topic two - from ideas to implementation
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Topic TwoFrom Ideas to ImplementationDot point summaries
DiscoveriesBy the beginning of the twentieth century, many of the pieces of the physics puzzle seemed to be falling into place. The wave model of light had successfully explained interference and diffraction, and wavelengths at the extremes of the visible spectrum had been estimated
The invention of a pump that would evacuate tubes to 10-4 atmospheres allowed the investigation of cathode rays X-rays would soon be confirmed as electromagnetic radiation
Patterns in the Periodic Table appeared to be nearly complete
Understanding of the atom
The nature of cathode rays was resolved with the measurement of the charge on the electron soon to follow. There was a small number of experimental observations still unexplained but this, apparently complete, understanding of the world of the atom was about to be challenged. Study of matter and electromagnetic radiationThe exploration of the atom was well and truly inward bound by this time and, as access to greater amounts of energy be available, the journey of physics moved further and further into the study of subatomic particles. Careful observation, analysis, imagination and creativity throughout the early part of the twentieth century developed a more complete picture of the nature of electromagnetic radiation and matter. Implementation
The journey taken into the world of the atom has not remained isolated in laboratories. The phenomena discovered by physicists have, with increasing speed, been channelled into technologies, such as computers, to which society has ever-increasing access. These technologies have, in turn, often assisted physicists in their search for further knowledge and understanding of natural phenomena at the sub-atomic level.RevisionImportant formulas:
During the nineteenth century, scientists:
Did not know the structure of an atom, i.e. they did not know about electrons
Heinrich Geissler invented a vacuum pump that was able to evacuate a glass tube to very low pressures.
Julius Plucker sealed a wire to either end of the Geissler tube and connected the tube to a high-voltage battery. He discovered that electricity passed through the vacuum. He also observed that as pressure was reduced in the tube, a green glow is visible around the anode.
Eugene Goldstein discovered that the green glow was the result of energy from the cathode. He called it cathode rays.
The problems studied and solved by Crooke:Cathode rays travel in straight lines
Problem: How do the cathode rays travel?Experiment: Used a bent tube OR the malthese crossObservation: Most intense glow was seen at the bendConclusion: Rays are travelling in straight lines
Cathode rays come from the cathode
Problem: Where do the rays come from?
Experiment: Placed a metal barrier (Maltese cross) in the centre of the tubeObservation: A shadow was cast away from the cathode. Conclusion: Rays came from the cathode. This also showed that the cathode ray had straight line propagation. This was inconclusive as it could be both a particle or a wave from this
Cathode rays are affected by magnetic fields
Problem: Were these rays affected by a magnetic field?Experiment: Brought a magnet close to the tubeObservation: The shadow of the cross movedConclusion: Cathode rays were affected by magnetic fields
Cathode rays are particles
Problem: Are cathode rays particles or waves?Experiment: Placed a light paddle wheel in the tube so that the beam struck one end of the paddleObservation: Wheel spins away from cathodeConclusion: Cathode rays are particles
Property of cathode rayThe wave theory (Hertz)The particle theory (Crookes)
Ability to penetrate solid objects metal foilsSimilar effect to X-rays passing through substancesAt the time, this could not be explained properly, though it is because electrons can penetrate substances as atoms are mostly empty space
Fluorescence produced when the ray strikes the gas or glassSimilar to the fluorescence produced by UV light in certain materialsCaused by the sudden deceleration of a charged particle, releasing energy
The apparent null effect of an applied electric fieldNo deflection was expectedCould not be explained until J.J. Thomson deflected the beam.
Affected by magnetic fieldsCannot be explainedThis happened because the particle has a negative charge
Travelled in straight linesEM radiation travels in straight linesParticles travel in straight lines
Mechanical effects able to turn the paddle wheelOne side of the wheel gets hit by photons. This energy makes that side of the wheel hotter, and causes the rotation The momentum caused by the fast moving particles as the collide into the paddle wheel causes the rotation
Controversies such as this one often occur in science when two models explain most but not all of a particular phenomenon. Debates centres around the relative merits of each model until more experimental results are available to resolve the controversy.
A cathode ray tube is an evacuated glass tube containing two electrodes. When a high voltage source is connected to the electrodes, streams of electrons flow from the cathode to the anode known as cathode rays. Electrons can be manipulated by:
Can either deflect electrons or accelerate them
Solid objects, which block the path of the rays (Maltese cross, paddle wheel)
Moving charges in magnetic fields experience a force which is perpendicular to the direction of motion and to the magnetic field. Because the force is perpendicular to velocity of the moving charge, it can change the charges direction, but not the charges speed.
