Ideas to Implementation Flashcards
1
Q
what is a cathode ray tube?
A
a vacuum tube wih electrodes at either end. the electrodes are conductive pieces of metal connected in series with some high potential power source
2
Q
when a potential difference is passed through a cathode tube, what happens?
A
electrons jump from the cathode(-ve) to the anode(+ve)
the electrons then collide with the gas molecules that are present inside the tube, causing them to absorb and emit the energy that was transfered by the electron from the collision
this energy is seen as light
3
Q
what is the appearance of light in a cathode ray tube dependant upon?
A
on both the chemical composition of the gas inside the tube and on the gas pressure
4
Q
why was their a debate as to the nature of cathode rays in the late 19th century?
A
experimental observations of cathode rays provided puzzling inconsistencies which led to the emergence of two competeing theories, one supporting that cathode rays were waves and the other supporting that the cathode rays were negatively charged particles
5
Q
who was it that settled the debate about the behaviour cathode rays and how did he do this?
A
A man named Thompson settled the debate once he was able to deflect the cathode rays with electric plates. this had been impossible up to this point as vacuum pumps before had not been strong enough and the electric field’s present had not been strong enough.
because emr was known not to be deflected by electric fields, and emr was a wave, this strongly supported that cathode rays were particles
6
Q
what properties of cathode rays fit the wave theory?
A
- they travel in straight lines
- produces a shadow when obstructed by large objects
- could pass through thin metal foils without damaging them
7
Q
what properties of cathode rays fit the particle theory?
A
- they could be deflected with an electric and magnetic field
- the cathode rays left the cathode at 90 degrees to the surface(instead of propagating out like a wave)
- they travelled considerably slower than light(which is a wave)
- small paddlewheels turned when placed in the path of the rays, indicating that the rays contituents must have momentum and hence mass
8
Q
what happens to a charge moving through a magnetic field?
A
it experiences a force
9
Q
if a charge is moving parallel to a magnetic field, what happens?
A
it doesnt experience a force
10
Q
what is the force on a moving charge in a magnetic field equal to?
A
F = qvBsin(theta)
11
Q
what do oppositely charged plats create between them?
A
a uniform electric field running between them from the positive plate to the negative plate
12
Q
when a potential is made between two oppositely charge plates, what expression gives the magnitude of the field?
A
E = V/d
where E is electric field strength in Newtons/ coloumb
V is potential difference measured in volts
d is the seperation of the charged plates
13
Q
what does spacing between electric field lines show?
A
how strong the electric field is, smaller spaces represent a strong electric field
14
Q
if a charge moves through an electric field, what will happen to its path?
A
it will form a parabolic path
15
Q
what are the field lines like for a positive point charge?
A
- field lines radiate from the centre of the charge outward, indicating the direction of movement of a positive test charge in the field
- field lines which are closer together show a stronger electric field
- field lines never cross
16
Q
what are the field lines like for a negative point charge?
A
- field lines radiate inwards toward the centre of the negative point charge, indicating the movement of a positive test charge in the field
- all of the properties that go with the positive point charge
17
Q
what is the expression for the magnitude of an electric field at a particular point in that field?
A
E=F/q
where F is force in newtons
E is electric field strength in Newtons/coloumb
q is charge measured in coloumbs
18
Q
what did JJ thompson do?
A
he determined the charge/mass ratio of an electron
19
Q
what did Thompson use to do this?
A
Thompson used a large glass vaccum tube containing:
- a cathode, connected to a high voltage DC power source
- anode double slit, connected to a high voltage DC power source
- electric plates
- electromagnets
- fluorescent screen with scale
20
Q
in his first experiment, what did thompson do?
A
thompson passed cathode rays through small slots in a cylinder anode making a near parallel beam that his the end of the glass tube in the centre, moving past two opposite electric plates and electromagnets which were perpendicular
first, he varied the strengths of both the electric field supplied by the plates and the magnetic field supplied by the electromagnets so that they would balance eachother out and thus the net force on the cathode rays would be zero and it would resume its straight line trajectory.
by equating the magnetic and electric force equations, thompson developed an expression for the velocity of the cathode rays in terms of E and B, v = E/B
21
Q
what was thompsons second experiment?
