single content Flashcards
What is an electric field?
The region where a charged object would experience a force
How is an electric field represented?
Using field lines
What does the direction of a field line show?
The direction of the force that would act on a small positive charge at that point
What does the spacing of field lines show?
The strength of the force.
The closer together the lines, the stronger the force
What is a uniform field?
A field for a force that has a constant size
What direction do field lines point when an object is positively charged?
Field lines point outwards because like charges repel
What direction do field lines point when an object is negatively charged?
Field lines point inwards because unlike charges attract
What happens to field lines as you move away from the charged object?
Field lines spread out as you move away, so the force becomes weaker
How does a ‘Van de Graff’ generator work?
A charge builds up on the dome due to electrons being rubbed off by the belt. If charge is big enough then the voltage comes high enough to ionise the air molecules so electrons jump down to earth = electric current
Dangers with static electricity when refuelling?
Fuel gains electrons from pipe so pipe is positive and fuel negative. Resulting voltage may cause a spark.
Solutions for static electricity when refuelling?
Earth the fuel tank with a copper rod or connect the tanker to a place by a copper conductor
How does paint spraying work?
Spray gun is charged and so are the paint particles, particles repel each other giving a fine spray. Object is charged opposite to paint so object attracts paint = even coat
What is an electrostatic dust precipitator?
A mechanism that remove smoke, dust particles etc. from chimneys
How do electrostatic dust precipitators work?
Dust particles gain charge as they pass through a grid
Dust particles are attracted to plates
Dust falls down the chimney when the particles are heavy enough or the plates are knocked
What does the pressure of a gas depend on?
mass of molecules
speed of molecules - hitting walls more often
how many molecules - more molecules to hit the walls
What happens when the volume of a gas decreases while the temperature stays the same?
The average speed stays the same but the molecules hit the walls more often
What does Boyles law state?
The pressure of a fixed mass of gas at a constant temperature is inversely proportional to its volume
pV=constant
pV=constant or…
p1V1=p2V2
How is gamma used for tracers in medicine?
Gamma radiation should be emitted from the source injected into the patient, so it can be detected outside the body
Why isn’t alpha used for tracers in medicine?
Would be too dangerous as the ionisation it causes could mutate cells and cause cancer
How is beta used for leak detection in pipes?
The radioactive isotope is injected into the pipe. The outside of the pipe is checked with a Geiger-Muller detector to find areas of high radioactivity - where the pipe is leaking
Why is beta used for leak detection in pipes?
It has a short half life so the material doesn’t become a long term problem. It has to be detected through the metal as well as the earth
Why is gamma used in radiotherapy?
Gamma kills cancer cells
Why is gamma used in food irradiation?
Because gamma keeps it fresh for longer as it kills bacteria or fungi
What happens when there are more nuclei in a sample in terms of half life?
When there are more nuclei, more are likely to decay in one half life, so the higher the count rate will be
What is the half life of a radioactive isotope?
• The average time it takes for the number of nuclei of the isotope in a sample to half
or
• The time it takes for the count rate from a sample containing the isotope to fall to half its initial level
What is the net decline of a radioactive sample?
The fraction of nuclei that have decayed in a certain time
How is the net decline of a radioactive sample portrayed?
Fraction, ratio, percentage
doesn’t have a unit
What is the half life required when sterilising medical instruments?
Long - years
What is the half life required when using medical tracers?
Short - days
What is the half life required when using a thickness monitor?
Long - years
What is the half life required when using industrial tracers?
Short - days
What is the half life required when using a smoke alarm?
Long - years
When does a nucleus become unstable?
When it possesses either too many or too few neutrons compared to the number of protons
What is radiation detected with ?
A Geiger counter
Define half life using the idea of activity
The time it takes for activity to half from its original value
Define half life using the idea of count rate
The time it takes for count rate to half from its original value
Define half life using the idea of undecayed atoms left in the source
The time it take for half of the nuclei present to decay
How is nuclear radiation used in medicine for imaging?
- radioactive tracers - imaging flows
* gamma cameras - imaging internal organs
What do radioactive tracers image?
Flows inside the body
What do gamma cameras image?
Internal organs
Why is imaging using radioactive radiation important?
To help with diagnosis
Why is nuclear radiation used in medicine?
For treatment; to destroy cancer cells
What processes using nuclear radiation help with diagnosis?
- radioactive tracers
* gamma cameras
What processes using nuclear radiation help with treatment?
- gamma radiotherapy
* radioactive implants
What are radioactive tracers used for?
To trace the flow of a substance through an organ
How do radioactive tracers work?
- a small amount of radioactive material is put into the patients body
- tracer is given time to move through body
- radioactive detector is positioned to detect radiation outside the body
- multiple images are taken to show progress of tracer over time
How is radioactive material put into a patients body to be used as a tracer?
