P2 Waves

Question 1

define how energy is transferred through mechanical waves

Answer 1

energy is transferred between 2 points through the vibration/oscillation of air/solid particles through which the energy travels

Question 2

describe transverse waves

Answer 2

oscillations are perpendicular to direction of energy transfer
oscillations are crests+troughs
eg. EM waves

Question 3

describe longitudinal waves

Answer 3

oscillations are parallel to direction of energy transfer
oscillations are compressions+rarefactions
eg. sound waves

Question 4

define amplitude

Answer 4

maximum displacement of a point on a wave from its undisturbed position
(horizontal line to peak)

Question 5

define wavelength

Answer 5

distance from a point on a wave to the closest equivalent point
(longitudinal: compression to compression / rarefaction to rarefaction)
(transverse: peak to peak)

Question 6

define frequency

Answer 6

number of waves passing a point per second
measured in Hz (= 1 wave per second)

Question 7

define wave period

Answer 7

time taken for a wave to complete a full cycle
(one wavelength)

Question 8

equation for wave period (wavelength)

Answer 8

period = 1 / frequency
(secs) (Hz)

Question 9

equation for wave speed

Answer 9

wave speed = frequency x wavelength
v = f x λ
(m/s) (Hz) (m)

Question 10

method of measuring speed of water waves RP (6)

Answer 10

• set up ripple tank: shallow water tank, lamp directly above+downwards, connect power supply+motor circuit, motor on wooden rod
• pour water into tank to 5mm deep, adjust rod so it’s touching water surface
• switch on electric motor+lamp, then adjust until clear+low-frequency waves can be seen on card
• find wavelength: place metre rule perpendicular to waves on card, record how many waves across 100cm, then divide length by no. waves
• find frequency: count number of waves passing a point in 10secs, then divide number by time
• find wave speed: v = f x λ

Question 11

suitability of method to measure speed of water waves RP

Answer 11

• motor ensures regular ripples are generated - increased accuracy

Question 12

method of measuring speed of waves through a solid RP (5)

Answer 12

• set up apparatus: attach string to vibration generator (connected to power supply), use a 200g hanging mass and pulley to pull the string taut. place wooden bridge under string near the pulley
• switch on vibration generator, adjust the wooden bridge until clear stationary waves can be seen
• find wavelength: measure across as many loops (half wavelengths) as possible, then divide length by no. loops to find half wavelength, double
• find frequency: equal to the frequency of the power supply
• find wave speed: v = f x λ

Question 13

suitability of method to find wave speed through a solid RP

Answer 13

• vibration generator ensures regular ripples are generated - increased accuracy

Question 14

describe EM waves (4)

Answer 14

• transverse
• transfer energy from source to an absorber
• continuous spectrum of long - short wavelength, grouped by wavelength+frequency
• they travel at same speed = 3 x 10⁸ m/s through vacuum/air

Question 15

describe frequency+energy of waves as wavelength gets shorter

Answer 15

• higher frequency
• higher energy

Question 16

name EM wave spectrum from long-short wavelength

Answer 16

radio, micro, infrared
visible light
ultraviolet, x-ray, gamma

Question 17

describe limit of EM waves detectable by humans

Answer 17

our eyes only detect visible light, so detect a limited range of electromagnetic waves

Question 18

why refraction occurs

Answer 18

when waves travel through one medium to another with different densities
wave speed changes, causing wavelength to change (as directly proportional)
-> wave fronts get closer together as speed decreases

Question 19

rule of refraction

Answer 19

-> a wave going through a less dense material will get faster + move away from the normal
-> a wave going through a more dense material will slow down + move towards the normal

Faster is Away , Slower is Towards

Question 20

effect of wavelength on refraction

Answer 20

shortest wavelengths are refracted the most (violet)

Question 21

what happens when white light is transmitted through a glass prism

Answer 21

colours making up white light have different wavelengths, so also different wave speeds
this means they are refracted at different angles, causing colours to disperse

Question 22

describe reflection/ absorption/ emission of light radiation for matt+black surfaces

Answer 22

• no visible light wavelengths reflected or transmitted
• therefore all wavelengths absorbed
• and all radiation emitted

Question 23

describe reflection/ absorption/ emission of light radiation for shiny+white surfaces

Answer 23

• all visible light wavelengths reflected
• therefore no wavelengths absorbed
• and no radiation emitted