A
thompson turned off the electric plates and kept the magnetic field applied. the cathode rays would become deflected and follow a circular path. these rays then hit the fluorescent scree at the very end of the apparatus indicating how the electrons were being deflected
the magnetic force provided the centripetal force, causing the rays to curve in a circular arc with a fixed and measurable radius which could be determined b measuring the displacement of the beam
Fcentripetal = qvB (the beams were being fired at 90 degrees to the fields, so sin(theta) would be 1)
22
Q
what was the final expression for the charge/mass ratio of the electron?
A
q/m = E/B^2r
23
Q
what did hertz use to measure and detect radio waves?
A
- induction coil
- reciever loop
- a spark gap
24
Q
what did hertz do to produce radio waves?
A
hertz used an induction coil to produce a high voltage oscillating spark. as these sparks oscillated back and forth across the gap in the induction coil, sparks were also noticed jumping across the air gap in the reciever loop, even when the reciever loop wasnt connected to any power source
25
what did hertz hypothesise that was causing the sparks in the reciever loop?
hertz suggested the oscillating spark in the induction coil set up changing electric and magnetic fields that propagated as an electromagnetic wave, as postulated by Maxwell. these waves, falling onto the reciever loop, set and electric and magnetic fields inducing a spark
26
what three things did hertz do which provided proof that the produced radio waves were em waves?
- he was able to reflect the waves of sheets of metal
- he was able to refract the waves with a prism of pitch
- he was able to polarise the waves by reorientating the spark gap of the reciever
therefore, he showed that his waves had the properties of reflection, refraction and polarisation, in common with light and all emr.
27
what two things did hertz need in order to measure the speed of the waves he had produced?
their wavelength and frequency
28
how did hertz find the frequency of the waves he produced?
he understood the geometry of the induction coil he was using. by knowing this, he identified the frequency as roughly 100MHz
29
how did hertz find the wavelength of the waves he produced?
-hertz reflected the waves off of a metal sheet, allowing the radio waves and the reflected waves to produce patterns of interference at a detector some distance away.
this produced a standing wave at the detector, showing points of constructive( antinodes) and destructive interference(nodes). he knew the distance between adjacent nodes was half a wavelength, and so from that he found the speed of the waves as 3x10^8m/s , supporting maxwells equations
30
how did hertz accidentally discover the photoelectric effect?
when observing the spark gap, hertz wanted to be able to view it more clearly. to do this, hertz enlcosed the reciever loop in a dark box. when he did this, he noticed the spark greatly diminish in size
31
what did hertz conclude regarding the observation he made with the diminished spark?
he concluded that this was because light or more specifically emr was affecting the size of the induced spark, but he didnt provide any explanation as to why this was happening
32
what did hertz do to investiage the photoelectric effect further?
he irridated the reciever with different frequencies of emr. he found that the spar was greatest when UV was shone on the reciever, but when a quartz screen obstructed the uv, blocking it, the spark become smaller again
33
what properties do black bodies have?
- they absorb all emr that falls onto it
- no emr passes through it
- no emr is reflected off them
- emr is emitted at a frequency chracteristic of the body's temp
34
what factors effect the type of radiation that is emitted from a black body?
- the material from which it is made
- its shape
- the nature of its surface
- its temp
35
at all temps, black bodys emit ....
radiation at all wavelengths, but the intensity of each wavelength varies
36
what was the ultraviolet catastrophe?
classical theory predicted that the radiation emitted by a black body should continuosly increase in intensity as teh wavelength becomes shorter, forming a continuous spectrum with intensities corresponding to an exponential curve
this prediction was obviously incorrect. if it were true, then that would mean that all objects in the universe would be emitting harmful xrays and gamma rays. it also violates the law of conservation of energy
this was the ultraviolet catastrophe
37
what did planck propose that solved the uv catastrophe?
planck hypothesised that radiation could only occir in small packets of every which he called quanta. this meant that radiation is no emitted or absorbed by a black body continuosly as classical physics said it should, but rather it is emitted or absorbed in little bursts of energy
38
overall, what did planck propose?
that black body radiation, and indeed all light, is quantised.
39
what is the expression for the energy contained within a single quanta?
E=hf
where h is plancks constant
f is frequency
E is the energy in joules
40
how did planck view his proposal of quantised energy?
he thought it was a mathematical trick
41
what list of ideas did the genius of einstein propose regarding quanta and plancks "mathematical trick"?