Marys note to self: Imagine someone is running in a straight line. The wind is blowing at them from the side, changing their direction. However, this does not make them go faster or slower. If the direction of the wind is from behind, then the runner will speed up, and if the wind is blowing towards the runner, then the runner will slow down. Therefore, the velocity will only change if the force is not perpendicular to the direction of the moving charge.
This force is called the centripetal force and it produces a circular motion.
The equation for centripetal force:
The force on a charge moving through a magnetic field:
Equating these formulas:This is applicable only when the force is perpendicular to the velocity of the moving charge.
The radius is: Proportional to the mass of the charge
Proportional to the charges velocity
Inversely proportional to the charged particles charge
Inversely proportional to the magnetic field strengthWhen a charged particle enters a magnetic field at an angle, it will move in a helical motion. When a charge enters perpendicular to the field, the resulting movement is circular If a charge enters parallel to the field, there will be no force on the charge Helical motion results when a charged particle enters a magnetic field between these two components
F = qvB sin Where: F = the force on the charge (N)
q = the charge in coulombs (C)
v = the velocity of the charged particle in ms-1B = the magnetic field flux density in teslas (T)
= the angle between the velocity and the magnetic fieldThe force is: Proportional to the size of the charge
Proportional to the velocity of the charge
Proportional to the magnetic field density
Proportional to the singe of the angle between the velocity and the magnetic field direction
Positive point charge The electric field strength decreases as the distance from the point charge increases The direction of the electric field points away from the point charge
Negative point charge
The electric field strength decreases as the distance from the point charge increases
The direction of the electric field points towards the negative point charge
Positive and negative point charges
The electric field strength is strongest between the charges, and as the distance from the charges increase, the field strength decreases
The direction of the electric field is from the positive point charge towards the negative point charge
Equal and oppositely charged parallel plates The electric field strength between the parallel plates is uniform in strength and direction The field direction is at right angles to the plates, and away from the positive plate to the negative plate
The field lines on the edge of the plates bow out slightly
Electric fields and charges
If a charge is placed at a particular point, and it experiences a force, then an electric field exists at the point.
The direction of the electric field at a point is defined as the direction of the force that acts on a positive electric charge placed at that point.
A negative charge placed in an electric field experiences a force in a direction opposite to the direction of the field.
Electric fields are directed from high potential to low potentialElectric field strength (E) is the amount of force that a positive one coulomb charge would be sujected to at a particular point.
F = qE
F = force on charge in Newton (N)
q = charge in Coulomb (C)
E = electric field strength in Newton per coulomb
The SI unit of electric field strength is NC-1
Charged plates produce an electric field, as charged objects experience a force when brought close to the plates. Electrical Potential Difference
An electric charge placed in an electric field has electric potential energy. The SI unit for electric potential energy is joules.
When a positive charge moves in the field direction, its kinetic energy increases, and its potential energy decreases. If it is moved in the opposite direction, its potential energy increases.
Conversely, when a negative charge moves in a direction opposite to the field direction, its kinetic energy increases and its potential energy decreases. If it is moved in the direction of the field, its potential energy increases.
The electric potential difference, measured in volts.
The potential difference between the two points in an electric field is the change in electric potential energy per coulomb or charge. So, if a charge, +q moves between two points in a field then the potential difference between the points can be found using the following formula:
V = W q
Electric field strength between parallel plates:
Work done on point charge = Force x distance moved
Vq = (qE) d
V = Ed
Where:E = Electric field strength (Vm-1)
V = Potential difference (V)
d = Plate separation (m)
The electric field is: Proportional to the potential difference between the plates
Inversely proportional to the separation between the plates
Equal in magnitude at all points in the region between the plates
Perpendicular to the plates
Note: This equation only applies to the electric field between parallel plates because work done is constant
J.J. Thomson resolved the cathode ray debate when he performed a series of experiments which established the nature of cathode rays. Thompson discovered that: Cathode rays are actually deflected by electric fields
Cathode rays are streams of charged particles
J.J. Thomson explained why cathode rays were seemingly unaffected by electric fields. In Crookes earlier experiments, he observed that cathode rays were not deflected by electric fields. Thompson explained that:
1. The discharge tubes Crookes used contained impurities
2. The cathode rays were ionising the gas inside these tubes
3. The ions were then attracted to the plate with the opposite charge
4. The line up of these ions neutralised the charge on the plate
5. Cathode rays pass through unaffected
Thomsons experiments which settled the debate and measured the charge-mass ratio of an electronCathode rays are deflected by electric fields
Problem: Are cathode rays deflected by electric fields?Experiment: Created a narrow beam of cathode rays using a slit. Tried to deflect the beam with an electric fieldObservation: Beam was deflectedConclusion: Cathode rays are in fact charged particles
Cathode rays are deflected by magnetic fields
Problem: Are cathode rays deflected by magnetic fields?
