Section 3: Waves Flashcards
What are waves?
Just vibrations. They transfer energy through a medium, but once they’ve gone through everything goes back to normal like they where never there.
What is a progressive wave?
A moving wave. It carries energy from one place to another without transferring any material.
What is a wave caused by?
It is caused by something making particles or fields (e.g. Electric or magnetic fields) oscillate at a source. These oscillations pass through the medium as the wave travels, carrying energy with it. A wave transfers this energy away from the source so the source of the wave loses energy.
What ways can you tell that waves carry energy?
- electromagnetic waves cause things to heat up
- x-rays and gamma rays knock electrons out of their orbits, causing ionisation
- loud sounds cause large oscillations of air particles which can make things vibrate
- wave power can be used to generate electricity
- waves can be reflected, refracted and diffracted
What is reflection?
The wave is bounced back when it hits a boundary
What is refraction?
The wave changes direction as it enters a different medium. The change in direction is a result of the wave slowing down or speeding up.
What is diffraction?
The wave it spreads out as it passes through a gap or round an obstacle
What is displacement and what is it measured in?
How far a point on the wave has moved from its undisturbed position.
Measured in meters
What is Amplitude and what is it measured in?
The maximum magnitude of the displacement, i.e. the distance from the undisturbed position to crest, or trough.
Measured in meters
What is wavelength and what is it measured in?
The length of one whole wave oscillation or wave cycle, i.e. The distance between two crests of a wave
Measured in meters
What is period and what is it measured in?
The time taken for one whole wave cycle
Measured in seconds
What is frequency and what is it measured in?
The number of whole wave cycles (oscillations) per second passing a given point. Or the number of whole wave cycles (oscillations) given out from a source per second
Measured in hertz
What is phase?
A measurement of the position of a certain point along the wave cycle.
What is phase difference?
The amount by which one wave lags behind another wave. It can be measured in angles or in fractions of a cycle
How are frequency and period linked together?
The number of whole wave cycles (oscillations) per second is 1/(time taken for one oscillation)
What is the equation for working out frequency?
F = 1/T
Frequency (Hz) = 1 / time in seconds
How can you find the speed of a wave?
You can you speed = distance / time
C = d / t
C = f x lambda
What’s speed do all electromagnetic waves travel through a vacuum?
3.00 x 10^8 ms^-1
How can you measure the speed of sound?
One of the easiest methods is to use two microphones in a straight line a distance D apart. The microphones should have separate inputs so the signals from each can be recorded separately. use a signal generator to produce a sound from the loudspeaker and use the computer to record the time between the first and second microphone picking up the sound. Do this by measuring the time delay between the first peak of the signal received by each microphone on a graph of voltage against time.
How can you measure the speed of a wave in water?
- set up a ripple tank
- record the depth of the water
- create ripples with a regular frequency
- dim main lights of the room and use a strobe light
- increase the frequency of the strobe light until the waves appear to be standing still. When this happens the frequency of the strobe light equals the frequency of the water waves.
- Use a ruler to measure the distance between two adjacent peaks.
What are the two different types of waves?
Transverse and longitudinal
What is a transverse wave?
In transverse waves the displacement of the particles or field is at right angles to the direction of energy propagation. All electromagnetic waves are transverse. They travel as vibrations through magnetic and electric fields - with vibrations perpendicular to the direction of energy transfer.
What are the two main ways of drawing transverse waves?
They can be shown as graphs of displacement against distance along the path of the wave. Or they can be shown as graphs of displacement against time for a point as the wave passes. Both of the graphs give a similar shape, so make sure you check the label on the x-axis.
What is a longitudinal wave?
In longitudinal waves the displacement of the particles or Fields is along the direction of energy propagation. The most common example is sound.
What does a sound wave consist of?
It consists of alternate compressions and rarefactions of the medium it’s traveling through.
What is a polarised wave?
A wave that oscillates in one direction only
What can a polarising filter be used for?
It can be used to polarise light and other waves. It only transmits vibrations in one direction. If you have two polarising filters at right angles to each other then no light will get through
What happens if the second filter isn’t at a right angle to the plane of polarisation?
It will just reduce the intensity of the light passing through it but still allows some light through
Why does Polarisation provide evidence for the nature of transverse waves?
Because it can only happen in transverse waves
What are a few ways we use polarisation in the real world?
In photography and Polaroid sunglasses. Also in improving TV and Radio signals
What is superposition and when does it happen?
Superposition occurs when two or more waves pass through each other. At the instant the waves cross, the displacements due to each wave combine. Then each wave continues on its way.
When do the waves cancel each other out?
If the displacements are equal and opposite
What is the principle of superposition?
It states that when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements
What does superposition mean?
It means one thing on top of another thing.
What is constructive interference?
When two waves meet, if the displacements are in the same direction, the displacements combine to give a bigger displacement. A crest + a crest gives a bigger crest.
What is destructive interference?
If a wave with a positive crest and a negative displacement, they will undergo destructive interference and cancel each other out. The displacement of the combined wave is found by adding the displacements of the two waves
What is total destructive interference?
If two waves with equal and opposite displacements meet, they cancel each other out completely
When are two points of a wave in phase?
If they are both and the same point in the wave cycle. When waves are superposed, the points in phase will interfere constructively with each other. Points in phase have the same displacement and velocity.
Which number represents the wave totally out of phase?
180 degrees
What is a stationary wave?
A stationary wave is the superposition of two progressive waves with the same frequency and amplitude, moving in opposite directions. Unlike progressive waves, no energy is transmitted.
How can you demonstrate a stationary wave?
Set up a driving oscillator at one end of a stretched string with the other end fixed. The wave generated by the oscillator is reflected back and forth. For most frequencies the resultant pattern is a jumble. However, if the oscillator happens to produce an exact number of waves in the time it takes for a wave to get to the end and back again, then the original and reflected waves reinforce each other. The frequencies at which this happens are called resonant frequencies and it causes a stationary wave where the overall pattern doesn’t move along it just vibrates up and down, so the string forms oscillating loops.
What are the stationary waves like in the experiment using a oscillator and string?
The stationary waves are transverse, so each particle vibrates at right angles to the string. Nodes are points on the wave where the amplitude of the vibration is zero. They just stay perfectly still antinodes are points of maximum amplitude. At resonant frequencies, an exact number of half wavelengths fits onto the string.
What happens at a node?
There is total destructive interference
What happens at an antinode?
There is constructive interference
What is the first harmonic?
This stationary wave is vibrating at its lowest possible resonant frequency, this is called the first harmonic. It has one loop with a node at each end. One half wavelength fits into the string, and so the wavelength is double the length of the string
What is the second harmonic?
It has twice the frequency of the first harmonic. There are two loops with a node in the middle and one at each end. Two half wavelengths fit on the string, so the wavelength is the length of the string
What is the third harmonic?
It is three times the frequency of the first harmonic and 1 1/2 wavelengths fit on the string
How many harmonics can you have?
You can have as many as you like. An extra loop and node are just added at each one. The wavelength goes up by 1/2 and the frequency increases by the value of the frequency of the first harmonic
How can you show stationary sound waves?
Powder in a tube. A loud speaker produces stationary waves in the glass tube. The powder laid along the bottom of the tubes is shaken away from the antinodes but left at the nodes
How can you find the speed of sound using a loud speaker and a glass tube?
The distance d between each pile of powder node is wavelength / 2 so the speed of sound is equal to c = f x 2df