By ingestion or injection
What type of radiation is used in medical tracing?
Gamma
Why is gamma used in medical tracing?
It is the least ionising and the most penetrating, meaning it can leave the body eventually
Why isn’t alpha used in medical tracing?
It can’t be detected outside the body and is very ionising so would damage cells as it moved through the bloody
What are the properties of gamma that ensure medical tracing is not harmful to the patient?
Gamma is not ionising, has a short half life and is not toxic
What does a gamma camera do?
Images internal organs
How does a gamma camera work?
- a radioactive isotope (gamma emitting) is objected into the patient
- the isotope concentrates in the organ
- a gamma camera is positioned over the area emitting gamma
What are gamma cameras made from?
A crystal scintillator (sodium iodide)
What happens when gamma hits a crystal scintillator?
It produces a burst of light
What do photo multiplier tubes do?
Convert energy in electrical signals - which are fed to a computer to produce an image
What does the lead grid do in a gamma camera?
Acts as a collinator - only allows gamma aligned with the holes to hit the crystal, making image sharper
What is a collinator used for in a gamma camera?
To make the image sharper, less blurred
What is the process happening inside a gamma camera to produce an image?
- gamma rays hit a crystal scintillator and crystal produces a burst of light
- bursts of light are picked up by a photo multiplier which convert energy to electrical signals
- the electrical signals are sent to a computer to produce an image
What can ionising radiation do to cells?
Low doses - damage cells
High doses - destroy cells
This can lead to cancer
How are cancer cells destroyed in a tumour?
Gamma radiation in narrow beams
Where is radiation emitted from in radiotherapy?
A radioactive isotope of cobalt
What is the half live of the radioactive isotope of cobalt?
Five years
Why are narrow beams used in radiotherapy?
So that no tissue before the area that is being treated becomes damaged
How do narrow beams of gamma destroy a tumour?
Lots of beams are shone from different angles to overlap on the problem area
Why is gamma used in radiotherapy?
It can damage deep inside the body - it is very penetrating - and can escape the body easily
Why is a half life of five years suitable for gamma radiotherapy?
It lasts for long enough so it doesn’t need to be replaced and the dose each patient receives doesn’t decrease
What are gamma knives used for?
To accurately focus many beams of gamma radiation on one or more brain tumours
What are the beams of radiation in gamma knives life?
- very low intensity
* approximately 200 beams - concentrated on a small volume very accurately
What are radioactive implants used for?
To attack the tumour from inside the body
How do radioactive implants work?
The sources of radiation are placed directly inside the tumour or nearly tissue
How long can radioactive implants stay in for?
Hours or days due to low dose rate sources
What type of radiation can radioactive implants emit?
Beta because they are more local to the tumour and are more ionising than gamma
What is a suitable half life for a radioactive implant?
A few weeks:
- enough time for radiation dose to be low for an extended period
- not long enough to stay active in patient after it’s needed
Advantages of radioactive implants?
- therapy over longer period compared to external beam therapy
- radiation more localised - especially with beta
- patient can spend less time in hospital
Disadvantages of radioactive implants?
- small risk of exposure of radiation to family and friends
* only early stage diseases and small tumours can be treated
What is nuclear fission?
The splitting of a large nucleus into two smaller nuclei (‘daughters’) while releasing energy
Which nuclei are split in fission reactions?
Usually uranium-235 but also plutonium-239
What is induced fission?
When a reaction occurs due to a neutron being fired at a uranium nucleus
What do fission reactions release?
- two or three neutrons at high speeds (these are used in further reactions)
- energy in the form of radiation and kinetic energy of the fission neutrons
What form of energy is released in fission?
- radiation
* kinetic - of the neutrons and the fragment nuclei
What happens in a chain reaction?
Each reaction causes more reactions which cause more reactions etc.
What is the energy produced in fission used for?
Heats water and turns it into steam -> turns turbine -> turns generator transforms kinetic to electrical energy
How is the output of a nuclear fission power station made stable?
The energy released must be constant and not grow rapidly like in an uncontrolled chain reaction
In a nuclear reactor, what is the function of the control rods?
To absorb surplus neutrons to keep chain reaction under control
In a nuclear reactor, what is the function of the moderator?
To slow the fission neutrons down
Why is water used as a moderator in a nuclear reactor?
Fission neutrons are slowed down by collisions with the atoms in the water molecules
What is a fission neutron?
A neutron released when a nucleus undergoes nuclear fission
In a nuclear reactor, what is the function of the coolant?
Water - transfers heat so radioactive water is not turned to steam
In a nuclear reactor, why is the reactor core in a thick steel vessel surrounded by concrete walls?