Experiment: Created a narrow beam of cathode rays using a slit. Tried to deflect the beam with a magnetic fieldObservation: Beam was deflectedConclusion: Cathode rays are not a form of electromagnetic radiation
Cathode rays move more slowly than light
Problem: Do cathode rays move like light rays?
Experiment: Put the beam through both magnetic and electric fields simultaneously. He balanced the fields until no deflection was apparent.
At this point, the magnetic field force is equivalent to the electric field force: FB = FEqvB = qE
v = E B
Discovery: Cathode rays move more slowly than light
Conclusion: Cathode rays are not a form of electromagnetic radiation
The charge to mass ratio of electrons
Problem: What is the nature of the particles in cathode rays?Experiment: To determine the nature of these charged particles, J.J Thomson passed the beam through a magnetic field. This caused the beam to move in a circle. qvB = mv2 r
Bq = mv r
q = v e m Br
q = E e m B2r
By measuring the radius of the beam (r), the electric field (E) and the magnetic field (B) he was able to calculate a value for the charge to mass ratio of the particles. Discovery: The charge to mass ratio was 1.76 x 1011 Ckg-1. This was 1800 times smaller than the hydrogen ionConclusion:The value of charge to mass ratio was the same no matter what material was used in the cathode. This suggested that they are fundamental particles common to all atoms. In 1897 he proposed that cathode rays were negatively charged particles called electrons.Summary: Thomson accelerated a cathode ray though an electric field and magnetic field which were set up in such a way that their deflections would be in opposite directions. He adjusted the voltage of the electric field until the cathode ray was allowed to pass through undeflected. This meant the net force was equal to 0 and hence v = E/B. He then turned off the electric field and found the CR travelled in a circular path, thus eventually reaching the above equations.
(1) An ion of mass 5.0 x 10-26 kg, carrying a charge of -3.2 x 10-19 C is placed in a uniform electric field of strength 5000 V/m. Find:a) The force on the ion
q = -3.2 x 10-19E = 5000
F = qE
= -3.2 x 10-19 x 5000
= -1.6 x 10-15 Nb) The acceleration that this force would produce
m = 5.0 x 10-26 F = -1.6 x 10-15 F = ma -1.6 x 10-15 = 5.0 x 10-26 x a
a = -3.2 x 1010 ms-2c) The work done by the field to move the ion through a distance of 10cm
d = 0.1F = -1.6 x 10-15 Work done = Fd
= -1.6 x 10-15 x 0.1
= -1.6 x 10-16 J
(1) An oil drop of mass 6.8 x 10-6 g is suspended between 2 parallel plates, which are separated by a distance of 3.5 mm.a) What is the electric field strength between the plates, given that V = 110Vd = 0.0035V = 110
E = V d
= 3.14 x 104 V/m
b) What is the charge that must exist on the oil drop?m = 6.8 x 10-6 g = 6.8 x 10-9 kga = g
F = mg qE = mgq x 3.14 x 104 = 6.8 x 10-9 x 9.8 q = 2.12 x 10-12 C
(1) An electron moving at 20 000m/s enters a magnetic field of strength 0.1T, at right angles to the field. Given that an electron has a charge of -1.6 x 10-19 C and a mass of 9.1 x 10-31 kg, find the magnitude of the force on the charge in the field.
