P12 Flashcards
Wave properties
What are transverse waves ?
Waves that oscillate perpendicular to the direction of energy transfer
What do waves transfer ?
ENERGY not matter
What is an electromagnetic wave ?
a wave that does NOT need a medium to travel through
- in a vacuum, all EM waves travel at 3 x 10^8 m/s
- ALL EM waves are transverse
What are longitudinal waves ?
Waves that oscillate parallel to the direction of energy transfer
Describe how to observe a transverse wave
- tie one end of a rope to a fixed point
- move the rope up and down
Describe how to observe a longitudinal wave
- tie one end of a slinky to a fixed spot
- move the slinky in and out
What is a mechanical wave ?
- waves that need a medium to travel through
- can be longitudinal or transverse
Describe the structure (?) of a transverse wave
- one wavelength is from peak to peak/trough to trough
- amplitude is from line of energy transfer to the highest point of a peak or lowest of a trough
- peak is the highest point of the wave
- trough is the lowest point of the wave
What is the amplitude of a wave ?
The maximum displacement from its undisturbed position
Describe the structure (?) of a longitudinal wave
- compression is when many close together
- rarefaction is when there is a gap
- one wavelength is from one compression/rarefaction to the next
Describe the relationship between energy transferred and amplitude
bigger amplitude = more energy transferred
Give the equation for frequency using time (with units)
Frequency (Hz) = 1/time period (s)
What are plane waves ?
Straight waves that move at the same speed and the same distance apart
Give the equation for wave speed (with units)
Wave speed (m/s) = frequency (Hz) x wavelength (m)
Describe the method for a practical measuring the speed of sound in air (give a flaw of this method also)
- gather equipment
[] two people (person A and person B)
[] stopwatch
[] trundle wheel (to measure distance between people)
[] cymbals
[] field or large area - person A to stand at one end of the field with the cymbals
- person B to use the trundle wheel to measure a distance of around 40-50 metres from person A
[] record the measured distance - person A bashes cymbals together
- as soon as person B SEES the cymbals clash, start the stopwatch
- as soon as person B HEARS the cymbals, stop the stopwatch
- use speed = distance/time to find the speed of sound in air
PROBLEM: relies on person B’s reaction time and eyesight
Describe the method for an investigation into the frequency, wavelength and wave speed of water waves using a ripple tank
- gather equipment
[] ripple tank on top of a stand
[] white paper
[] pencil
[] ruler
[] timer
[] phone camera
[] power pack
[] vibrating bar
[] lamp - set up ripple tank
[] white paper beneath tank
[] lamp on top of ripple tank to cast shadow
[] pencil perpendicular to paper long edge to act as a marker
[] ruler on opposite side to pencil, vertical along long paper edge, with 0cm mark where the vibrating bar touches the water
[] power pack connected to vibrating bar
[] vibrating bar at one end of the ripple tank so can dip into water and produce plane waves
[] timer near the set up - set timer for a minute
- turn on vibrating bar
- start recording and start timer, making sure the timer, the pencil and the ruler are all in frame
- stop recording after a minute
- to find frequency, play back recording
[] count number of wavefronts passing the pencil tip in 10 seconds
[] divide this number by 10 to get a more accurate frequency in Hz - to find wavelength, pause the video at any time and measure the distance using the image of the ruler between 10 wavefronts
[] divide this length by 10 and convert to metres - to find wave speed, use wave speed = frequency x wavelength equation
Describe the method for an investigation into the wavelength, wave speed and frequency of waves through a solid
- gather equipment
[] pulley
[] mass (eg. 100g)
[] string
[] signal/frequency generator
[] vibrator
[] metre rule - set up
[] pulley with string through it attached to mass hanging off edge of surface
[] masses used to keep string taught
[] vibrator connected to string and the frequency generator - turn on the generator and adjust frequency until there is one loop of a standing wave in the string
[] this is HALF of a whole wave
[] use a metre rule to measure the length of the half-wave end to end and multiply by two to get the full wavelength - record frequency at this point and the length of one full wave in metres
- adjust frequency on generator again until there are two loops on the string as standing waves
[] measure the length of both loops as this is one wavelength - again record the frequency and wavelength
- repeat for 3 loops
[] to calculate one full wavelength, divide the total length of all 3 by 3 and then multiply by two - repeat until 5 loops
- calculate the wave speed at each stage using the recordings of frequency and wavelength at each stage
What does the wave speed of waves through a string depend on ?
taughtness of string and length of string
- NOT frequency or wavelength
What is reflection ?
when incident waves are rebounded from a surface
- the angle of incidence = the angle of reflection
What is refraction ?
change in SPEED and DIRECTION (not frequency) of travelling waves when crossing the boundary of one medium to another, both with different densities
What can happen when a wave is directed at a substance ?
