Physics waves

Definition of a wave

something that transfers energy without transferring matter

What is the difference between mechanical and electromagnetic waves?

Mechanical waves need a medium to travel through whereas electromagnetic waves don’t

Name some examples of mechanical waves

sound, water, seismic

What is a transverse wave?

A wave where the oscillations are perpendicular to the direction of energy transfer

Give examples of transverse waves

water, EM waves, vibrations on a string, seismic s-wave

What is a longitudinal wave?

A wave where the oscillations are parallel to the direction of energy transfer. The oscillations alternate between compressions and rarefactions.

Give examples of longitudinal waves

sound, seismic p-waves

Draw a labelled transverse wave

Draw a labelled longitudinal wave

What is the line through the middle of a transverse wave called?

The undisturbed position

Are mechanical waves transverse, longitudinal or both?

both

Are electromagnetic waves transverse, longitudinal or both?

transverse

Amplitude

the maximum displacement of a point on a wave from its undisturbed position.

The bigger the amplitude the more / less energy the wave carries?

more

Wavelength

The distance from a point on the wave to the equivalent point on the adjacent wave.

Frequency (f)

The number of waves passing a point per second

What is the unit for frequency?

Hertz, Hz

Period (T)

The time taken for a wave to pass a point

What is the equation that links frequency and time period?

T=1/f or f=1/T

Draw a labelled displacement-time graph

Wave speed

Distance travelled by the wave per second. Measured in m/s.

What is the equation linking frequency, wave speed and wave length?

wave speed = frequency x wave length

What are the symbols for wave speed frequency and wave length?

How do you measure wave speed using a ripple tank? (PAG)

• Increase strobe frequency until the waves appear to ‘freeze’ in place

• Frequency of the wave = frequency of the strobe

• Measure the wavelength (between two adjacent ripples) with a ruler

• Use the equation wave speed = wave length x frequency

How do you measure the wave speed of sound?

• Stand far away from a wall and clap

• Measure the time taken for you to hear the echo of a sharp clap

• Measure the distance between you and the wall and then multiply it by 2

• Wave speed = 2 x distance travelled / time heard between clap and echo

What is the speed of sound in air?

Ranges from 330 - 340 m/s

What is the speed of sound in a vacuum and why?

0 m/s because sound is a mechanical wave meaning it needs to travel through a medium therefore it can’t travel in a vacuum because there is no medium.

How do you measure a wave speed in water?

• Measure out a known distance using a metre ruler

• Time how long it takes for a wave to travel that distance using a stopwatch

• Use the equation speed = distance / time

What type of waves are sound waves?

Longitudinal and mechanical

What do the amplitude and frequency link to in soundwaves?

Amplitude = loudness

Frequency = pitch

Does sound travel faster in solids liquids or gases? Why?

Solids because their particles are closer together allowing the mechanical vibrations to transfer energy more efficiently.

What changes / stays the same when sound waves travel through different mediums?

The wave length and wave speed change.

The frequency stays the same

What can reflecting sound be used for?

Echo ranging (mapping sea bed from a boat), ultrasound

What frequency is infrared?

0-20Hz

What frequency can humans hear?

20-20000Hz

What frequency is ultrasound?

20000< Hz

How do humans hear things?

1. Sound waves cause the eardrum to vibrate

2. This causes small bones in the ear to vibrate which amplifies the vibrations

3. The vibrations are transmitted into the inner ear which is filled with fluid

4. Vibrations of the fluid are detected by cilia (small hairs)

5. Different hairs detect different frequencies and pass signals to the brain

What type of wave is ultrasound?

mechanical and longitudinal

How can ultrasound waves be used to build up an image?

• A transducer produces and detects pulses of ultrasound waves

• Each ultrasound wave pulse from the transducer is partially reflected and they return to the transducer at different times

• The further away from the boundary the transducer is, the longer a wave takes to return

• These returning waves can be used to build up an image on the screen

What are some uses and advantages of ultrasound?

• Used in medicine for prenatal scans of a fetus - non ionising

• Used to image organs, ligaments or muscles - non ionising

• Used in industry to investigate flaws in objects - non destructive

• Used in echo sounding to work out distance between ocean surface and sea floor

What are the layers of the earth?

• crust - solid layer of rock around the earth (about 50km thick)

• mantle - solid layer of rock

• outer core - molten due to high temps (only liquid layer) - slightly less pressure

• inner core - solid due to high pressure

Seismic waves

Waves produced by an earthquake

Seismology

study of earthquakes

What are earthquakes recorded by?

seismometer

Where do earthquakes happen?

inside the crust

Focus

The point where the earthquake originates from

Epicentre

The point on the surface nearest to the focus

What is a primary / P-wave?

The initial tremors which last about 1 minute - longitudinal waves

They are faster than S-waves

What is a secondary / S-wave?

Second tremors a few minutes later - transverse waves

They travel slower than P-waves and can’t travel through liquid

What do you need to remember for drawing an S-wave shadow zone?

S-waves can’t travel through the liquid outer core as they are transverse waves?

How do you draw a shadow zone for P-waves?

P-waves are strongly refracted at each boundary, creating a shadow region due to difference in path between passing through the core and not passing through thr core.

What are the uses of radiowaves and why are they useful?

Used for communication, radio and TV.

They are useful because they travel through earth’s atmosphere

What are the uses and dangers of microwaves?

Used or satellite communication and microwave ovens

They are dangerous because they heat water molecules easily.

What are the uses and dangers of infrared?

Used for thermal imaging, short range communication and heating.

Excessive exposure can damage your skin/ eyes.

What are the uses and dangers of visible light?

Used for illumination and fibre optic communication

Can damage eyes

What are the uses and dangers of ultraviolet?

Used for sterilisation, forensics and tanning beds

It can cause sun burn, skin cancer and eye damage

What are the uses and dangers of x-ray?

Used for medical x-rays, airport security and dentistry

It can cause DNA mutations which lead to cancer

What are the uses and dangers of Gamma rays?

Used to kill cancer cells, as a medical tracer or for sterilisation

Can cause DNA mutations which lead to cancer

Which of the EM waves are ionising?

UV, X-ray, Gamma

Gamma has the shortest/longest wavelength and the lowest/highest frequency.

shortest wavelength highest frequency

What are X-rays used for in a medical setting?

diagnosis and therapy

How does an x-ray work?

• x rays pass through soft tissues such as skin/muscle/organs but they are absorbed by hard tissue (bones/teeth)

• x rays detected at the photographic plate are used to build up an image

How do radiographers protect themselves from the ionising rays?

• wearing a lead apron/vest

• leaving the room to increase distance

• having a lead lining around the x-ray tube

How can you use x-rays to see soft tissues?

Patients can be given a contrast medium e.g. barium meal. The barium compound is a good absorber of x-rays.

How does x-ray therapy work?

• X-rays are ionising therefore they can be used to target cancerous cells in the aims of destroying them

• we can use a very narrow beam to try and target only cancerous cells and thick plates of lead between x-ray tube and healthy tissue

• x-rays for therapy are shorter in wave length and higher in energy (more energy) than x-rays used for imaging

Dose

the amount of ionising radiation a patient is exposed to - measured in Sv

What things does the doctor need to consider when doing x-rays?

• benefits vs. risk

• the smallest dose while still getting the procedure done effectively

What does the radiation dose depend on?

• the type of radiation used

• how long the body is exposed to it

• the energy per second absorbed by the body from radiation