waves (notes 1) - cp physics
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waves
definition
A wave is a wiggle in space and time.A wave is an oscillation (a repeated disturbance).
what does the waving?
1. mechanical waves the wave disturbance occurs in a medium (substance)e.g., pressure disturbance in air (sound waves) disturbance of water surface (water waves)
2. electromagnetic wavesrequire no mediume.g., light, x-rays
There are two directions associated with waves.
1. propagation direction
Where the wave goes.Where the energy goes.
what direction is the waving done in?
2. oscillation direction
The direction in which individual particles (or fields) move.
If propagation and oscillation are at right angles, it’s a transverse wave.
oscillationpropagation
wave on a string (PHeT)
crowd doing the wave (video)
There were once water ripples on Mars.
Some bats can detect the
ripples made by frogs when
calling for mates.
The ripples are like a target,
with a juicy meal at the center!
Anatomy of a transverse wave
wavelength
wavelength = horizontal distance between any two identical points on the wave. Units: m Symbol: (“lambda”)
Anatomy of a transverse wave
amplitude
amplitude = vertical distance from center line to wave peak. Units: m
If propagation and oscillation are in the same direction, it’s a longitudinal wave.
oscillation propagation
ancient video on sound
slinky
Anatomy of a longitudinal wave
compression = region of high pressure
rarefaction = region of low pressure
high pressure low pressure
Anatomy of a longitudinal wave
wavelength
wavelength = horizontal distance between any two identical pressures. Units: m Symbol:
compression rarefaction
amplitude = maximum pressure – average pressure.
pre
ssure
amplitude
avera
ge
the wave equation
frequency = number of waves passing a fixed point each second.
Symbol: f
Units: Hz (“Hertz”)
the wave equationLet’s count the number of waves passing in 1 sec:
t = 1 s
t = 0 s
2.25 waves
the wave equation
frequency = number of waves passing a fixed point each second.
f = 2.25 Hz in the example from the previous slide.
the wave equationWe can also calculate the frequency if we know the wave’s speed, .
t = 1 s
𝑑
𝑑=𝑣𝑡
the wave equationWe can also calculate the frequency if we know the wave’s speed, .
t = 1 s
𝑑number of wavelengths
𝜆
the wave equation
number of wavelengths
frequency = number of wavelengths in 1 sec, so set = 1 sec:
Therefore,
or
the wave equation
the wave equation
the wave equationExample1. What is the frequency of red light?
(=700 nm, speed of light = 2.99 x 108 m/s)
Make sure everything’s in SI units:
Solve: , therefore