- the energy of light is not evenly spread out over the wavefront, but is concentrate in bundles or packets of energy, or photons
- each photon has an energy equal to hf
- photons are the smallest units of light
- intensity of light depends on the number of photons, more photons higher intensity
- all photons of the same frequency have the same energy
- photons have zero rest mass and travel at c in a vacuum regardless of their frequency
42
how did einstein explain the photoelectric effect?
- to produce the photoelectric effect, the energy contained in the light photons must be equal to, or greater than, the energy required to overcome the forces holding the electrons to the lattice. the energy required to realse the electron from the lattice is called the work function
- if the energy o the photon is greather than the work function, the additional every of the photon, above the work function level, will provide the kinetic energy of the photoelectrons
- a photon can only give up all or none of its energy to one electron, cannot give halfs or fractions, all or nothing principle
43
what is eintsteins photoelectric equation?
Ek = hf - (work function)
44
how does einsteins contribution to quantum theory relate to black body radiation?
Eisteins theories came about directly through the work undertaken by planck in explaining the ultraviolet catastrophe and black bodies
45
how did einsteins work on quantum theory effect physics at the time and assess its impact
- he expanded on plancks ideas and turned what was previously a mathematical trick such as quantum theory into a set if udeas with concrete principles and modelling.
- einsteins with this work could properly explain the photoelectric effect, the ultraviolet catastrophe and the nature of black body radiation
- einsteins work has led to further discoveries including the heat capacity of solids and the compton effect/scattering
- line spectra of atoms and molecules cold now be better explained in terms of einsteins and plancks ideas of the photon and quantised light
- einsteins work opened a door for futher research into quantum ideas. our entire knowledge of physics on the atomic scale today is based on the quantum theory that began with einstein and a few other great individuals
therefore, einsteins made an astounding contribution to quantum theory and its relation to black body radiation
46
how does the photoelectric effect work?
when light strikes a metallic surface, it is the energy of the photons whichwill activate or energise the electrons in the metal. when a photon strikes a metal, all or none of the energy will be passed on to the electron and then be emitted by a spark
47
what does the particle model of light state?
light energy travels in photons, small particles or quanta that have mass dependant on their energy
48
when i increase the frequency of a photon, what happens?
it gains more energy, and these energys can only occur in multiples of plancks constant
49
when i increase the amplitude of a photon, what happens?
the intensity of the light increases as the number of photons increases
50
what is the equation for photon energy?
E=hf
where h is plancks constant
where f is the frequency
51
what is the formula for the speed of a wave?
v = f • λ
52
the energy of a photon is proportional to.....
its frequency
53
the speed of a wave is proportional to.....
the waves frequency and wavelength
54
what is a photocell?
a photocell, or photoresistor, is a light controlled variable resistor where electrons initiating an electric current originiate by photoelectric emission
55
what does a photocell consist of?
consist of a cathode and anode made from a highly resistive semiconducting material
56
what are photocells used for?
photocells are used to detect lght as the intensity of the light that is detected is directly proportional to the iniated electric current
57
how does a photocell work?
when light falls onto the cathode, the released electons are accelerated to the anode by a potential difference between them resulting in a current
58
what are some of the industrial uses of photocells?
- automatic street lights
- motion detectors
- house alarms
- pollution detectors
- radiation detectors
- light motors
59
by monitoring potential, resistance and current flow in a photocell, ....
a quantifiable measurement of light is possible because these properties change when a semiconductor experiences the photoelectric effect
60
what is a solar cell?
a device which converts light energy into electrical energy.
61
how does a solar cell work?
it uses the junction between a p type and n type semiconductor when exposed to sunlight who are both connected via an external circuit
within the junction, free electrons from the n layer move across the juction to the p layer to fill some of the holes present. this produces an electric field at the junction, where the p type layer is slightly negatively charged, and the n type layer is slightly positively charged
this forms a barrier in the form of an electric field that resists further movement of charges across it. this region in the middle of the pn junction is called the depletion zone. the barrier is of high electric resistance and acts as an insulator.
when photons hit a solar cell, its energy frees electron-hole pairs via the photoelectric effect. this moves an electron to the conduction band, producing a hole in the valance band. theses both act as charge carries. the electric field inherent in the p-n junction will send the electron to the n side, and the hole will tend to migrate to the p side of the solar cell. if an external circuit is provided, these electrons will flow through the circuit from the n to the p side, releasing enery and doing work
62
what were plancks views regarding his country during world war one?
he was a devout nationalist and believed in the righteousness of the german cause and its inevitable victory.
