PAPER 4 Flashcards
What is a wave?
an oscillation that transfers energy
What are sound and water waves?
- mechanical waves
- need a medium (matter) to travel through unlike electromagnetic waves
What are longitudinal waves?
- soundwaves
- direction of vibration of individual air molecules is the same as the direction of the wave
What are transverse waves?
- direction of vibration is at right angles to the direction of travel of the wave
- if you make a transverse wave on a spring, the individual coils move up and down, but the energy is transferred horizontally
- ripples on water
What is amplitude?
- distance from the middle to the top (crest) or bottom (trough) of a wave
What is wavelength?
- distance from one point on a wave to the same point on the next wave
- metres
What is frequency?
- number of waves, or oscillations, per second
- Hz, hertz
What is the time period?
- the time for one wave to pass a given point
- seconds
apply formulae relating velocity, frequency and wavelength
wave velocity (m/s) = frequency (Hz) x wavelength (m)
Describe absorption of wave energy
Provide 3 examples
Common examples of absorption of wave energy:
- waves hitting the beach usually give most of their energy to the sand
- sunlight landing on a face is mostly absorbed, warming the skin
- sound waves hitting thick curtains give up their energy and the sound is muffled
Describe transmission of wave energy
Provide 3 examples
Common examples of transmission of wave energy:
- sea waves passing a shallow area continue with their energy mostly unchanged
- light passing through a glass window continues with over 95% of its energy
- ultrasound waves scanning a baby pass from flesh into bone and continue with enough energy for the machine to detect the echo
Describe reflection of wave energy
an echo
What is ultrasound?
- a sound of a frequency greater than 20,000 Hz
- has a very small wavelength, so it can be focused into a beam
- a transmitter beams ultrasound waves into the mother
- the waves reflect from the different boundaries
- the machine calculates the distances using time and velocity, and uses those distances to produce an image
Describe the ear
- designed to detect, amplify and convert sound into an electrical signal
- sound waves enter the ear canal and cause the eardrum to vibrate
- three small bones transmit these vibrations to the cochlea, which produces electrical signals which pass through the auditory nerve to the brain, where they are interpreted as sound.
Why does hearing (audition) change due to aging?
- hairs in cochlea have a natural frequency
- if a vibration is applied to cochlea hair at their natural frequency, they will vibrate with a very big amplitude (resonance)
- hair in the cochlea have different lengths and resonate at different frequencies of sound
- range of frequencies you can hear depend on the range of lengths of hair in your cochlea
- as you age, the hair in the cochlea get shorter, making it more difficult to hear higher frequencies
What are electromagnetic waves?
- transverse
- are transmitted through space where all have the same velocity
- transfer energy from source to absorber (some transfer information, eg. microwaves sent to satellites)
- consist of oscillating electric and magnetic fields
Outline the electromagnetic spectrum
Radio-waves Micro-waves Infrared Visible light Ultraviolet X-rays Gamma Rays
R-G = decreasing wavelength and increasing frequency
Give examples of some practical uses of electromagnetic waves
Microwaves = communication with satellites, WiFi and bluetooth
Radio waves = TV and radio stations (add sound and picture information to radio waves)
Infrared - remote controls to televisions via pulses & cooks food in an oven/grill
Micro wave - in a microwave, the water and fat in food absorb the microwaves, which heats up the outside of the food (conduction transfers energy to the middle)
Visible light - lasers in CDs, DVDs, and Blu-ray disks
Ultraviolet - helps to produce vitamin D (strong bones) & helps to detect forged bank notes
How can ultraviolet be potentially hazardous?
- can damage or kill cells
Good:
- kills bacteria in water (sterilises it)
Bad:
- can damage DNA in skin cells (cells may grow rapidly and cause skin cancer)
- ultraviolet exposure can cause cataracts, which makes corneas cloudy
How can X-rays be potentially hazardous?
- can damage or kill cells
Good:
- can kill skin cancer or other cancer cells
Bad:
- can damage your cells and cause cancer
- a radiographer operating on an X-ray machine stands behind a lead screen or in another room whilst the machine is on
How can gamma rays be potentially hazardous?
