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Physics - Section 3 - Waves - Part 1 - Coggle Diagram
Physics - Section 3 - Waves - Part 1
progressive waves
definition and energy transport
progressive waves carry energy from one place to another without transferring material
caused by oscillations at a source that propagate through a medium or field
source loses energy as waves carry energy away
ways to tell waves carry energy
EM waves heat things, X-rays/gamma rays ionise, loud sounds induce vibrations
wave power can generate electricity
basic wave quantities
cycle, displacement, amplitude, wavelength, period, frequency, phase
cycle is one complete vibration; displacement x is distance from undisturbed position
amplitude A is maximum displacement; wavelength is length of one cycle
period T is time for one cycle; frequency is cycles per second (f = 1/T)
phase locates to a point in the cycle; phase difference measures lag (degrees/radians or fractions of a cycle)
reflection, refraction and wave speed
reflection: wave bounces at a boundary
refraction: wave changes direction entering a different medium due to speed change
wave speed relations
speed c = distance / time (c = wavelength x frequency)
for EM waves in vacuum, c = 3.00 x 10^8 m/s; but c = frequency x wavelength denotes speed for that wave in its medium
longitudinal and transverse waves
characteristics and examples
transverse waves: vibrations perpendicular to energy transfer
all EM waves are transverse, water ripples, waves on a string with crests and troughs
longitudinal waves: vibrations parallel to energy transfer
e.g. sound with compressions and rarefractions
polarisation and filtering
polarisation occurs only for transverse waves
fence example: only vibrations aligned with fence orientation pass through (filtering)
practical uses
polarising filters, polaroid sunglasses reduce glare
TV and radio aerials require aligned polarisation (orientation of rods affects signal strength)
superposition and coherence
principle of superposition
when waves cross, resultant displacement equals vector sum of individual displacements
each wave continues unchanged after crossing
interference patterns
constructive interference when crests add to larger crests and troughs add to larger troughs
destructive interference when crest meets equal trough, cancelling displacements
partial cancellation occurs if amplitudes differ
phase, path difference and coherence
path difference determines interference outcome
constructive when path difference = n x wavelength (n is an integer)
destructive when path difference = (n + 1/2) x wavelength
coherence required for clear interference
coherent sources have same wavelength, frequency and a fixed phase difference
in-phase points share displacement and velocity; phase difference of 0 or multiples of 360 degrees are in phase
stationary waves
formation and energy
stationary (standing) waves arise from two progressive waves of same frequency travelling in opposite direction
result transmits no net energy; pattern oscillates in place (resonance)
nodes, antinodes and resonant conditions
nodes are points of zero amplitude; antinodes are points of maximum amplitude
resonant frequencies occur when an exact number of half-wavelengths fits the string between fixed ends
harmonics and factors affecting resonance
harmonics: first (fundamental) has one loop with nodes at ends; second harmonic has two loops and one node in the middle of the ends; 3rd loop has 3 loops, two nodes in middle of ends, etc.
factors affecting resonant frequency
greater mass per unit length -> lower frequency due to slower wave speed
lower tension (looser string) -> lower frequency due to slower wave speed
longer string -> lower resonant frequency (longer half-wavelength)
first-harmonic frequency depends on length L, tension T and mass per unit length u
frequency increases with the (T / u) and decreases with L