q = -1.6 x 10-19v = 20 000
B = 0.1
F = qvBsin = -1.6 x 10-19 x 20 000 x 0.1sin90 = 3.2 x 10-16 NThe cathode ray tube:
The electrodes in the electron gun: A filament linked to a power source heats the cathode The heat allows some electrons to escape (thermionic emission) The electrons are attracted by the anode, and accelerate towards it Two anodes accelerates the electrons to a greater speed
Through this, the electron gun can control the number of electrons passing throughThe deflection plates or coils Located between the anode and the screen
There are horizontal and vertical deflection plates
Voltage of the plates can be varied so that the beam could be focused up and down, and left to right
An appropriate combination of these deflections could move the beam to any point on the screen
The fluorescent screen When the electron beam hits the screen, it forms a bright spot as fluorescent emits light when high energy electrons strike it If deflection occurs rapidly, we see a line because our retinas allow vision to persist for a few milliseconds
When the electrons build up on the screen they begin to repel incoming charges, so a coating of graphite paint links the edge of the screen to the final anode, completing the circuit
The horizontal and vertical deflections are achieved by electric fields as they are required for its high speeds Only one electron gun is used for the colour of the screenThe Time base
The horizontal deflecting plates is linked to the time base, which sweeps the spot across the screen at a constant speed The frequency can be varied
High frequencies straight line
Low frequencies dots across the screen
The voltage pattern being studied
This is usually applied to the vertical deflection plates. If a steady DC voltage is applied, the time base line will move up or down the screen, depending on which plate is positive
If an AC voltage is applied, the spot will be moved up and down as well as sideways, resulting in a sine curve
The horizontal and vertical deflections are achieved by magnetic fields from pairs of current-carrying coils because they allow for a wider deflection angle The time base is fixed
Colour TV has three independent electron guns for red, green and blue
A DC induction coil that transformed the current into high voltages was connected to a series of discharge tubes. Each discharge tube contained different pressures of gas.
Concentration of gas (mmHg)Striation Pattern observed
40 Flashes of purple light at the cathode
Flashes of purple at the anode
Black in the middle
10 Bright purple at the anode/cathode
Pink stream of light in the body
6 Pink-purple body
A dark gap near the cathode
3 Pink glow at the cathode
Orange pink striations
0.55 Bigger gap
Orange pink body
0.14 Less striations
Do not touch the terminals when the induction is turned on to avoid electrocution
Stay at least 3m away due to minimise X-ray exposure
Turn on the equipment for a short amount of time
Beware of implosion
The different striation patterns of the cathode ray are due to the different pressures. At a high pressure where there are more gas molecules, the electrons are subjected to more collisions and thus lose kinetic energy. As electrons collide with surrounding atoms of the gas molecules, it causes glowing due to the excitation of the electrons in the valence band. As it loses kinetic energy, it is reaccelerated due to the potential difference set up and thus regains kinetic energy and undergoes further collisions with the other gaseous atoms/molecules. This forms a series of dark and bright regions observed in the tube
As pressure decreases, there are less gas molecules around, meaning less collisions. As a result the length of the dark spaces increases as observed. At very low pressure the electrons can travel through the tube without significant collisions until it strikes the glass and glows.
Background informationIn 1873 a Scottish physicist called James Clark Maxwell worked with magnetic and electric phenomena and found that a changing electric field and magnetic field resulted in a wave that could propagate through space which he called EM waves He predicted that:
There were a whole spectrum of electromagnetic field
These all travelled at the speed of light and had similar properties such as
Travelled in straight lines
Heinrich Hertz and Radio waves
Hertz was the first to confirm Maxwells prediction by producing EM waves of frequencies outside the visible spectrum. He set up a high voltage AC induction coil where he connected a transmitting antenna that consisted of two brass roads separated by a small gap. He then set up a circular detecting loop with also, a small gap
When hertz turned on the coil, a spark was visible in both the transmitter loop and the receiver loop that was not connected to any electrical supply. He reasoned that the spark in the transmitting antenna emitted a form of electromagnetic radiation that induced a current in the receiver loop, causing the spark
The photoelectric effect This is the phenomena that when light of a threshold frequency hits the surface of a metal, photoelectrons will be released. When hertz tried to make better observations, he placed the detector in a dark box, but this only caused the length of the spark to decrease. He then used a quartz prism to break up the light from the transmitter spark and discovered that the spark was made more vigorous when violet let was shone. He then followed through and radiated the spark with UV radiation, causing a more intense spark. He had actually discovered the photoelectric effect in this process but failed investigate it.
After discovering this new EM wave (which were radio waves) he then demonstrated they had properties similar to light.