63
what did planck sign in support for the german military effort during world war one?
the manifesto of the ninety three, which was a document signed by prominent german scientists stronly supporting germany military actions
64
after germanys defeat in world war one, what did Planck believe was necessary in order to uplift the status of germany once again?
he believed that the increased respect for german sciece would inevitably lead to greater respect for germany as a whole
65
how was planck coping under nazi germany during WW2 and what position was he appounted to that was so prestigious?
again, german science had become marginlised but, still a devout nationalist, planck still cooperated with the system to strive for a bigger and better germany
in 1930, planck had been appointed as president of the kaiser wilhelm institute, Germany's most prestigious scientific facility
66
WOAH, what happened to planck in 1937 and what did he speak out about?
he resigned from his position at the institute because all the jewish teachers from german universities were losing their jobs, he also spoke out ahainst anti semetism and nogaotiated to atleat have jewish sciencists spared
plancks opposition to anti-semetism led him to refuse any work on any war research projects
67
SO ,based on plancks ideas and beliefs, what was his view on science and social and political forces?
they are not seperate from one another
68
how was einstein different to planck?
unlike planck, einstein was a devote pascifist who took out swiss citizenship in 1901 because of his opposition to german militarism. whereas planck signed the masnifesto of ninety three, einstein signed and three others signed a COUNTER manfiesto
69
what was special about einstein that made him be opposed to the nazi tyranny occuring in germany?
he was a jew and had been a target for prejudice even before the rise of national socialism.
70
where was einstein when hitler was rising to power?
he was living in california
71
what did einstein do to spread the news about the nazi tyranny occuring in germany?
he decided not to return home but to travel across europe detailing what was occuring in germany. he made it adubandanly clear that science should not be used by governments in waging war and the murder of men
72
what did einstein do in 1934 and how did he change?
he permanently imigratted to california, where his previous pascifist beliefs had become overshadowed by an advocation of the re-armament of the democratic government to oppose the nazi tyranny
73
what did einstein do in 1939?
eintein warned president roosevelt of the potential for a german atomic bomb, this led him to help in the manhattan project, which saw the worlds first development of nuclear weapons. this contributed strongly to ending the war
74
so, based upon einsteins previous views and actions leading up to the end of world war 2, what was his view on the link between science and social and political forces?
eintein previously stongly objected that science should be removed from social and political forces
BUT
once he did the manhattan project, his views changed and suddenly realised that science is intrinsically linked to political and social forces, even if that is the ideal situation
75
in a way, planck and einstein are representative of a...
wider debate in science that continues even today as to the role of governments agenda should be in terms of scientific research, but, like todays scientists, they realised that they can never be seperated
76
what is an energy band?
a series of energy levels in which electrons in a lattice can occupy. these levels combine to form energy bands within the crystal
77
what are the two energy bands found in crystal lattices?
the valence band and conduction band
78
what is the forbidden energy gap ?
the seperated band gap between the valence and conduction bands
79
what is the relationship between band structure of a conductor and its electrical resistance?
in conductors, the conduction and valence bands overlap, this means that electrons in their normal valence positions can without any energy be in the conduction band and move freely between atoms
the valence band in conductors is hence only partly filled
hence as it is so easy for electrons to move into the conduction band, these is little resistance
80
what is the relationship between band structure in a semi conductor and its electrical resistance?
in semidoncutors, the gap between the conduction and valence bads is very small, less than five electron volts. this means they are seperated by a very small energy gap
therefore, additional energy in the form of light, heat or an electric field is required to prove the necessary energy for an electron to jump the gap to the conduction band and be available for conduction
therefore, semiconductors have moderate resistance
81
what is a the two types of semiconductors?
intrinsic: semicondutcotrs that are made from pure semiconductor crystals consisting of only one element
extrinsic: semiconductor crystals with deliberate impurities consisting of small quanties of a group 3 or 5 element. these semiconductors contain an extra energy level insdie the forbidden energy gap for electrons to exist, reducing the energy required to get into the conduction band
82
what is the relationship between band structure in an insulator and its electrical resistance?
in insulators, the distance between the conduction band and valence band is very large, which correponds with a very large amount of energy necessary to jump the gap
in order to conduct electricity, electrons in an insulator must gain enough energy to jump from their normal valance band positions over the large forbidden energy gap and into the conduction band, because this process requires a great deal of energy input it is very difficult to cause insulators to doncut, and so they have high electrical resistance
83
what are holes?
holes are effectively regions of empty space or an empty energy level that acts as a positive point charge.
