Waves Wave Properties 1172014 Physical Waves Need a

Waves & Wave Properties 11/7/2014

Physical Waves • Need a medium (material) to transmit energy through. • Light waves are an exception. They use the space-time fabric itself! • NOT position vs. time graphs! • Y vs. X (Real Shape frozen in time) • 2 Types Travelling • Standing • l A photograph, frozen in time, but showing all places, of a travelling water wave.

What Makes a Wave? A Disturbance in the Medium � Wave pulse (like Slinkies!) � Something that creates a displacement along the medium. � An energy input that “wiggles” the medium What is a Medium? � The medium has to be elastic, like Hooke’s Law. � All materials are made of atoms which are springy! � Space is a medium whose electromagnetic properties are springy. � Take AP Physics C to find out more about this. We will a little.

2 Types of Physical Waves Transverse Longitudinal �Energy transfer is 90 degrees to displacement of medium. �Makes the SHAPE of a sine/cosine wave. �A water wave is an example. �Energy transfer is ALONG the direction of displacement of the medium. �Think slinky pulse. �SOUND is our most well known example. �Air molecules don’t exert forces on each other until they bump straight into each other!

2 Types of Transverse Waves Travelling Standing � Energy transfers from one place to another via a medium. (TRANSMITS) � Energy spreads out in spheres, but are often limited to 1 D motion or 2 D. � Transmit energy at a speed: � Energy Reflects at an interface between two media. � Frequency of Energy input must match reflected wave. � Constructive Interference creates anti-nodes, . � Locations of destructive interference are Nodes � All other places are in between. � Created by Resonance. � Microwaves � V=fl=l/T

Standing Wave Terminology

Resonance � A condition when the INPUT energy frequency MATCHES the n. r. f. of a system/object. THINK MYTHBUSTERS EARTHQUAKE MACHINE � n. r. f. – “natural resonant frequency”. � Size (related to mass and density) � Shape (Square, Round, linear) � Material (as in bonds, IMFs, tension) � Density � Creates a standing wave by perfectly timed constructive and destructive interference. � If energy input matches energy output (due to friction or heat transfer) then it has a steady Amplitude (Etotal=constant) � Amplitude grows if more energy goes in than out. � Amplitude is “damped” if energy output is greater.

Resonance Graphs

Physical Waves and SHM � V=fl � v=l/T � l found from y vs. x � T found from y vs. t

DEMO: Water pendulum �Notice something oscillating in SHM like a pendulum can make a real wave (y vs. x). �Frequency/Period is determined by equation: �The wavelength depends on speed I pull paper!

Applying Wave Equation

Motion Graph (x vs. t)

Speed of Waves �Related to the medium they travel through. �V water ripples (capillary or “cat’s paw” waves)~1 -2 m/s �V sound in air = 330 m/s +0. 6 Tc �Depends on Density and “elasticity” �“Bulk Modulus” �Speed ~ √(Elasticity/Inertia) �V sound in steel= 6100 m/s!!! (17 x speed of sound in air!) �V of light= “c”= 3 x 10^8 m/s. (Approx. 1 million x faster)

Finding the speed of sound. Reflection from a Wall (ECHO) �Speed of sound is constant under constant conditions like temperature and moisture content. �Stand a known distance from a wall. �Clap hands. �Listen for echo, claps hands, listen clap hands listen. �Another person clap their hands along with initial clap and echo clap, making a rhythm. �This rhythm is related to period. Use a stopwatch to time 10 claps. Divide by 10, now you have Period and distance (2 x for echo travel) �V=distance over Time.

4 Wave Properties �Superposition- overlapping waves add displacements. �“Monsterwellen” photo, cargo ships �AM radio waves. “Wave Envelope” �Reflection- Waves reflect at the interface between 2 boundaries. (Often some of the wave transmits). �Refraction- Wave direction changes when passing through a different medium. �Interference- Waves overlapping “in-phase nl”, or “out of phase (l/2) can double in Amplitude or “cancel”.

Superposition • Displacements at all x positions add together, . • Causes a waveform. • Vowel Sounds on Oscilloscope • AEIOU

Reflection • Phet animation. “Waves on a String” • Think “Conservation of Energy”

Refraction • The path of a “wavefront” changes when it passes through a different medium with a different wave speed. • • Snell’s Law Animation using “Huygen’s Principle”- All waves reform as spheres at boundaries. http: //www. walter-fendt. de/ph 14 e/huygenspr. htm

Interference • When the path length difference between 2 waves is a WHOLE number mulltiple of a wavelength, then: • Constructive interference • If the path length difference is a multiple of ½ wavelength then: • Destructive Interference. • Speaker Demo • Diffraction Patterns