They could be reflected and refracted using parabolic reflectors
No gap was produced in the second loop unless gaps were parallel, thus they were polarised
Could be diffracted from a prism made of pitch
They travelled at the speed of light, this was done by using the equation v = f He reflected the generated waves off a metal sheet by moving the receiver up and down at intermediate angles. This causes destructive and constructive interference which result in a standing wave. From this the wavelength of the standing wave The frequency of the wave was the same as the frequency of the oscillated current
A DC induction coil was connected to a transformer. A radio set to static frequency was placed 50cm away from the induction coil. The induction coil was turned off and on repeatedly.
This resulted in static or crackling noise when the induction coil was turned on. This was a result of the interference due to the radio waves produced by the induction coil being received by the radio antennae.
Classical PhysicsClassical physics refers to the works of physicists up until the end of the 1800s that include the works of Galileo, Newton and Maxwell.
Modern PhysicsThis is the work at the beginning of the 1900s that use the quantum theory for explanation and include physicists such as Einstein, Borhs, Lewis and Planck.
Black body radiation A black body is a perfect absorber and emitter of radiation. Therefore the emitted EM waves produced by a hot black body was entirely due to its own temperature, not because it reflected radiation form other sources
An example of a perfect black body is a hollow furnace with a cavity where the radiation is emitted from the cavity due to its temperature.
Using a black body allows us to determine the connection between the temperature of the body, the wavelength of the emitted Em waves and their intensity
Black body radiation is the radiation released by a black body in the form of EM waves. This is radiative cooling as the energy released cools down the object to that of its surrounding. According to classical physics, this was due to the oscillation of the atoms in a hot object. However it predicted that as the wavelength of the radiation decreased, the intensity would increase without limit. This became known as the UV catastrophe as it violated the law of conservation of energy
Experimental results show that the radiation emitted at each temperature has a large range of frequencies and at each temperature there is a peak amount of energy given off.
Note: The graph at 5000k represents the ideal black body of the sun. We see it as
yellow because it emits a dominant wavelength of yellowMax Planck Planck proposed a theory in 1900 that was able to reproduce the graphs above. His theory made an assumption that the energy of the oscillation of atoms were quantised, meaning that they were not just any value, but rather a discrete value or a multiple of this minimum value
The energy of each quantum is given by This theory is known as the quantum theory
The photoelectric effectThe photoelectric effect is the phenomena where photoelectrons are released from the surface of a metal when light of a threshold frequency hits the surface. The electrons absorb the energy from the incident radiation and thus overcome the potential energy barrier that normally confines them to the metal if the radiation has enough energy. This potential energy barrier is known as the work function of the substance. The work function of every substance differs depending on the material Classical physics suggested that
The kinetic of electrons from a metal surface were determined by the intensity of the incident light wave
The greater the intensity of light, the electrons from the surface of the metal should be ejected from the surface with a greater intensityExperimental results show:
The kinetic energy of the electrons ejected depends on the frequency of the light falling on the metal
The kinetic energy of electrons was not dependent on the intensity of light
The light intensity was found to affect the number of ejected electrons for light of a threshold frequency or greater.
If the light had a frequency lower, then no electrons were released, regardless of the intensity.
Einsteins explanation using the quantum theory:
Light is quantised, meaning it exists as discrete packets of energy called photons. The energy of these photons are dependent upon their frequency given by Plancks equation E = hf When a photon collides with an electron at the surface of a metal, the electron either absorbs all of the energy or none of it The intensity of the light is the amount of photons per square unit
This accounts for the experimental results of no electrons being released when greater intensity is shone since each photon still does not have the required energy
To produce the photoelectric effect where the electron absorbs the energy of the photon, the energy contained in the photon must be equal or greater tan then energy required to overcome the work function
The maximum kinetic energy emitted is equal to the initial energy minus the work function
KE max = hf work function
Ultimately Einstein was able to make a connection between black body radiation and the photoelectric effect by using and supporting Plancks quantum theory.
Einstein made a significant contribution to the quantum theory by using it and supporting it and thus giving it a real basis of acceptance. He incorporated Plancks quantum theory to propose photons and essentially explain the photoelectric effect. This model used Plancks theory by quantising light as existing in discrete small packets of energy given by E = hf, rather than a continuous stream of energy. Using this model, Einstein was able to establish the duality property of light as both a particle and a wave. This led to increased acceptance of Plancks ideas and the theory behind the black body radiation. Through this it changed the fundamental scientific thinking and impacted greatly on the scientific community
Einsteins model of light states
Light consists of particles called photons
The energy of each photon is quantised and given by E = hf
On collision with electrons, it follows the all or nothing principle
Through this light can be thought of a steam of particles as packets of energy that behave like particles, rather than a continuous stream.
