paper 3B tuning points Flashcards
cathode ray is a beam of…
electrons
a discharge tube is a glass tube containing a ______ and ______.
the gass in a discharge tube is at _____ pressure.
anode and cathode
low
what are the 4 propities of cathode rays that were demostrated by J.J.thomson
1.) they have enegy mass and momentum
2.) they have negetive charge
3.) same propity no matter what gas is used or what the cathode ray is made of
4.) have a very high charge to mass ratio (specific charge) - much higher then hydrogen ions
(they assumed same charge so cathode rays must have been very small)
1.) have enegy, momentum, mass
2.)-ve
3.) same propity
4.) ↑ specific charge
explain the 3 types of glow in discharge tube and how they are formed
- naturally occuring free e- within the gas accelerate towords the anode
- the e- collide with the gas atoms ionising them
- +ve ions are attracted to and accelerate towards the cathode, they the collide with the cathode releasing free e-, these e- then accelerate towards the anode and some combine with the ions near the cathode emitting photons
- as free e- are accelerated towadrs the anode some collide with gass atoms ionisiong them, the e- exited within the atom then dexite emitting photons near the anode
- some e- accelerating towards the anode are going so fast that they miss the anode and collide with the glass wall of the tube, this exites the molecules in the glass, wich then dexite emitting photns (making teh galss glow
explain how a electron gun works
(this includes the explanation of thurmionic emission)
- a metal plate is heated, transfurming thurmal energy to the free e- within the metal
- when the e- gain enough Ek they break free from the metal surface
- once the e- are emitted they can then be accelerated by the E in the electron gun towards the anode
- some e- pass through a small hole in the anode
- this creats a narrow beam of e-
- once the e- pass through the hole there is no longer any force acting upon them thereofre the e- travel at a constent velocity
when dealing with an e- in a electric field
wd=__=__
Ek = ev
same as wd=QΔV
eV defonition
1 ev is the Ek carried by an e- after it has been accelerated through a potential diffrence of 1V
decribe the set up of a fine beam tube used to measure the specific charg of an e-
there is:
- e- gun
- uniform magnetic fields generated by 2 magnetic feild coils
- low pressure gass within the galss bulb
explain the porcess of measureing the specific charge of an e- within a fine beam tube
- electron gun fires e- beams into a low pressure gass bulb.
- hydrogen atoms within the e- path absorb enegy
- the e- within the hydrogen atoms are exited and then dexite emitting photons causing the e- path to glow
- these e-‘s initial motion are perpendicular to a unifom magnetic field across the bulb that is generated by 2 circular magnetic field coils
- the uniform magnetic field causes the e- to follow a a circular path as the magnetic force acts perpendicularly to the e- direction of motion thereofre acting as the centrpetal force.
- the specific charge can then be calulated by equating the F=F and subing in Ek=eV to find the equation:
e/m = 2V/B^2 x r^2
what do you use to measure the specific charge of an e-
fine beam tube
gass within the bulb a a fine beam tube is at what pressure
low
why is a glowing path created by an e- in a fine beam tube
hydrogen atoms within the e- path absorb enegy
- the e- within the hydrogen atoms are exited and then dexite emitting photons causing the e- path to glow
what is the equation used to find the measure the specific charge of an e-
derive it
e/m = 2V/B^2r^2
F=F
mv^2/r=BeV
v^2=B^2e^2r^2/m^2
1/2mv^2=eV
v^2=2eV/m
2eV/m = v^2=B^2e^2r^2/m^2
e/m = 2V/B^2r^2
who found the specific charge of an e-
who found the absalute charge of an e-
thomoson
milikans
what is stokes law/viscous drag force equation
this acts in the _______ direction to velocity of the object
F=6πηrv
draw milikans experoment set up
see pg 242
explain milikans set up
- a atomiser created a fine mist of oil drops that were charged by friction as they left the atomiser (could be +ve or =ve)
- some frops fell through a hole in the top plate and could be veied through a microscpe
- milikans could adjust the potential diffrence between the 2 plates wich created a electric field between teh 2 plates
when the switch is turned off what are ethe forces acting on the drop
↓weight
↑viscous force
when the drop reaches terminal velocity the 2 forces are ______
now rearange the 2 forces acting here to fine r of the drop
equal
F=F
mg↓=
you get r^2 = 9ηv/2ρg
when the E is turned on milikans adjusted the E untill the drop was _____
now what are the 2 forces acting on the drop
rearange to find Q
statinary
↓mg=↑QV/d
weight = electrostatic foce
mg = 4/3πr^3ρg
so
↓4/3πr^3ρg=↑
QV/d
Q=4πr^3ρgd/3V
what did the milikans experoment sugest
- charge exists in descrete packets of 1.6x10^-19c
- this is the fundimental unit of charge
- this charge is carried by one e-
(this also ment that the mass of the e- could be calculated proving it was the lightest partical known at that time)
newton had his _______ ______ and thought light was a ______ __ ______
cupuscular theory, partical
what was one of newtons majur arguments for the curpuscular therory
why was this argument false
light was know to travel in straight lines yet waves were know to bend when going around obsticals (diffraction)
equipment at that time were not able to detect the diffraction of light
at the time of newton and huygens light was only known to _____ and ______
reflect and reftact
newtons cupuscular theroy was based on _____ _____ _____ ____ that all particals will naturaly travel in ______ lines
newtons laws of motion, straight
newton beleived that reflection was due to a force that ______ the particals away from the surface
draw this diagram
newton beleived that refraction occured because corpuscules travelled ______ ins a _____ meadium.
