Waves and Energy
Energy review
• Energy- the ability to do work
Remember- Potential and Kinetic
• Mechanical- movement
• Heat- Convection, Conduction and Radiation
• Chemical- like a battery
• Electromagnetic- electric circuit or light
• Nuclear- fission in a nucleus
• Gravitational- a ball falling
Law of conservation of ENERGY
• What is it? (It’s the same as the Law of
Conservation of Mass/Matter…)
What is a wave?
• Wave- a disturbance that moves energy from
place to place.
• Some require a MEDIUM- which is the
material a wave travels though- solid, liquid of
gas. The energy of the wave cause the
particles to move.
• What type of material would make a wave
move SLOW? What about FAST?
Do all waves need a medium?
• Waves that require a medium are called MECHANICAL
WAVES.
• Waves that DO NOT require a medium are called
ELECTROMAGNETIC WAVES.
• Waves are created when a source of energy causes a
medium to vibrate, or electric/magnetic fields are
created by charged particles
• A vibration is a back and forth or up and down motion,
also called oscillations
2 Categories of Waves
Based on HOW the move!
• A wave that moves at a right angle to how the
material moves- TRANSVERSE wave
• Waves that vibrate in a parallel directionLONGITUDINAL wave
Rest Position
• Equilibrium, natural position, place a wave
would be if nothing had disturbed it, starting
point
Parts of a wave
• Amplitude- the MAXIMUM position the
medium can become away from its REST
position (transverse)
• OR how compressed or rarefied the wave
becomes
Wavelength
• Or the distance between two similar parts of a
wave
• Transverse- crest-crest or trough to trough
• Longitudinal- compression to compression
Frequency
• The number of times a complete wave passes
a given point in one second- measured in
HERTZ (unit Hz)
Math time! Speed of waves
• Speed= wavelength x frequency
• Frequency (Hertz, Hz) = speed/wavelength
• Wavelength= speed/frequency
• Can you draw the triangle for this equation?
Practice problems!
• A wave has a wavelength of 2 m and a
frequency of 3 Hz. What is the wave’s speed?
• The speed of a wave on a rope is 50 m/s and
its wavelength is 10 m. What is the wave’s
frequency?
• The speed of a wave on a guitar string is 142
m/s and the frequency is 110 Hz. What is the
wavelength of the wave?
Doppler Effect- vroom!
• is the change in FREQUENCY of a wave based
on the observer and the source.
• Examples- sirens, trains, cars, how we
measure distances of stars…
• Compared to the initial frequency (start), the
received frequency (what you hear) is higher
during the approach, identical at the instant of
passing by, and lower after it passes.
All about Angles
• The approach of a wave to a surface- Angle of
Incidence
• Angle of a wave bouncing of a surface- Angle
of Reflection
More about Waves
•
Reflection- The bouncing back of a
wave when it can not pass through the medium
• Refraction- The bending of a wave
when it enters a new mediumchanges its speed
• Diffraction- The bending/ spreading of waves as they
move around a barrier or pass
through an opening
Interference!
• Constructive- The interference that occurs
when two waves combine to make a wave
with an AMPLITUDE LARGER than either of
the original 2 waves
• Destructive- (the opposite) interference that
occurs when 2 waves combine and make a
new wave with an AMPLITUDE SMALLER than
the originals
Types of Waves
•
•
•
•
Sound
Water
Seismic
Light
• Let’s look at them one at a time…
SOUND!!!!
• Velocity can depend on frequency and the
medium it is travelling though, can travel
though all mediums- ex 1200 ft/s in air
• So can if go through a vacuum?
• Longitudinal
• Use your EARS to sense
• The denser the material, the greater the
speed
(shorter distance between particles in the medium)
- High notes= high frequency, Low notes = low frequency
More SOUND!
• Resonance- increase in amplitude of a vibration that occurs when
external vibrations match an object’s natural frequency
• Pitch- how sound is perceived as high or low
• Intensity- amount of energy carried through a unit by means of a
wave
• Decibels-a unit used to compare the loudness of sounds (dB)
• Sonic boom- is a sound associated with a shock wave where the
object moving is travelling faster than the speed of sound, creates
an explosion like sound
Water Waves (Liquid/Currents)
• Speed can vary depending on other forces,
temperature, gravity, density
• Can be Longitudinal and Transverse.
• Transmitted in water or anything in a liquid form
• The radius of the circles decrease and the depth
increases (long. waves)
• Feel or see the force of water
• Shallow water= large amplitude
and slow frequency
Seismic Waves- EARTHQUAKES
• Velocity varies depending on the material
• 3 kinds- P waves- Longitudinal
S waves- Transverse
Surface Waves- Both
Seismic waves travel though rocks, sediment, and
whatever else crosses its path and can be felt
P waves have the highest frequency (most energy,
S- waves are slower, and surface waves are the
slowest
Electromagnetic Waves
• Transverse waves, don’t necessarily need a
medium to travel though- can travel though all
materials unless they are opaque
• The denser the material, the harder it is for
light to travel (particle block movement)
• You can see light, but also feel
• Low frequency = red light, High frequency =
Purple/Violet light
The Electromagnetic Spectrum!