- refraction
- transmission
- absorption
- reflection
What type of wave is a sound wave, and how is one produced ?
- mechanical, longitudinal
- vibrations
Describe a method for an investigation for the speed of sound in air using a wall
- gather equipment
[] cymbals
[] trundle wheel
[] BARE wall
[] stopwatch
[] person A and person B - person A to stand at a measured distance (using trundle wheel) from wall, facing it
- person A bangs cymbals together
- person B starts stopwatch when SEES person A bang cymbals
- when person A hears the echo if the cymbals, person B to stop the stopwatch
- use speed = (2 x distance)/time to find speed of sound in air
What makes the pitch of a sound higher ?
more frequency
What makes the volume of a sound louder ?
increased amplitude of waves
How can you investigate the effect of increased amplitude of sound waves ?
tuning fork + oscilloscope
What is the hearing range of the human ear in Hz ?
20 Hz - 20 000 Hz
Why is the frequency range of the human ear limited ?
- sound waves converted to vibrations when passing through a solid
- this only works over a limited frequency range
What is echo sounding ?
- boats have transmitters on their bottoms
- transmitter emits pulses of high frequency sound waves that travel towards the ocean/sea floor
- when waves are reflected back up, detected by a receiver
- formula distance = 1/2x velocity x time used to determine distance to sea/ocean floor in metres
What are ultrasound waves ?
Sound waves with a frequency above 20 000 Hz/20 kHz
Give two uses of ultrasound
- ultrasound (pre-natal) scans
- industrial imaging
What is the benefit of ultrasound scanning over x-rays ?
- is NON IONISING, so has no health risk compared to x-rays
- ultrasound reflects at boundaries between tissue, so can be used to scan soft tissue and organs
Describe how an ultrasound scan is taken
- ultrasound scanner = transducer (emits + detects pulses of ultrasound)
Describe how ultrasound is used in industrial imaging to detect flaws or cracks in metal
- transducer connected to oscilloscope placed on metal surface
- transducer detects reflected ultrasound waves
- if there is a flaw etc., there will be a difference in time between the return of waves reflected at the boundary and below the boundary
[] this shows up on the oscilloscope trace
Give the equation for the depth of a boundary
depth (m) = 1/2 x wave speed (m/s) x time (s)
Give the equation for distance travelled by a wave
distance (m) = speed (m/s) x time (s)
What are seismic waves ?
- waves produced during an earthquake
[] travel outwards from the epicentre (where an earthquake originates) - two types: P-waves and S-waves (primary and secondary)
- can travel through the Earth
What is a P-wave ?
- primary wave
- longitudinal wave
- cause initial tremors in an earthquake lasting about a minute
- can travel through the liquid outer core and are refracted at the boundary twice (when entering and leaving)
[] the second refraction is further around, so can’t reach the shadow zone of the Earth
What is an S-wave ?
- secondary wave
- transverse wave
- causes tremors a few minutes after the P-waves
- travel more slowly than P-waves and shake from side to side
- can’t travel through liquid outer core
- cannot be detected in the shadow zone
What are L-waves ?
- arrive last in an earthquake
- travel more slowly than either P or S waves
- cause violent tremors up and down in the Earth’s crust
List the three types of seismic waves from least violent to most violent
- P waves
- S waves
- L waves
What is the shadow zone ?
An area of the Earth from ~105-142 degrees where seismographs only are able to detect longer seismic waves (hence only weak P-waves, no normal P or S waves)
What does the existence of the shadow zone prove and why ?
- that Earth has a liquid outer core beneath the mantle
- BECAUSE:
[] P waves are refracted twice at the boundary between mantle and outer core, and the change in direction means they can’t reach the shadow zone - S waves can’t travel through liquid, and so don’t reach the shadow zone at all, stopped by the liquid outer core
Why do the directions of P and S waves change when travelling through the Earth ?
- bend as travel
- speed changes gradually with depth and so direction changes gradually with depth also