- can damage or kill cells
Good:
- can kill cancer cells
- can kill bacteria on food
Bad:
- can damage or kill the cells in your body
How are infrared waves used for medical imaging?
- A thermal imaging camera produces a thermogram (an image that shows regions of different temperatures) (thermograms can show problems with blood flow in blood vessels)
- Pixels inside a CCD (charge-coupled device - eg. A phone camera) absorb infrared and produce an image
- Colours are added by a computer
- Skin will emit more infrared radiation if it is hot due to injury or infection
How are X-rays used for medical imaging?
- Bones absorb many X-rays (soft tissues such as skin and muscle do not)
- Photographic film darkens when it absorbs X-rays and shows the details of a person’s internal structure
- A CCD (charged-coupled device) can detect X-rays
- Colours on an X-ray show differences in intensity due to different densities of the material that the X-rays have travelled through (higher density material = absorbs more X-rays)
- Computerised tomography = when a computer uses X-rays to make an image that looks like a slice through your body = produces CT scans
How are gamma rays used for medical imaging?
- used as tracers to treat problems with organs
- tracers are also used to find leaks in underground pipes (more gamma rays will be detected from a point above a leak than in the rest of the pipe)
- a doctor injects a patient with a tracer (a radioactive substance that emits gamma rays)
- patient’s organ absorbs the tracer
- the doctor diagnoses problems from the CCD images
What is the relationship between radio waves and oscillations?
radio waves can be produced by, or can themselves induce, oscillations in electrical circuits
How do different substances absorb, transmit, refract, reflect electromagnetic waves?
different substances may absorb, transmit, refract, or reflect electromagnetic waves in ways that vary with wavelength
Why are electromagnetic waves refracted?
- when an electromagnetic wave goes from air to a denser medium at an angle, it slows down and bends towards the normal
- larger difference in density = larger change in direction
Why are electromagnetic waves reflected and refracted?
- earth is curved
- if you want to send radio waves over very long distances, you reflect them from the ionosphere (a layer of the atmosphere)
- high-frequency radio waves, or micro waves have a smaller wave length so you can use them for satellite communication
Why are electromagnetic waves transmitted and absorbed?
- depends on wavelength
Describe a convex lens
- magnifying
- refracts rays to a principal focus or focal point
- focal length = the distance from the optical centre of the lens to the focal point, when the rays going into the lens are parallel
Describe a concave lens
- spreads light out
- cannot set fire to anything
- fixes short-sightedness
How do you construct a ray diagram?
- shows what happens when electromagnetic waves hit a surface or travel through matter
- Draw lines to represent the rays
- Draw a normal at 90o to the surface at the point where the ray hits it
- Measure the angles from the normal to the rays
explain how colour is related to differential absorption, transmission and reflection
- each frequency of light travels at a slightly different speed in glass
- each frequency of light is refracted by a different amount
- colours with a higher frequency are refracted at greater angles (this spread light out into a spectrum - dispersion)
- light can be scattered from particles (why milk appears white bc. the particles scatter all wavelengths) (ink appears black bc. particles absorb all wavelengths)
- specular reflection - regular surface reflection (mirror)
What is an isotope?
atoms of an element with different numbers of neutrons
What do unstable nuclei emit?
Radioactive decay
What is alpha radiation?
- particle
- nucleus of a helium atom
- 4 2 He (equation symbol)
- large relative mass
- +2 charge
- high ionising power
- short penetrating distance
What is beta radiation?
- particle
- a fast-moving electron
- 0 -1 e (equation symbol)
- a neutron can decay to make a proton and an electron
- small relative mass
- -1 charge
- medium ionising power
- medium penetrating power
What is gamma radiation?
- electromagnetic wave
- no equation symbol
- no relative mass
- no charge
- low ionising power
- long penetrating power
What is neutron radiation?
- a particle in the nucleus
- 1 0 n (equation symbol)
What is ionising radiation?
- the radiation emitted by a radioactive material
- can remove electrons from atoms to produce positively-charged ions
- must transfer energy to an atom to ionise it
Why do atoms emit radiation?