draw this diagram
pushed
see pg 244
faster, denser
see pg 244
huygens thought light was a _____
wave
explain huygens principle
draw this
- every point on a wavefront may be considered as a point source of secondary wavelates
- these wavelets then spread out in the forward direction at the same speed of the wave
- the new wavefornt is the surface that is tangential to all the prior secondary wavelets
see pg 244
draw the diagram of reflection
refraction according to hygens theroy
see pg 244
see pg 244
what did hygens predict (2)
- light will slow down when in denser meadiums
- light should diffract around tiny objects and two coherent light sources should interfer with each other
why did scientists of the period prefer newtons corpuscular theroey over huygens wave theory
- there was no experomental data to prove light was a wave untill yungs dubble slit experoment
- dubble refraction was only thought to be explained by newtons theory at the time (it was explained by curpuscules having ‘sides’)
- newton has a large reputaion
what was the problem with demonstraiting that light sources could interfernce
how did young oslve this?
how was then able to prove light was a wave as…
light sources needed to be coherent and this was difficalt to do as light is emitted in random bursts
- young placed 1 point source of light behind a narrow slit
- the light was then spread out through diffraction
- this spread out light acted as 2 cohrent saources when incedent on 2 slits
- it was abserved on a screen that the light shown through the slit and then the 2 narrown slits produced dark and light fringes
- this could only be produced by the light overlaping and interfering.
the fact that light spread out form the 1 slit and 2 slits showed that light diffracted - diffrection and interfrence are only propities of waves thereofre light must have been a wave
huygens proposed light was _______ ____ but this couldnt explain ________ _______.
young relised that _______ ____ could exlpain it tho
longitudinal wave, dubble diffraction
transverse waves
Fizeau measured the _____ ____ _____
speed of light
draw fizeaus experoment
explain how Fizeau measured the speed of light
see pg 245
- Fizueau passed a beam of light through a gap between 2 cog teeth to a reflector (about 9km away).
- the cog was the rotated at exactly the right speed so that the reflector beam was blocked by the next cog
- fizeau could then use:
1.)frequency of rotaion (f)
2.)nomber of gaps and
3.)distence traveled
to calculate the speed of light
why is fizeaus measurement of the spead of light significant
because this was used by maxwell to surport his theroey that light was a EM wave as his speed of EM waves and Fizeaus speed of light were similar
maxwell predicted __ _____ and ther _____
Em waves, speed
maxwell created a ______ ______ of _____ and _______ fields. this modle said that a change to these fields would create a __ wave.