To withstand high temperature and pressure. Walls absorb gamma radiation that escapes steel vessel
In a nuclear reactor, why does the coolant circulate through sealed pipes to and from a heat exchanger?
So contaminated water is not turned to steam and enters the atmosphere
In a nuclear reactor, why may the control rods need to be inserted completely into the reactor core?
So there isn’t too much energy being released from uncontrolled fission
What is nuclear fusion?
When two small nuclei collide and fuse to form a larger nucleus, releasing energy
What process powers the sun and the stars?
Nuclear fusion
How is the gas in a fusion reactor heated?
By passing an electric current through it
What happens when the gas in a fusion reactor gets very hot?
It forms a plasma of small nuclei
How is the plasma contained in a fusion reactor?
Using a magnetic field to prevent it from touching the container walls
What is one of the products when hydrogen nuclei fuse?
Helium
What happens in a fusion reactor is the plasma touches the sides of the container?
The plasma would cool down, and fusion would stop
In fusion, what do nuclei need to fuse?
Enough kinetic energy
In fusion, what happens if nuclei do not have enough kinetic energy to fuse?
They will repel each other and not fuse
Advantages of nuclear fission?
- concentrated source of energy
- no polluting gases - energy is released from nuclear reactions not combustion
- supply of uranium for many years
- reliable - 24/7 (doesn’t depend on weather like renewables)
Disadvantages of nuclear fission?
- produces radioactive waste
- chain reaction must be controlled
- non-renewable
Advantages of nuclear fusion?
- produces very little radioactive waste - safer
- no chain reaction to control - safer
- even more concentrated energy
- fuel readily available (hydrogen in water)
- renewable
- hydrogen is cheap
Disadvantages of nuclear fusion?
• not yet viable - fusion on earth needs more energy input than output (need 5 million amp current!)
Which is a more concentrated source of energy: fission or fusion?
Fusion
Which type of nuclei fuse in the sun?
Hydrogen
Which type of nuclei are formed in the sun?
Helium
What equation shows Newtons 2nd law of motion?
F = m a
How is thinking distance decreased?
Friction reduced - wet roads, worn tyres or brakes
How is braking distance decreased?
Larger mass of car (more inertia means its harder to stop)
How is thinking distance shown on a graph?
Area of the rectangle underneath the graph
How is braking distance shown on a graph?
Area of the triangle underneath the graph
What are the two SUVAT equations?
v = u + at
v2 = u2 - 2as
What are the two ways of increasing the turning effect of a force?
- increase the size of the force
* increase the distance from the pivot
What is the turning effect of a force called?
A moment
What is the equation to calculate the size of a moment?
M=Fd
What are moments measured in?
Nm
What does the pressure of a liquid depend on?
The depth and the density
What is the equation for the pressure at a certain depth due to the column of liquid?
pressure = depth x field strength x density
What is the nature of liquid pressures at different places, but at the same depth?
The pressure acts in all directions and is the same depth
What is the nature of liquid pressures at different depths?
The greater depth, the higher the pressure so the water is pushed down with a greater force and has a bigger horizontal range
What is a gear?
A rotating machine with cogs that mesh with another toothed part to transmit a moment
What do gears act as?
A force multiplier
What is a force multiplier?
Machines designed to increase the size of the force
What is a distance multiplier?
Machines designed to increase the distance or speed with which something moves
What effort and load do force multipliers involve?
- small effort (input force)
* larger load (output force)
Equation for mechanical advantage?
load / effort
Example of a force multiplier?
Lever with a distance from pivot to effort 3 times greater than distance from pivot to load - mechanical advantage is 3
How is a bike an example of a distance or speed multiplier?
Slow movement of pedals produces a much faster rotation of the wheels
What effort and load do distance multipliers involve?
- small movement of effort
* large movement of load
How is energy shown to be conserved in gears?
Work done by input gear = Work done by output gear
Why can a machine not be a force and distance multiplier at the same time?
As energy is conserved, if force is doubled the distance is halved and vice versa
What are levers used to do?
Lift heavy masses with the least amount of effort
What three things can gears do when passing power from one wheel to another?
- increase speed - first gear has more teeth than second so second has to turn faster to keep up
- increase force - second gear has more teeth than first so second turns slower but with more force
- change direction - when gears mesh they always turn in the opposite direction
In a hydraulic system, a smaller force on the master (effort) piston will produce what force on the slave (load) piston?
A larger force as the pressure of the liquid is constant - acting as a force multiplier
What is the ratio of in levers?
Lengths
What is the ratio of in gears?
Number of teeth
What is the ratio of in hydraulics?
Cross sectional areas
Equation for levers?