84
how can holes in a lattice structure carry current in a semiconductor?
the hole moves when bonds in the lattice switch around and so the position of the hole changes as electrons fill it. this snowballing effect allows the hole to carry electrical current and hence makes the semicondutor conductive
85
when the electron fills a hole, the hole moves in the ....
opposite direction
86
in a conductor, what are the relative number of free electrons that can drift from atom to atom?
the number is very large, easily fascilitating the flow of current. this is due to the overlap between the condution and valence bands and the valence band only being partly filled
87
in a semiconductor, what are the relative number of free electrons that can drift from atom to atom?
there are some electrons able to drift from atom to atom in the conduction band. this number can be increased if the energy in the form of heat, light or a strong electric field is applied to the lattice structure
88
in an insulator, what are the relative number of free electrons able to drift from atom to atom?
there are almost no electrons available to drift from atom to atom in the conduction band of insulators
89
what was the first element used in the transistor?
germanium
90
why wasnt silicon used instead of germanium for the first couple of transistors?
suitable industrial techniques were developed to purify the germanium to the ultra-pure level required for semicondutors
also, silicon could not be manufactured pure enough to make reliable chips. the techniques used to purify germanium could not be applied to silicon
91
why was silicon superior over germanium in the transistor?
- silicon is much more abundant than germanium, actually the 2nd most abundant elecment on the planet
- silicon retains its semiconductiing properties at high temps, germanium beomces too conductive with only modertate heating making germanium chip performance highly dependant on temp
- silicon forms an oxide later that protects from corrosion
- silicon can be easily doped into N and P types, and made into thin, flat layers
92
what is doping?
the process involving adding a group 3 or group 5 element as an impurity into an instrinsic semicondutor lattice to reduce the energy input required for the semiconductor to become conductive
93
what are p type semiconductors and how do they work?
semiconductors which hve been doped with a group 3 element, leaving behind a positive hole in the silicon lattice
group 3 atoms have 3 electrons in the valence band, when such an atom replaces a silicon atom, there is one issing elecron in the tetrahedral structure, which requires 4 electrons from each atom, this means a hole is produced.
the movement of holes in a p type semicondutor act as charge carriers
94
what are n type semiconductors and how do they work?
n type semiconductors are semiconductors which have been doped with a group 5 element to add extra electrons to the crystal structure
group 5 elements have 5 electrons in their valence band, when such an atom replaces a silicon atom, there is an extra electron present in the tetrahedral structure
the moevement of negatively charged lectrons in a n type semiconductor act as charge carriers. under the influence of an electric field, these electrons will be free to move and will constitute an electric current
the extra electron from each dopant atom does not participate in bonding, and is moved into the conduction band where it can be a charge carrier
95
which american inventor developed a triode in a vacuum tube that could be used to amplify signals on telephone lines as they were transferred across america?
Lee De Forest in 1906
96
what were the issues with the vacuum tubes used to amplifty these signals?
- extremely unreliable
- used to much power
- generated to much heat
- recieved signal was too weak and could not produce a loud enough sound
- high failure rate
- high power consumption
- high weight
- long warm up time
97
which device was invented to address the current shortcomings in communication technologies, escpecially for the telephone business in america?
for the business to grow, a device that may be built from the foundation of a semiconductng material called a transistor was made, a promising and attractive alternative
98
therefore, how did the shortcomings in available communications technology lead to the development of the transistor?
it led to the rapid development of the transistor that would ahve otherwise taken many years longer
99
through what two devices has transistors dramatically changed society?
through microchips and microprocessors
100
in what ways(theres a lot) have transistors positively impacted upon society?
