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

A photograph, frozen in time, but showing all

places, of a travelling water wave.

l

What Makes a Wave? A Disturbance in the Medium What is a Medium?

Wave pulse (like

Slinkies!)

Something that

creates a

displacement

along the

medium.

An energy input

that “wiggles”

the 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 1D

motion or 2D.

Transmit energy at a speed:

V=fl=l/T

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

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 +0.6Tc

Depends on Density and “elasticity”

“Bulk Modulus”

Speed ~ √(Elasticity/Inertia)

V sound in steel= 6100 m/s!!! (17x speed of sound in

air!)

V of light= “c”= 3x10^8 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 (2x 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

• A E I O U

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/ph14e/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