c = f
E = hf
therefore E = hc/
E = mc^2, therefore hf = mc^2*This equation shows the dual properties of light as it has mass and frequency
Believed science research should not be removed from social and political forces
Was a pacifist and signed an anti-war counter manifesto
He saw the moral imperative in opposing Hitlers regime
His pacifist ideas were compromised when he encouraged the US to further research on the atomic bomb. This show how political and social forces impacted on his views as prolonging the war or allowing Germany to win would desist world peace
Believed science research is removed from social and political forces
Planck was a nationalist whom continued to operate his research for the benefit of his nation
He remained in Germany and signed a manifesto defending Germanys war conduct
Did not see the moral imperative in opposing Hitlers regime
His views impacted significantly on social and political forces in supporting any government of Germany and doing research on weapons even in hostile conditions.
A photocell is any device that converts energy from sunlight into electric energy. A photocell consists of an anode and a cathode coated with a photosensitive material. It uses the photoelectric effect to liberate electrons from the cathode which are then accelerated to a positive anode. This can be used for light meters in measuring the amount of light as the photo current would be directly proportional. .
Some electrons in solids are shared between atoms and can move freely because they are delocalised. These can conduct electricity through the lattice
The atoms held by these lattices are in strong covalent bonds as electron pairs are shared between atoms and are held by strong covalent bonds. This sharing means that electrons are held tightly and are not available to conduct electricity
Valance band this is the outer most energy band filled with the valence electrons that are still in bondsConduction band this is the energy band after the valence band that contains no electrons originally. When electrons move into this band they can easily conduct electricity
Energy gap this is the energy level that lies in between the valence and conduction band. An electron needs this quantum of energy before it can move into the conduction ban
Conductor In conductors, the valence band and the conduction band virtually overlap and thus there is no energy band gap. This allows for electrons to very easily pass through and conduct
Semi conductors In semi conductors, there is a small gap between the two bands such that if an electron gains enough energy through thermal or light energy, it may jump into the conduction band.
Note: As the temperature of a semi conductor is increased, the resistivity decreases because the electrons gain energy which allows them to pass through to the conduction band. The lattice is held very strongly by covalent bonds and thus does not impede the electron flow. However in conductors, increasing temperatures increases resistivity as it it makes the positive ions vibrate more rapidly causing more collisions
Insulators In insulators, there is a large energy gap where at normal temperatures, it is very difficult for electrons to move into the conduction band
In semi conductors, if an electron gains sufficient energy to jump to the conduction band, it leaves a hole. This then allows a nearby electron to move to this hole when a potential difference is supplied so that it leaves a hole behind. Thus another electron can jump over and leave behind its own hole and so on. Through this, the creation of holes allow the movement of electrons which is the equivalent to an electric current. Holes move in the opposite direction and are considered positive.
Thus both electrons and holes help to carry current. Electrons do so in the conduction band while holes do so in the valence band
On a subatomic level of the diagram on the left
Conductors many electrons in the conduction band
Semi conductors few electrons in the conduction band
Insulators the least electrons in the conduction band or even none
Transistors are devices that use semiconductors. Early transistors used germanium because it was able to be purified to a high degree as semi conductors require this. Even though silicon is more abundant (thus cheaper) it could not be used because there was no efficient method to industrially purify it to the extent needed for it to replace germanium
Advantages of silicon over germanium
More abundant, thus cheaper
Retains semi conducting properties at higher temperatures than germanium
Can handle higher electric currents as germanium broke down when too much current passed through it.
The process of adding a small amount of impurity to increase the conductivity of a semi-conductor is called doping that produces an extrinsic semi conductor.
N type semi conductor It is doped with a group V element as it has 5 valence electrons such as Phosphorus. This allows 4 electrons to participate in covalent bonds, while the excess electron automatically goes into the conduction band and conducts electricity.
P-type semi conductor It is doped with a group III element as it has 3 valence electrons such as boron. This allows the creation of holes that allows electrons from neighbouring atoms to move to this hole and create a hole current that increases the conductivity of a semi conductor
N typeP type
There are more negative charge carriers then positive holesIn p-type semiconductors there are more positive holes than negative charge carriers.
