topic 6 waves

what do waves do

transfer energy in the direction they are travelling

• they can all be absorbed, transmitted or reflected

• particles vibrate to transfer energy when waves pass through a medium

transverse waves

amplitude definition

height from rest position

wavelength definition

distance of point on 1 wave to same point on next wave

what are transverse waves perpendicular to

energy transfer

transverse waves example

The ripples on a water surface are an example of a transverse wave.

longditudinal waves

what is longitudinal waves direction of energy transfer

parallel vibration to direction of energy vibration

example of longitudinal waves

sound waves in air and seismic shock waves

what is rarefaction

region where the particles of the medium are spread furthest apart

compression definition

where the particles are packed closely together

longitudinal wavelength definition

gap between 2 rarefactions or compressions

what is wave frequency

the number of waves passing a point each second in Hz

what is the wave speed

the speed at which the energy is transferred (or the wave moves) through the medium in SECONDS

method to measure speed of sound waves in air

a person fires a starting pistol and raises their hand in the air at the same time.

A distant observer stood 400 metres away records the time between seeing the action (the light reaches the time keeper immediately) and hearing the sound (which takes more time to cover the same distance) - shows light travels faster than sound

The speed of sound can be calculated using the equation: speed = distance/time

flaws of this method

humans do not use stop clocks identically to one another/human error

The values recorded will be dependent on the reaction time of the observer, and will not be entirely accurate

relationship between frequency and wavelength

inversely proportional

• freq double = wavelength halves

Required practical - measuring waves in a ripple tank

1. Set up the ripple tank as shown in the diagram with about 5 cm depth of water.

2. Adjust the height of the wooden rod so that it just touches the surface of the water.

3. Switch on the lamp and motor and adjust until low frequency waves can be clearly observed.

4. Measure the length of a number of waves then divide by the number of waves to record wavelength. It may be more practical to take a photograph of the card with the ruler and take measurements from the still picture.

5. Count the number of waves passing a point in ten seconds then divide by ten to record frequency.

6. Calculate the speed of the waves using: wave speed = frequency × wavelength.

hazards from measuring waves in tank

electrical components near water risk of shock or damage to components

ensure to secure electrical components before adding water taking care not to splash

measuring waves in a solid

1. Set up a string attached to a vibration generator.

2. Pass the string over a pulley and attach a hanging mass to keep the string tight.

3. Measure the length of the vibrating section of the string with a metre ruler.

4. Turn on the signal generator.

5. Change the frequency slowly until a clear stationary wave is seen.

6. Count the number of loops in the wave.

7. Record the frequency shown on the signal generator.

8. Repeat for different frequencies.

9. Calculate the wavelength using:

10. Calculate the wave speed using:

Control Variables

• length of string

• type/thickness of string

• tension in the string (same hanging mass)

• amplitude / power of vibration

• same apparatus setup

what happens down the electromagnetic spectrum

wave length decreases and freq increases

what type of waves are electromagnetic waves and what do they transfer energy from and to

transverse waves

from source to absorber

what do EM waves all have same in vacuum or through air

speed

which light can humans see

visible light

what is the speed of light

3 × 10^8

uses of radiowaves

radio communication

bluetooth

TV

Why: Long wavelength → can travel long distances and around obstacles easily. Can carry information over large areas.

uses of microwaves

Why: Can pass through the atmosphere for satellites. At certain frequencies, water molecules absorb energy efficiently → heats food.

satellites

microwave ovens

uses of infra red

monitor temperatures

thermal cameras

Why: Long wavelength → can travel long distances and around obstacles easily. Can carry information over large areas.

uses of visible light

Why: Short wavelength → can be guided through thin fibres with very low loss. Carries information as pulses of light.

fibre optics - carry data

uses of ultra violet

Why: High energy → excites electrons in materials (makes lamps glow) and stimulates skin to produce vitamin D/tan.

sun tan

invisible pens

fluorescent lights

uses of xrays

Why: Very high energy → can penetrate soft tissue (X-rays) for imaging and destroy cancer cells (gamma rays) in treatments.

medicine in radiography

uses of gamma rays

Why: Very high energy → can penetrate soft tissue (X-rays) for imaging and destroy cancer cells (gamma rays) in treatments.

medical tracers

what is radiation risk measured in

sieverts

why are high freq waves dangerous n examples

they transfer lots of energy so can be harmful - UV XRAYS AND GAMMA

why r low freq waves not so dangerous

waves like radio waves do not transfer much energy and pass soft tissue without absorption

what does uv do to skin

damages surface cells, leads to sunburn and increases skin cancer chance

relation between velocity, frequency and wavelength, in transmission of sound waves from one medium to another

• When a sound wave moves from one medium (like air) to another (like water), its speed changes.

• The frequency of the sound stays the same, but because speed changes, the wavelength changes.

These three - speed, frequency, and wavelength—are related by the equation:

speed = freq x wavelength

So if you know any two of these variables, you can find the third.

why does speed change but frequency stays same in different mediums

Because frequency is determined by the source of the sound, not the medium

what can happen to waves at the boundary between 2 different materials

Waves can be reflected at the boundary between two different materials

or can be absorbed or transmitted at the boundary between two different materials

using an oscilloscope to measure the speed of sound

1. set up the oscilloscope so the detected waves at each microphone r shown as separate waves

2. start w both microphones next to the speaker then slowly move 1 away until the 2 waves are aligned on the display, but have moved exactly one wavlength apart

3. measure distacne between microphones to find 1 wavelength

4. then calculate the speed of the sound waves passing through air

measuring the speed of water ripples using a lamp

using a signal generator attached to the dipper of ripple tank, u can create water waves at a set frquency

1. dim the lights and turn on the lamp - you’ll see a wave patternmade by the shadows of the wave crests on the screen below the tank

2. the distance between each shadow line is equal to 1 wavelength, measure the distance between shadow lines that r 10 wavelengths apart then divide by 10

3. calc speed of waves

4. this method allows u to measure the wavelength without disturbing the waves

investigating waves on string

1. set up the equipment shown below, then turn on the signal generator and vibration transducer. the string will then vibrate

2. u can adjust the frequency setting on the signal generator to change the length of the wave created on the string, u should keep adjusting the freq until there is a clear wave travelling through the string

3. u need to measure the wavelength of the wave, u should measure the length of all the half-wavelengths to get mean half-wavelength then double

4. freq of wave is whatever the signal generator is set to

5. find speed w the formula