- invention paved the way for the development of miniature electron circuits and profoundly intergrated circuits ( a single silicon chip with many transistors within the same chip). these circuits paved the way for the rapid development of solid state based electronics which are at the foundation fo all modern electronics devices today
- invention of microprocessors enabled the building of small, efficient computers that now have widespread applicaitons throughout society as well in scientific researh. devices could not be made that required less space nad less power which had for more reliablity then existing devices. the use of microprocessors and microchips have made possible a continuous decline in real price of computers and a continuous increase in processing power
- transistors have allowed the automation of repetitive tasks which has led to a higher quality of life. thie spread of computers led to the widespead adoption of the internet, which as increased societies connectedness and increased information accessiblity
- fast computers and tinu electronics have connected the world in an incredible amoutn of waters. for eg, microprocessors used incommunication technology have increased reliablity of efficiency. they are now so advanced that through GPS, we are no able to pin point any location on earth wthing a few meters. this has had a profound impact on missile and war technology, satellite technology and communication technology. it is now possible to speak and communicate with others all the way around the world, wirelesly and cheaply
- computers are now used in many different professions with an intergral role such as in law, engineeringm medicine and in sall business such as resturants, real estate agencies etc to store and analyse data
- microschips are found in mobile phones which have eneabled instananeous commincation between people in cars, betwee traveling salespersons and head office, between parents and young children away from home. mobile pones allowed contact between peple who dont have access to a landline phone
101
what are some of the negative effects of transistors on society?
- computers, for eg, have made many traditional jobs redundant, leading to more unemployment that is only temporary
- increase uf of computers had led to an increase in computer crime and child obesity
- reduced social and ethical codes
102
what is my final assesment of the impact of transistors on society?
the negative side effects are temporary and minimal and are necessary for progress. therefore, overall, transistors have had an extremely positive impact on society
103
what did the sir william and lawrence bragg use to determine the crystal lattice structure?
the technique of x-ray crystallography
104
what is x-ray crystallography?
technique used for determining the atomic and molecular structure of a crystal, in which the crystalline atoms cause a beam of incident x rays to diffract into many specific directions
105
what equipment did the braggs use to identify the crystal lattice structure?
xray tube
collimator
crystal target
photographic film
106
what did the braggs realise in their experiment?
that the distances between the atoms in the structure were similar in length of the wavelengthof the x rays that was being emitted, and therefore the crystal target would act as a 3D diffraction grating
107
how does a diffraction grating work?
a diffraction gratting uses small obstructions with seperations similar to the wavelength of the wave in question placed side by side to produce a predictable interference pattern that is directly linked to the spacing within the diffraction grating
108
from the interference pattern that the braggs obtained, what else could they figure out and whats the importance of this to science?
the distance between the lattice layers which is of great importance to science and the understanding of the crystal lattice structure
109
so, what was the method used by the braggs to determine the crystal structure?
firstly, they shone a beam of x rays through a collimator (used to focus the rays in to a beam).
these rays then hit the crystal target which acted as a diffraction gratting producing patterns of interference that produced regions of light and dark areas on photographic film( constructive and decstructive interefence respectively)
the braggs observed the regions of maximum and zero intensity and how they were occuring in certain drections. the pattern only wouldoccur if the x-rays where diffracted by a series of parallel planes of atoms, forming the crystal lattice.
110
whats the structure of a metal?
metals have a crystal lattice structure in their solid state. this means that they exist as a 3 dimensional grid of atoms arranged into layers. it is a repeating structure where each atom occupies a well difined equilibirum dristance from its neighbours
in the case of metals, free electrons called a delocalised cloud of electrons eist in between the lattice layers that conduct electricity.
111
why is there no net movement of charge in an isolated metal?
metals consist of free electrons that exist in the spaces between metal ions in the lattice forming a delocalised sea of electrons. the electrons in the metal are moving at enourmous speeds, and hence often collide with otehr electrons or with impurity atoms and imperfections of the lattice
in doing so, they lose some of their energy which is converted into heat. after a collision, the electrons accelerate again until another collision. because of this random movement, there is no net movement of charge
112
so, whats the science behind the conduction of electrons in a metal?
when an electric field or potential is applied to the metal, a force is exterted onto the electrons that results in a net drift of electrons towards the positive potential, hence these electrons then conduct and will move freely between the lattice layers
113
whats the science behind resistance in a metal?
resistance to curent is a result of the collisions of electrons with impurity atoms or imperfections in the metal. this why when a current is applied to a metal it gets warmer. this is because the free moving electrons in the lattice are taking part in collisions with other electrons and/or impurit atoms, relasing energy in the form of heat
114
so, based on my new understanding of resistance, explain why increasing the temp of a metal means more resistance?