These negative charges are excess electrons in that are the major charge carriers in the conduction band and hence increase the conductivity These positive holes are the major charge carriers in the valence hand that increase its conductivity
Doped with group V elementsDoped with group III element
Note: even though there is a lack of electrons or an excess of electrons in a p/n type semiconductor, the overall charge is still neutral
Thermionic devices These are those that utilise thermionic emissions (the emission of electrons from a material that is heated to a sufficiently high temperature Thermionic devices that are used to control and amplify current are called thermionic valves (a device where electrodes are enclosed in a glass vacuum tube) Diodes these contain two electrodes and rectify current. They act as a switching device as electrons are only liberated when the anode is more positive than the filament. If AC is inputted, it will become DC as when the anode becomes negative, the electrons are repelled back and no current flows
Triodes these contain three electrodes and amplify current. They contain a third electrode in which a small increase in the input voltage draws many more electrons from the cathode filament which leads to a bigger output current
Solid state devices These are those that utilise the properties of semi conductors for the same purpose as the thermionic valves.
The positive terminal repels the holes towards the junction and the negative terminal repels the electrons towards the junction which allows the flow of current
When the current is reversed, the holes and electrons do not meet at the junction and so no electrons are carried across
A transistor is the solid state equivalent of a triode valve. It uses p-n-p or n-p-n that similarly amplifies current
Thermionic devicesSolid state devices
Large amounts of heat producedSmall around of heat produced
Consumed a lot of power and is expensive to useConsumes less power and is more economical
Slow due to warm up time neededVery fast
Requires a vacuum and an evacuated glass tube making it fragileDurable
Less reliable due to risk of damage and losing vacuum with a limited life spanIndefinite life span
Expensive to produceMass production is possible
Therefore for the above properties, solid state devices replaced thermionic devices for most applications.
Technology available in the 1940s included Radios
The shortcoming of these technologies included that they
Were physically bulky and fragile
Consumed a lot of power
Had valves which had limited life times and prone to burning out
Limited in response time
Unreliable and inefficient
It became recognised that thermionic valves were not efficient due to their shortcomings. This lead to the research for other materials that could replace them in their respective applications. This came across the discovery of semi conductors and the development of solid state devices. With further research, semi conductors went from germanium to silicon and then doping was invented, increasing the versatility of transistors and the scope of applications for which they can be used
The invention of the transistor has allowed for microchips and microprocessors to be developed. These have impacted profoundly on society as millions of transistors can be incorporated into one silicon chip for integrated circuits
This has lead to the impacts of
Miniaturisation of electrical appliances allowing for convenience, faster transfer, storage and processing
Increased the standard of living, e.g. the development of TV, cars, mobile phones
Faster communication using the telephone, fax, TV, internet
It has allowed for technologies that are capable of storing and processing more information at faster speeds such as in medical diagnosis, treatment machines, commerce, industrial designs and entertainmentSome negative impacts are
Loss of privacy
Invention of weapons for mass destruction
Assessment: Therefore the transistor has had a major positive impact on society by pushing it into the technological era with a higher standard of living for humans
Solar cells convert light energy into electrical energy using semi conductors with the photoelectric effect. A solar cell consists of a p-type and a n-type semiconductor joined to ech other. At the p-n junction, free electrons from the n-type diffuse to the p-type and fills the holes. This creates a potential difference as shown above.
When light hits the
N type, the electrons are liberated into the metal grid (external circuit) where they are attracted repelled by the potential difference and move
P type, the electrons are attracted by the potential difference and pass through the n type layer
There is a build up of electrons in the metal grid and when connected to an external circuit, a current will flow
The Braggs directed a collimated beam of X-rays towards a crystal. The X-rays were scattered by different atomic layers in the crystalline lattice onto a photographic film forming a diffraction pattern by interference. X rays were used because their wavelength which are approximate to the small gaps between atoms in crystals. Hence a crystal was found to be made of atoms arranged in a regular three dimensional pattern.
Assessment of their contribution
They are responsibility for creating the technique of X ray crystallography This allowed the work of other scientists to investigate the crystal structure of metals, inorganic compounds and ceramics.
Metals possess a crystal lattice structure made up of positive ions in a sea of delocalised electrons. The crystal lattice is defined by a repeated 3D unit where they are arranged in a geometrical pattern.