mathmatical modle, magnetic and electric fields
what did maxwell predict
- there will be a spectrum of EM waves
- EM waves will travel at the same speed with diffrent frequency
give maxwells equation of speed of EM wave
c=1/rootu0ε0 = 2.998x10^8
what gave credibility to the assumption that light as well a ultra vilet and infered rediation are Em waves
the fact its value for speed of EM waves was very simular to Fizeaus value for speed of light
Hertz discoverd ____ _____
radio waves
draw a diagram of the experoment hertz used to show radio waves have a magnetic component
see pg 146
hertz noticed a radio wave was produced when…
a high voltage spark jumped across a gap air
how did hertz prove radio waves had a magnetic component
- when the radio waves were incedent on a loop of wire hertz noticed a spark jumped across a small break in the wire
- this showed that the wire underwent electromagnetic induction that generated ε in the coil - this shows that there was a change in flux linkage that could onlly be induced by the magnetic component of the radio wave
hertz set up _______ ____ waves at a ____ ______ to measure the _____ of radio waves
stationary waves, at a fixed frequency, speed
how was hurtz able to calculate teh speed of radio waves
- he found the wavelenth by mesureing the distence bewtween nodes
∴ using the frequancy
he was able to calculate spee c
c=λf
hertz was able to show the _____ __ _____ ______ were the same speed as the ____ __ _____ and matched ______ ______
what meaduim must hertz experoment be done in for this comparison
speed of radio waves, speed of light, maxwells prediction
vacuum
in the wave theory of light:
enegy is propotinal to _____ (E∝ ).
the energy carried by the light would be ______ _____ over the wavefront
each free e- on the surface of the meatal will gain a __ __ _____ from each incoming wave ∴ _______ each e- will gain enugh _____ to be able to be emitted
intensity I
spead evenly
bit of enegy, gradualy enegy
the wave thery of light cannot explain…
1.) why Ek ∝ f and does not increase w/ intensity
2.) thresh hold frequency
a black body is a _____ ______ and _____
therfore it…
perfect absorber and emitter
absorbes all wavelenth of light and emits all wavelenth of light
object emit __ _____ due to there _______
Em radiation, tempreture
draw the graph of power over wavelenth (black body radiation) (when dealing with a very hot black body)
see pg 248
when dealing with black body radiation:
when does the wave theory modle work and when does it not.
- wave theroy modle works for long wavelenth
- but wave theory modle predicts a infinetly high peak in power when λ leads to 0
who came up with the photon modle of light
einstein
plank suggested EM waves can only be released in ______ _______ or _______
what is the eqaution for the engy carried by one of these packets
what did eistein then add to this…
how did einsien apply this to the photoelectric effect
what is the diffrence bewteen planks sugestion and einstines surgestion about light
descrete packets, quanta
E=hf=hc/λ
eistein thought EM could only exist in desctete packets of enegy called photons.
einsein thought these photons would have a one on one partical like interaction with e- on the metal surface transfering all of there enegy to that one specific e-.
plank said EM radiation can only be released in descret packets whilse einstine said EM waves can only exist in descete packets of enegy
why was einseins photoelectic work significant
- demonstarated light was a stream of particals
- showed that photons are the fundimental unit of electromagnetic radiation
debroglie surgestion:
if light showed ______ ______ the particals like e- should be able to show ______ ______
give his equation
this eqiation relates what to what
partical propities, wvae propities
λ=h/mv
partical propities like p to wave propities like λ
explain electron difraction
give the eqaution used in this
derive it
when accelirated e- in a vaccumed tube pass through gaps in a graphit cristal a diffraction patten was produced
λ=h/root2m(e)V
λ of e- accelerated in a vaccume tube have about the same λ as _____
x rays
why do e- microscops have a greater reselution then light microscpes
beacuse accelerated electrons have a smaller λ then visible light
∴ can resolve greater details
(this is because larger λ then a gap will bounce of entially ∴ the gap would be seen as a flat wall 0)
what does TEM stand for
what does STM stand for
transmission electron microscope
scaning tunalling microscope
what adversly effects the image quality in a TEM
- a thick speciman as this will cause greater loss of speed to some of the e- that pass through the speciman ∴ reducing λ∴reducing resalution
- faster the e- the lower the λ ∴ higher the reolution
- e- in the e- gun are produced through thurmionic emission, as e- will need diffrent amounts of enegy to be emitted from the metal surface there will be Ek distrabution
draw STM diagram
how doeas a STM work
what are the 2 modes of a STM
see tuning points book or look up
- a very fine porbe is moved across the suface of the sample and a potential diffrence is aplied acoss them
- e- then ‘tunnel’ form the speciman surface to the probe
- the results in a weak electrical current
- the smaller the distence the greater the strenth of the current