as the temp of a metal is increased, the atoms in its structure gain more kinetic energy and vibrate more
this means that free flowing electrons will have more of a chance in colliding with impurity atoms and hence resistance increases
115
whats the final way to increase the resistance of a metal which id probably forget?
adding more impurity atom to the metal. impurities in a metal distor the lattice structure and electrons can collide with the impurity, increasing resistance
116
low resistance in a metal =
obstruction is minimal, no collisions/hardly any
117
high resistance in a metal =
obstruction is high and collisions are frequent and common
118
what is a superconductor?
a substance capable of becoming superconducting at sufficiently low temperatures. no resistance to current
in a superconductor the electrons are not impeded by the lattice structure and hence there is no energy loss
119
what are the two types of superconductors and what distinguishes them?
type 1: common elements
type 2: consisting of mutliple combination of elements
120
which type of superconductor have a higher critical temp than the other?
type 2 superconductors
121
which three scientists came up with the BCS theory?
John Bardeen, Leon Cooper and J. Robert Schrieffer in 1957
122
what is the BCS theory?
a quantum mechanical effect that can explain, in simplified classical terms, superconductivity
123
how does the BCS theory work?
firstly, one electron passess by positivelty charged ions in the lattice of the superconductor causing the lattice to distort and create an area of increased positive chage concentration. this distortion is due to the attraction between the positive ions in the lattice to the electron
this distortion of the lattice forms a phonon, packets of sound energy present in the vibrating lattice.
thi phonon then attracts another traling electron passed the highly positive region, effectively binding the two electrons together into a cooper pair where they are unimpeded through the crystal lattice
because the two electrons are interacting with eachother they interact less with the lattice, and os travel through it very easiliy with very little resistance
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in terms of the BCS theory, why does superconductivity stop when temp increases?
due to the more vibrations experienced by the lattice at high temps, a superconductor must be cooled down to allow these cooper pairs to form. when the temp exceeds the critical temp, superconductivity is lost as vibrations within the lattice become too strong for the cooper pairs to exist
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what are the limitations of the BCS theory?
-cannot account for the superconductivity of type 2 superconductors whose critical temp exceeds that of 30 Kelvin up to even 92 kelvin. this is because the theory predicts 30 kelvin as being the maxtemp for cooper pairs to form
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what are the pros of the BCS theory?
extremely succesful in explaining type 1 superconductivity, where critical temps are below 30 Kelvin
127
what are the advanatges of using superconductors?
-operate with very little energy loss and so are extremely efficient, and thus they do not produce any waste heart because they are perfect conductors
narrow wires can be made to trasmit huge currents with no power loss, perfect for transmission
- superconductors are capable of generating very strong magnetic fields per unity of wieght, and so useful in MRI scanners, motors, generators
- environmental benefirts accrue from the high efficiency of power transmission, generation, distribution and use of electric power using superconductors. this saves the environment from less pollution due to power generation in fossil fuels power plants
- because superconductors are so efficient, their use can reduce the size and weight of various devices such as motors, transformers, batteries, generators, thereby increasing efficiency
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what are the disadvantages of superconductors?
- there exists a critical current density and magnetic flux density, that, once reached, breaks down the superconductivity of the material
- it is very difficult and very costly to cool superconductors below their critical temps, they require a constant supply of something like liquid nitrogen given that such low temps are required to keep them operational. in some uses, low temp may be undersirable, operation costs are prohibitively high
- current high temp type 2 supers are all ceramic based materials. these materials are brittle and are subject to breaking and cracking very easliy. therefore they cant be stretched into thin wires and thus you cant make coils out of them
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what is the meisenner effect?
the phenomenon where magnetic flux is expelled from a superconductor.
it disallows the magnetic field lines to penetrate its interior once it is superconducting.
IT IS NOT A RESULT OF LENZ LAW
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because of the meissener effect, what can sueprconductors act like?
magnetic shields
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whats the science behind the meissener effect?
when a magnet is near the superconductor, surface currents are induced in the superconductors surface to create an exactly equal magnetic field to oppose the field from the magnet (because superconductors have no resistance)
this is not a result of lenz law because if the magnet is already on the super before it has reached its Tc, it will still levitate
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what is also interesting about a magnet during the meissener effect?
the magnet appears to be locked in position aloft the superconductor. this effect is a result of magnet pinning which is a quantum mechanical effect which stops the magnet from moving horizontally off the superconductor
133
whats the application of superconductors that will allow people to shuttle across cities at extremely high speeds?
maglev trains
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what are the two different systems for maglev trains?
electromagnetic suspension and electrodynamic suspension
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how does electromagnetic suspension (EMS) work?