Metals have a large number of electrons in the conduction band relative to other materials. Electrons in the conduction band can move freely when an electric field is applied. These electrons are delocalised because they are shared by all the ions Chemical impurities disrupt the lattice integrity which impedes the free movement of electrons. Similarly, free electron movement is impeded by vibrations in the lattice. The vibrating lattice collides with free moving electrons which hence increase the resistance of the substance. Increasing the temperature of the metal increases the amplitude of the vibrations and thus a higher resistance. Lower temperature decreases the resistance; it was this idea that led to superconductivity
As the temperature of a metal lowers there will be less and less vibrations and thus the resistance lowers. Hence if the temperature continues to fall it will reach a point where the resistance will be absolute 0. At this point, electrons will be able to travel in pairs through the crystal lattice unimpeded by the any electrical resistance. There are two types of superconductors
Type 1 These are the pure metals whos critical temperature is close to 0K
Type 2 these are the metal alloys or oxides/ceramics which have a relatively higher critical temperature than type 1Critical temperature the temperature at which a substance exhibits superconductivity propertiesSuper conductivity this is the state where a substance has 0 resistance
The BCS theory explains conventional superconductivity
When a substance has reached its critical temperature, the electrons in the lattice form cooper pairs which allow them to bypass the obstacles in the crystal lattice which usually are responsibility for the resistance under normal conditions
As the first electron passes through the lattice, the lattice distorts due to the electrical attraction inwards and emits a phonon. This produces a temporary positive region which attracts the second electron in the pair
This pair absorbs the phonon and repels the first electron through the lattice
Then another electron from behind repels this electron and so on. Cooper pairs are constantly forming and broken down
The BCS theory gives a clear explanation of how electrons can move through a metal at its critical temperature unaffected by electrical resistance. However, it is limited because
This only applies to type 1 superconductors
It is simplified and doesnt go anymore into specific details
Advantages They have zero resistance and hence can carry large currents with no heat loss and thus allowing themselves to be used with 100% efficient power distribution, generation and storage
They will be able to carry high voltages with narrow and light weight conductors
New superconductive films may result in the miniaturisation and increased speed of computer chips Particle accelerators that use superconducting electro-magnets are cheaper to run because they use less electricity to produce the needed magnetic fields.This makes life more convenient and of a higher quality
However achieving and sustaining such low temperatures with current technology is too costly The materials of superconductivity are often brittle, hard to manufacture and difficult to make into wire
May be chemically unstable in some environments
Superconductors must be monitored to carry a current below its critical current density otherwise it will return into a normal conductor
MaterialTypeCritical temperature (oK)
Tin niobium alloyAlloy18
Meissner effect This is the phenomenon of magnetic levitation where a magnet is able to hover above a superconductor When a magnet is brought near a superconductor, there is a change in magnetic flux which induces a current on the surface of the superconductor by Lenzs law. These eddy currents will produce its own magnetic field which balances out the field from the magnet. This causes the levitating and since there is no resistance, the magnet will continue to hover as the current flows continuously
Maglev trains utilise magnetic levitation to create a frictionless motion at very high speeds. Their system minimises friction and the use of superconducting magnets are considered superior to conventional means due to less energy being wastedEMS uses conventional electromagnetic mounted under the train on structures that wrap around the guide way to provide lift and to create a frictionless running surface. This system is unstable due to the varying distances between the magnets and guide way. The lifting force is provided by the repulsive force between the electromagnets of the train and the guide wayEDS -uses superconducting magnets on the vehicle and electrically conductive coils in the guide way to levitate the train. This system requires very low temperatures and thus not practical
The maglev trains move by continuous series of vertical coils of wire mounted inside. When the train passes these coils, the motion of the train induces a current making them electromagnets and keep it moving due to the repulsion of the electromagnets inside the train.
ComputersSuperconductive film can be used as connecting conductors in computer chips. Since there is no resistance, they will not produce heat
They will reduce the size of microchips
Increase the speed at which microchips will work hence increasing the speed of information and operating systems
Impact on society
They will bring upon a new generation of technology Small devices can be made thus more convenient
Power generators Superconductors can be used to replace conventional wires in motors and generators
Smaller size and no need for iron core
Produce less noise pollution
Requires less maintenance
Impacts on society
Impacts greatly as the supply of electricity can increase
There will be less need for fossil fuels and thus reduce emission of green house gases
TransmissionThey can be used for transmission lines
Very efficient transmission lines as there will be no energy loss as heat
Reduces the need for new power stations
Large currents can be carried in relatively thin wiresElectron beam
Describe how doping a semiconductor can change its electrical properties
Identify that the use of germanium in early transistors is related to the lack of ability to produce other materials of suitable purity
Compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors and insulators
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