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) connected to electronic display
- when pulses are emitted, some partially reflect at the boundaries between soft tissue at different times
- transducer receives this information at different times and transmits the ultrasound waves to the display to build up an image of the different tissue boundaries inside the person
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