- Electrons can be ‘excited’ by many things - eg when passing an electric current through an atom of gas
- when electrons move from a higher to a lower energy level, they emit radiation
- an emission spectrum = shows a set of frequencies of radiation emitted by an atom when excited electrons move to lower energy levels
- frequency of radiation emitted depends on the difference in energy of the energy levels
- energy change can take place in one go or two or more
- 2 changes = emitted photons will have less energy, lower frequencies and longer wavelengths
- largest energy difference = from an energy level just below ionisation
- highest energy photons from a hydrogen atoms are in the ultraviolet part of the electromagnetic spectrum, but carbon atoms can emit X-ray photons
- Gamma rays = highest energy radiation = emitted from nuclei
- Protons and neutrons occupy energy levels in the nucleus & energy levels = much higher so radiation emitted is of a higher energy
How are electrons in atoms arranged?
- different atoms have different energy shells
- electrons usually occupy the lowest possible energy level at the smallest distance from the nucleus
- inner electrons can become ‘excited’ when they absorb energy from radiation, and rise to a higher energy level
- when this energy is lost by the electron, it is emitted as radiation
- when outer electrons are lost = ionisation
What happens when atoms absorb electromagnetic radiation?
PHOTON MODEL
- electromagnetic radiation = emitted and absorbed as packets of energy (photons)
- energy of each photon = proportional to the frequency
- only a photon of exactly the right energy can ‘excite’ an electron to a higher energy level
- when light of all frequencies is passed through hydrogen gas, some frequencies are absorbed
- an absorption spectrum = shows a set of frequencies of radiation absorbed by an atom when excited electrons move to higher energy levels
- only a photon that has enough energy can completely remove an electron from the atom = the atom is ionised - photons of ultraviolet, X-ray and gamma ray frequencies have enough energy to ionise atoms
What is a half-life?
- the time it takes for half the unstable nuclei to decay, or for the activity to halve
- atoms decay; they do not disappear (if they emit alpha or beta particles, they change to the atoms of a different element, and eventually become stable
How do you measure the activity of a radioactive material?
- random decay
- Geiger counter (measures the activity, ie count rate)
- activity = radiation (photons or particles) emitted per second
- activity = measured in becquerels (Bq)
What is irridation?
- happens when there is radioactive material outside your body but it can travel into your body
- ionising radiation can damage the DNA in your cells, which can cause cancer
- your body can repair any damage from small doses of radiation, but too much exposure increases the risk of cancer
What is contamination?
- happens when you take radioactive material inside your body or if it is on your skin
- once you are internally contaminated, you cannot remove the radioactive material from inside you
- ionising radiation can damage the DNA in your cells, which can cause cancer
- your body can repair any damage from small doses of radiation, but too much exposure increases the risk of cancer
explain why the hazards associated with radioactive material differ according to the half-life involved
- Doctors may inject a patient with a radioactive isotope that is absorbed by the organs of your body, and detected by a gamma camera.
- The camera makes images of your organs, which helps doctors to diagnose disease.
TOO SHORT
If the half-life of the isotope is too short, then the tracer will decay before they can use the gamma camera.
TOO LONG
If the half-life is too long, then the tracer will continue to emit radiation for a long time and increase the risk of cancer.
What is nuclear fission?
- “The splitting of a nucleus into 2 smaller nuclei, with the release of energy and neutrons”.
- for fission to occur the unstable nucleus must usually first absorb a neutron
- Unstable nuclei have too much energy, due to an imbalance between the number of protons to neutrons.
- Such nuclei will undergo radioactive decay.(emit α, β and/or γ radiation)
- Such unstable nuclei are naturally occurring.
- Stable nuclei can be made unstable by adding neutrons to them.
- If large nuclei are made unstable, they have so much instability, they simply break apart.(as opposed to emitting α, β and/or γ)
- In a nuclear reactor, fission is induced when a Uranium-235 nucleus is hit with a neutron.
- The two smaller nuclei (daughter nuclei) are themselves unstable, and become stable by undergoing radioactive decay(emit α, β and/or γ radiation)
- nuclear waste = very dangerous, has very long half-life & must be buried and guarded