Fb/Fa = Lb/La
Equation for gears?
Fb/Fa = Nb/Na
Equation for hydraulics?
Fb/Fa = Ab/Aa
What is the atmosphere?
A thin layer of air around the earth
What happens at a greater altitude?
- the less dense the atmosphere
* the lower the atmospheric pressure
Why is the atmospheric pressure lower at greater altitudes?
- at higher altitudes there is less air above a surface
- so there is a smaller weight of air acting on the surface
- p=F/A and F is reduced so p will be smaller
How is upthrust created?
When the bottom surface of an object is submerged in a liquid it experiences a greater pressure than the top surface
When does an object float?
When its weight = upthrust / weight of fluid displaced
What is upthrust equal to?
The weight of the liquid displaced
What does force equal in terms of the change in momentum?
Force = change in momentum / time taken
When are ultrasound waves reflected?
When they reach a boundary between two different media
How do you determine how far away a boundary is?
How long the reflections take to reach the detector
What surface is the best emitter of infrared radiation?
Matte black surfaces
What surface is the worst emitter of infrared radiation?
Silver (shiny) surfaces
Best materials to emit infrared radiation? Best to worst
Matte black, shiny black, white, silver
Which kettle will cool down faster: black or metallic?
Black because it will emit more radiation
What direction does heat energy flow in?
From a hot object to a cooler one
What do good emitters also act as?
Good absorbers
What surface is the best absorber of infrared radiation?
Matte black surfaces
What surface is the worst absorber of infrared radiation?
Silver surfaces
Best materials to absorb infrared radiation? Best to worst
Matte black, shiny black, white, silver
Why are shiny surfaces the worst absorbers?
They reflect most of the radiation away
Why do solar panels have a black outer layer?
So it can absorb heat
Why are firefighters suits shiny?
They are the worst absorbers, the the best reflectors
Why is the back of a solar panel silver?
Reflect heat back onto pipes
Why are premature babies wrapped in silver blankets?
Its the worst emitter, so reduces the amount of heat that they lose
Why are radiators in car engines black?
It’s the best emitter
What is a perfect black body?
An object that absorbs all the radiation that hits it
What does a perfect black body not do?
Reflect or transmit any radiation
What is the radiation emitted by a black body called?
Black body radiation
What is black body radiation?
The radiation emitted by a black body
What are the two major types of lens?
- concave
* convex
What can concave lenses also be known as?
Diverging lenses
What can convex lenses also be known as?
Converging lenses
When is there a principal focus?
When the light rays are parallel
In a convex lens, how does the thickness of the lens affect the focal length?
The thicker the lens, the shorter the focal length
What is the shape of a converging lens?
It is thicker at the centre than the edges
What is the shape of a diverging lens?
It is thinner at the centre than the edges
How is magnification calculated?
magnification = image height / object height
If the image is larger than the object what is the magnification equal to?
greater than 1
If the image is smaller than the object what is the magnification equal to?
less than 1
What can the image in a convex lens be?
- real or virtual
- magnified or diminished
- upright or inverted
How is the image transformed in a camera?
(convex lens)
- diminished
- inverted
- real
How is the image transformed in a projector?
(convex lens)
- magnified
- inverted
- real
How is the image transformed in a magnifying glass?
(convex lens)
- magnified
- upright
- virtual
What type of images do convex lenses give?
- virtual
- upright
- diminished
What happens in a convex lens when the object moves further away from the lens?
The image gets smaller and moves away from the lens
How is short-sightedness corrected?
The eyeball is elongated so needs a concave lens
How is long-sightedness corrected?
Convex lens
Why does white light spread out into colours after it passes through a glass pyramid?
The different wavelengths experiences different changes in speed as they enter and leave the glass so refract by different amounts
Which colours are refracted the most and the least?
- red is refracted the least
* violet is refracted the most
What are the primary colours of light?
Red, green, blue
What are the secondary colours of light?
Yellow, cyan, magenta
What does green and blue light create?
Cyan light
What does green and red light create?
Yellow light
What does green and red light create?
Magenta light
What does a filter do?
Only allows a small range of wavelengths to pass through
What does a red filter do?
Only allows red light through and absorbs blue and green light
What are opaque objects?
Those that do not transmit light
What does the colour that an objects appears depend on?
Which wavelengths are most strongly reflected
Why do objects appear white?
They reflect all of the wavelengths of visible light equally
Why does a red object look black through a blue filter?
A red object only reflects red light and the blue filter only transmits blue light, so only blue light shines on the red object and it is absorbed so it appears black
How is a light ray reflected by flat foil?
Reflected in a single direction (specular reflection)
How is a light ray reflected by crumpled foil?
Scattered (diffuse reflection)