- therefore an image can be produced by scaning he surface and measuring the strenth of the current porduced to determin the distence form probe tip to speciman
1.) the probe can be fixed and the distence can veriry verying the strenth of the current
2.) the current can be fixed and the distence can be veryed by controling the probe useing pesoelectricts
michelson morley:
most physicists believed in the idea of _____ ____. they thought everything moved reletive to a _____ _____ called the ____
absalute motion, fixed bacround, either
what did michelson and morley try to measure
what equipment did they use
the absalute speed of the earth
interferometer
michelson and morley thought that the speed of light measured in a lab moving parrelel to the light would either be…
c+v or c-v
where v is the absalute motion of the earth
draw a interferomitor with lables
describe how the experoment was carried out
see pg 252
- monochomatic light hits a partial refractor
- light is split into 2 beams traviling at right ables to each other
- the beams are reflected by 2 mirros and travel the same distence
- the beams meet at the partial reflector and interferd to create a interfrrence patten that was observer by the observer
- this is recoreded
- the experoment is rotated 90 degrees and repeted
why is there a second glass block in the michelson moreley experoment
to ensure that both beams travel through the same amount if air and glass
what was the expected outcome of the michelson morley experoment
- the light moving parrelele to the motion of the earth should take longer to reach the mirror then the light moving perpendicularly to the earths motion
- therefore rotating the interferomitor should create a tiny shift in the interference patten observed
what was the result of the michelson morley experoment
what conclusions were draw
- there was no shift in the ineterference patten
- its impossible to detect absalute motion (the either doesnt exist)
- speed of light is the same for all observers
what is a inertial refrence frame
anything moving at a ______ ______ is in a inertial frame
a frame were newtons 1st law is obeyed
constent velocity
what are einseins postulates of special reletivity
1.) physical laws have the same form in all inertial frames
2.) the speed of light in free space is inverient
what is time dialation
were an observer moving at a constent velocity will measure a longer time interval
proof of time dialation from moun decay:
what do we expect
decribe the experoment
what happend
- most mouns with a short half life (around 2us) are expected to decay between the top of the earths atmosphear and the surface
- measure speed v of mouns
- place a detector at a high altatude and measure the moun count rate
- use another detector to measure the moun count rate at
- compear teh 2 figures
there was a greater count rate at the second detecor then expected as the mouns experenced slower time from there point of veiw as a result of time dialation therefore less mouns decayed
what is lenth contraction
a rod moving in the same direction as its lenth looks shorter to external observers
special reletivity:
the ____ and ____ of an object _____ with ______.
draw mass over speed diagram
draw mass of Ek diagram
therefore why can no massive object travel at the speed of light
mass and energy, increases, speed
see pg 255
see pg 255
because as a massive object approaches the speed of light mass leads ∞ wich is impossible
explain how bertozzi demonstrated that mass increases with speed
- e- were accelerated and smashed into a aluminuim disc a set distence away
- time taken to reach the disc was recoreded to find the speed of the e-
- as the enegy was increase the speed tailed off towards a maximum value appraching 3x10-8
- this showed that as energy increase mass increased as velocity couldnt increase past c
- bertozzi used heat generated by the colliions ot calculate Ek befor eimpact
- when plotted this matched the curve predicted by eintien closely
exam quetion (3mark):
- how are cathode rays produced in a gas discharge tube
- e- are pulled off from gas atoms in the tube as a result of the high pd. (ionising them)
- ions that are produced near teh cathode are accelerated towards the cathode. when the collide with the cathode free electrons are produced
- in both cases free electrons are then accelerated towards teh anode by the high pd
(cathod rays are fast moving e-)
draw it
explain the process of image formation in a TEM
(dubble check about the wide parrlel beam bit)
- an e- gun fires a beam of electrons
- e- in the middle of the lens are uneffected tho e- on the edge are bent by the condenser lense into a parrelel beam incedent on the specimain
- after the e- pass through the specimain a objector lense then forms an image of the sample
- the prejector lense then casts a second larger image onto the fluecent screen
exam question (6marks):
explain refraction through newtons and hygens theroys
newton:
- light is made up of cupuscules
- at the boundry force acts to attract the partical to
- this force only effects the vertical component of the velocity
- since the vertical velocity has increased the cupuscle changes direction
hugynes:
- wave front is incedent on boundry
- the secondary wavelets from the wavefont slows down as they enter the denser meduim
