Dtrough = −A.

Sinusoidal Waves

Sinusoidal Waves• The distance spanned by one cycle of the motion is called    the wavelength λ of the wave. Wavelength is measured in    units of meters.• During a time interval of exactly one period T, each crest    of a sinusoidal wave travels forward a distance of exactly    one wavelength λ. • Because speed is distance divided by time, the wave    speed must be

  or, in terms of frequency

Sinusoidal Waves

• The angular frequency of a wave is

• The wave number of a wave is

• The general equation for the displacement caused by    a traveling sinusoidal wave is

This wave travels at a speed v = ω/k.

Waves in Two and Three Dimensions

• Suppose you were to take a photograph of ripples    spreading on a pond. If you mark the location of the    crests on the photo, these would be expanding    concentric circles. The lines that locate the crests are    called wave fronts, and they are spaced precisely    one wavelength apart.• Many waves of interest, such as sound waves or    light waves, move in three dimensions. For    example, loudspeakers and light bulbs emit    spherical waves.• If you observe a spherical wave very, very far from    its source, the wave appears to be a plane wave.

Waves in Two and Three Dimensions

Waves in Two and Three Dimensions

Sound Waves

Sound Waves

• For air at room temperature (20°C), the speed of sound   is vsound = 343 m/s.• Your ears are able to detect sinusoidal sound waves   with frequencies between about 20 Hz and about   20,000 Hz, or 20 kHz. • Low frequencies are perceived as “low pitch” bass   notes, while high frequencies are heard as “high pitch”   treble notes.• Sound waves exist at frequencies well above 20 kHz,   even though humans can’t hear them. These are called   ultrasonic frequencies. • Oscillators vibrating at frequencies of many MHz   generate the ultrasonic waves used in ultrasound   medical imaging.

Test your hearing

• www.phys.unsw.edu.au/jw/hearing.html

EXAMPLE 20.6 Sound wavelengths

QUESTION:

EXAMPLE 20.6 Sound wavelengths

Electromagnetic Waves

• A light wave is an electromagnetic wave, an oscillation   of the electromagnetic field.• Other electromagnetic waves, such as radio waves,   microwaves, and ultraviolet light, have the same physical characteristics as light waves even though we cannot sense them with our eyes.• All electromagnetic waves travel through vacuum with   the same speed, called the speed of light. The value of   the speed of light is c = 299,792,458 m/s.• At this speed, light could circle the earth 7.5 times in a   mere second—if there were a way to make it go in   circles!

The Index of Refraction

• Light waves travel with speed c in a vacuum, but they    slow down as they pass through transparent materials    such as water or glass or even, to a very slight extent,    air. • The speed of light in a material is characterized by the    material’s index of refraction n, defined as

Power and Intensity

EXAMPLE 20.9 The intensity of a laser beam

QUESTION:

EXAMPLE 20.9 The intensity of a laser beam

EXAMPLE 20.9 The intensity of a laser beam

Intensity and Decibels• Human hearing spans an extremely wide range of    intensities, from the threshold of hearing at ≈ 1 × 10−12    W/m2 (at midrange frequencies) to the threshold of pain    at ≈ 10 W/m2. • If we want to make a scale of loudness, it’s convenient    and logical to place the zero of our scale at the threshold    of hearing. • To do so, we define the sound intensity level, expressed    in decibels (dB), as

where I0 = 1 × 10−12 W/m2.

Intensity and Decibels

The Doppler Effect

• An interesting effect occurs when you are in   motion relative to a wave source. It is called the   Doppler effect. • You’ve likely noticed that the pitch of an   ambulance’s siren drops as it goes past you. A   higher pitch suddenly becomes a lower pitch.• As a wave source approaches you, you will observe   a frequency f+ which is slightly higher than f0, the   natural frequency of the source.• As a wave source recedes away from you, you will   observe a frequency f− which is slightly lower than   f0, the natural frequency of the source.

The frequencies heard by an observer moving at speed v0 relative to a stationary sound source emitting frequency f0 are

The Doppler Effect

The frequencies heard by a stationary observer when the sound source is moving at speed v0 are

EXAMPLE 20.11 How fast are the police traveling?

QUESTION:

EXAMPLE 20.11 How fast are the police traveling?

Snapshot and History Wave Activity

• PHET and Matlab• phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.h

tml• Turn on the ruler and create a snapshot graph (amplitude vs.

displacement graph)• Select a green bead and center the left side of the ruler on it• Turn on the timer• Choose oscillate and pause the simulation to collect amplitude

and time data for that green bead• Record your data in Matlab• Plot and fit your data to create a history graph (amplitude vs.

time)