EMS uses the attractive forces between iron core electromagnets and ferromagnetic rails
electromagnets which first have current flowing through them are placed on the underside of the carriage which creates a magnetic field that is attracted to the stationary ferromagnetic reaction rails that are installed on the underside of the guideway
this attraction results in a distance of just 10mm between the stator and support magnet, which sallows the carriage to hover 150mm above the top of the guideway --- levitation
propulsion is achieved using an onboard linear motor. the polarity of the stators at the track will quickly change its polarity continuosly to move the maglez train. stators at the side are excited sequentially. the electromagnets onboard 'chase' the current forward along the track via attractive and repelling poles
speed is controlled by the frequency of the alternating current that is fed into the stator coils
136
how does electrodynamic suspension (EDS) work?
EDS uses trains that have a looped superconductor on either side of the train. the superconductor is charged with electric current when it is madem and because it is looped (physically), current flows through it continously. this sets up a strong constant magnetic field
figure 8 shaped coils which are situated along the sides of the track experience a change in flux when the train is in motion, and hence a current is induced in these coils in such a way that repel the train from the bottom and attract the train from the top, causing the train to float
the train first starts by sliding on wheels, then these wheels are retracted like the landing gear of a plane because it needs motion in order to illicit changes in flux for levitation
additional superconducting electromagnets on the track serve to propel the train. these electromagnets are situated along all of the side of the track and are fed AC. these are often called levitation coils
magnets infront of the train attract the trains magnets while magnets on the track behind the train repel the train
by constanly changing the polarity of the tracks coils, the train is attracted and repelled in the same direction, causing the maglev to mvoe rapidly along the track
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why are superconductors vital for maglev trains?
- they are light, as compared with heavy permanent magnets which would provide the same strength
- conventional electromagnets would lose to much energy as heat waste due to electrical resistance
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what are the advantages of maglev trains?
- very high speed
- reliability
- safety
- minimal maintenance; little wear and tear
- low environmental impact
139
what would the advantages be by using superconductors in computers?
will result in little, if any, waste heat production. this is especially useful because a great deal of energy is lossed as heat in comps. heating also makes it difficualt for procesors to operate properly, as it changes the properties of the silicon presently used. superconductors would result in a far more efficient processor that can function at much high speeds
140
what could replace transistors?
superconducting quantum switches or SQUID (superconducting quantum interferne device), which would make processors faster still
141
what are the three components necessary in theconstruction of a cathode ray tube used in television displays and oscilloscopes?
- electron gun
- fluoroscent screen
- deflection plates/coils
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describe the function of an electron gun
consists of a cathod, anode collimator and heating filament, serving to produce a fast moving stream of electrons by thermionic emission
143
descirbe the function of deflection plates/coils in a cathod ray tube device
- used to change the direction of the electron beam by producing unidirectional electric and magnetic fields
- two sets of parralele plates or coils so that the cathode ray can be deflected both vertically and horizontally
144
describe the function of a fluorescent screen in a cathod ray tube device
-coated with layers of fluorescent material that emit light when struck by the moving electrons, the electrons sweep across the screen to form an image
145
how are electron guns, deflection plates/coils and a fluorescent screens used in conventional TV displays?
- three guns are used (red, green and blue)
- appropriate signinal is given to each gun to develop a suitable colour image
- deflection coils are used as they allow for greater delfection angles
- current in the coil varies so the beam can sweep across the screen 50 times a second
- fluorescent screen consists of fluorescent painted dots which emit light when excited
- cathod rays pass through coloured filters before striking the dots to give a colour image
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how are electron guns, deflection plates/coils and a fluorescent screens used in CRO's?
- one gun produces a single stream of electrons, focusing and directing them
- two pairs of deflection plates are used (one vertical and one horizontal)
- horizontal plates provide a time based voltage (they plot voltage vs time)
- vertical plates are given the input voltage (which measures amplitude)
- fluorescent screen traces cathode ray movement( visible output illustrative of voltage)