EM Waves
• They are TRANSVERSE waves without a medium.
(They can travel through empty space- vacuum)
• They travel as vibrations in electrical and
magnetic fields.
• Have some magnetic and some electrical
properties to them.
• Speed of electromagnetic waves = 300,000,000
meters/second (Takes light 8 minutes to move
from the sun to earth {150 million miles} at this
speed.)
Things to know about LIGHT
Waves or Particles
• Electromagnetic radiation has properties of
waves but also can be thought of as a stream of
particles.
• Example: Light as wave or particles (photons)
• Light as a wave: Light behaves as a transverse
wave which we can filter using polarized lenses.
•
When directed at a substance light can knock
electrons off of a substance (Photoelectric
effect)
Spectrum
• The waves have been ordered in INCREASING
frequency-
Radio Waves 103 m
• A. Have the longest wavelengths and lowest
frequencies of all the electromagnetic waves.
• B. A radio picks up radio waves through an antenna
and converts it to sound waves.
• Used by cell phones
• C. Each radio station in an area broadcasts at a
different frequency. # on radio dial tells frequency.
• D. MRI (MAGNETIC RESONACE IMAGING)
– Uses Short wave radio waves with a magnet to create an
image
How a RADIO works
– AM=Amplitude modulation—waves bounce off
ionosphere can pick up stations from different
cities.
– (535kHz-1605kHz= vibrate at 535
to 1605 thousand times/second)
– FM=Frequency modulation—waves travel in a
straight line & through the ionosphere--lose
reception when you travel out of range.
– (88MHz-108MHz = vibrate at 88million to
108million times/second)
Microwaves 10-2 m
• Microwaves—have the shortest wavelengths
and the highest frequency of the radio waves.
– Used in microwave ovens.
• Waves transfer energy to the water in the food causing
them to vibrate which in turn transfers energy in the
form of heat to the food.
– Used by cell phones (sometimes)
– RADAR (Radio Detection and Ranging)
• Used to find the speed of an object by sending out
radio waves and measuring the time it takes them to
return.
Infrared 10-5 m
• Infrared= below red
• Shorter wavelength and higher frequency than
microwaves.
• You can feel the longest ones as warmth on your
skin
• Heat lamps give off infrared waves.
• Warm objects give off more heat energy than
cool objects.
• Thermogram—a picture that shows regions of
different temperatures in the body.
Temperatures are calculated by the amount of
infrared radiation given off. Therefore people
give off infrared rays.
Visible .5 X 10-6 m
• Shorter wavelength and higher frequency than infrared
rays.
• Electromagnetic waves we can see.
• Longest wavelength= red light
• Shortest wavelength= violet (purple) light
• When light enters a new medium it bends (refracts).
Each wavelength bends a different amount allowing
white light to separate into it’s various colors ROYGBIV.
• Sonoluminescence- when a bubble is popped in a
liquid (by sound) and emits light- form of cavitation
• Bioluminescence- when an organism can make and
emit light
Ultraviolet 10-8 m
• Shorter wavelength and higher frequency than
visible light
• Carry more energy than visible light
• Used to kill bacteria. (Sterilization of equipment)
• Causes your skin to produce vitamin D (good for
teeth and bones)
• Used to treat jaundice ( in some new born babies.
• Too much can cause skin cancer.
• Use sun block to protect against (UV rays)
X-ray 10-11 m
•
•
•
•
Shorter wavelength and higher frequency than UV-rays
Carry a great amount of energy
Can penetrate most matter.
Bones and teeth absorb x-rays. (The light part of an x-ray
image indicates a place where the x-ray was absorbed)
• Used to examine for broken bones or foreign objects
• Too much exposure can cause cancer
– (lead vest at dentist protects organs from unnecessary
exposure)
• Used by engineers to check for tiny cracks in structures.
– The rays pass through the cracks and the cracks appear dark on
film.
Gamma 10-12 m
• Shorter wavelength and higher frequency than
X-rays
• Carry the greatest amount of energy and
penetrate the most.
• Used in radiation treatment to kill cancer cells
• Can be very harmful if not used correctly.
- What is the relationship between frequency and
wavelength?
- What is meant by ‘spectrum’?
- What does ROY G BIV mean?
- Which color is more energetic, red or yellow?
- Which type of wave travels faster, gamma or radio?
- Why are microwaves more dangerous than radio
waves?
- If I were in a car accident, which section of the EM
spectrum would be the most useful and WHY?