AP Physics

Waves and Energy Transfer

TEKS Objective: 8A

The student knows the characteristics and behavior of waves. The student is expected to:

(A) Examine and describe waves propagated in various types of media and describe wave characteristics such as velocity, frequency, amplitude, and behaviors such as reflection, refraction, and interference.

Two Main Topics

Wave propertiesWave behavior

Waves transfer energyA wave pulse is a single disturbancehttp://www.colorado.edu/physics/phet/

simulations/stringwave/stringWave.swfA repeated disturbance is a continuous

wave pattern

Wave properties

1. Mechanical Waves – transfer energy, require a medium and obey Newton’s Laws of Motion

What is a medium? Sound waves – called compressional or

longitudinal waves Transverse wave – move through the

medium, making an “s” shape

2. Measuring Waves

Speed – how fast the wave is moving

Unit: m/s

Formula:

v f

2. Measuring waves continued:

Amplitude – maximum displacement from equilibrium

Wavelength – the distance need to make one complete cycle or wave, usually measured from crest to crest (λ), usually measured in meters.

2. Measuring waves continued:

Period – the time needed to make one complete wave or cycle

(T) unit is usually the secondFrequency – the number of waves or

cycles in a time unit (usually one second)

(f) Unit is the Hertz (Hz)

2. Measuring Waves continued:

Period and frequency have an inverse relationship

1

1

fT

Tf

3. Parts of a Wave

Crest – highest point of a waveTrough – lowest point of a waveAmplitude – maximum displacement from

line of equilibriumWavelength – λ the distance needed to

complete one wave or cycle

Parts of a Transverse Wave

4. Two types of waves:

Transverse – particles vibrate perpendicular to the direction of the wave motion

Longitudinal or compressional – particles vibrate parallel to the direction the wave motion. Examples: sound waves, primary earthquake waves

http://physicsclassroom.com/mmedia/waves/lw.html

Example Problem

A sound wave has a frequency of 262 Hz and a wavelength of 1.29 m.

a) What is the speed of the wave?

b) How long will it take the wave to travel the length of a football field (91.4m)?

c) What is the period of the wave?

Homework

Page 335 #1-7

Wave Behavior

When waves interact with boundaries they can:

ReflectRefractDiffractInterfere

Reflection

Is when a wave bounces back off a boundary or barrier

Parts of Reflection

Normal – the imaginary line perpendicular to the barrier at the point of reflection

Angle of incidence – angle between the incident ray and the normal

Angle of reflection – angle between the normal and the reflected ray

Parts of reflection continued:

Law of Reflection – for a smooth surface, the angle of incidence equal the angle of reflection

i r

Refraction

Bending of a wave at the boundary between two media as the wave moves from one medium to another, velocity and wavelength change, but period and frequency remain constant

http://physicsclassroom.com/mmedia/optics/bp.html

Diffraction

Waves moving around a barrier.Examples: sunset, miragehttp://micro.magnet.fsu.edu/primer/java/

scienceopticsu/diffraction/basicdiffraction

Diffraction continued:

Interference

The superposition of waves, waves pass through each other unchanged, it is only a displacement of matter

Two types: Constructive and Destructive interference

Constructive Interference

When two waves combine to produce a wave with a larger amplitude, the meet crest to crest

Destructive Interference

When two waves combine to produce a wave with a smaller amplitude, they meet crest to trough

Total destructive interference

Standing Waves

When two waves meet with equal and opposite amplitudes

Parts of a standing wave: node and antinode

Node is the area of no apparent motionAntinode is the area of maximum

displacement

http://physicsclassroom.com/Class/waves/U10L4b.html

http://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swf

Homework

Page 337 #8-10Review problems page 346 #32-47