Question 24

why do black/matt surfaces get hot

Answer 24

• black/matt surfaces are good absorbers of light radiation
• short wavelength light radiation (absorbed) transferred to long wavelength infrared radiation (emitted)
• radiation emitted causing surface to heat up

Question 25

method for investigating effect of surface on infrared absorption/emission RP

Answer 25

* Leslie cube on heat-proof mat, filled with hot water, put lid on * cube has surfaces w/ different finishes: shiny black, shiny silver, matt black, matt white * surfaces absorb heat energy from water+ emit the heat as infrared radiation * use infrared detector to measure amount of IR emitted from each side/surface -> detector must be same distance from each surface -> give time between readings for detector to settle

Question 26

describe what radio waves are made up of

Answer 26

oscillating electric/magnetic fields therefore can be produced by oscillations in electric circuits

Question 27

describe how radio waves are produced with electricity

Answer 27

* high-frequency AC supplied to a transmitter, causing electrons in the antenna to oscillate * oscillating electrons produce oscillating magnetic fields aka radio waves (with same frequency as AC) * when waves reach the receiver they are absorbed, causing electrons in the receiver to oscillate - produces a current of same frequency as the radio waves

Question 28

factors affecting how dangerous EM waves are (3)

Answer 28

* type of radiation: high frequency wave means high energy so more damage * size of dose absorbed: higher dose means higher energy so more damage * tissue which has absorbed the radiation

Question 29

danger of micro waves for humans

Answer 29

heats water molecules in body cells - can damage/kill them

Question 30

danger of infrared for humans

Answer 30

felt as heat - can cause skin damage/burns

Question 31

danger of visible light for humans

Answer 31

can damage eyes - blindness

Question 32

danger of UV for humans

Answer 32

* cause mutations to DNA - cancer * cause skin to age prematurely - increased risk of cancer

Question 33

danger of x-rays for humans

Answer 33

* cause mutations to DNA (ionising) - cell copying errors - cancer * radiation sickness - organ failure, death * damages eyes

Question 34

danger of gamma rays for humans

Answer 34

* cause mutations to DNA (ionising) - cell copying errors - cancer * radiation sickness - organ failure, death * damages eyes

Question 35

define radiation dose

Answer 35

a measure of the risk of harm from an exposure to radiation depends on: type of radiation / amount absorbed / type of tissue affected measured in milliSieverts (mSv)

Question 36

suitability of radio waves for transmitting tv/radio signals +how

Answer 36

they can travel long distances * long-wave radio signals diffract+follow curvature of the earth - national broadcasting * short-wave radio signals reflect off the ionosphere - intercontinental broadcasting range limited by horizon as waves travel in straight lines

Question 37

suitability of micro waves for transmitting signals to satellites

Answer 37

they are not strongly absorbed by earth's atmosphere as they have short wavelength, so can reach satellites in space * waves sent by a transmitter to a receiver on a satellite orbiting earth * satellite then transmits waves back to earth + picked up by receivers

Question 38

suitability of micro waves for cooking

Answer 38

* water molecules in food absorb microwave radiation * water heats up and cooks the food

Question 39

suitability of xrays for medical imaging

Answer 39

* xrays absorbed by dense body parts (bones) and transmitted through soft body tissues * scanners can create negative images of internal body parts

Question 40

suitability of gamma rays for radiotherapy/sterilisation

Answer 40

gamma rays (+ high doses of xrays) can damage/kill living cells as high energy - used to kill cancer cells or bacteria

Question 41

suitability of infrared for electric heaters/cookers and cameras

Answer 41

* infrared absorbed by surfaces of food/objects - cause heating * objects emitting infrared can be seen at night using IR cameras

Question 42

suitability of visible light for fibre-optic communications

Answer 42

* data is coded into light pulses * light travels down a long fibre by *total internal reflection* by glass

Question 43

suitability+process of UV for energy-efficient light bulbs

Answer 43

fluorescent lights absorb UV radiation and emit the energy as visible light * an electric current is used to excite the gas * when atoms de-excite they emit UV waves * the waves are absorbed by the coating on the bulb * the coating 'fluoresces' + emits visible light

Question 44

effect of changes in nuclei on EM waves

Answer 44

EM waves can be generated or absorbed over a wide frequency range eg. gamma rays originate from changes in the nuclei of atoms (radioactive decay)