Download 1 - Optus

9.4 From Ideas to Implementation
1.
Increases understandings of cathode rays led to the development of television.
1.1
Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they
were charged particles or electromagnetic waves.
Wave model Travelled in straight lines.
Opaque objects had shadows.
Pass through thin foils with no damage.
Particle model-Rays left the cathode at right angles to the surface.
Deflected by magnetic fields.
Paddle wheels turned in the path, (they had mass and momentum.)
Travelled slower than light.
1.2
Explain that cathode ray tubes allowed the manipulation of a stream of charged particles.
Inside the cathode ray tube is a cathode and an anode target, which are separated by an extremely high
potential difference. As the electric circuit is turned on, the cathode rays (charged particles) will flow from
the cathode to the anode. The cathode ray tubes allowed the streams to be manipulated as other apparatus
could be placed in and around the tube itself.
1.3
Identify that moving charged particles in a magnetic field experience a force.
Charged particles experiences forces such that: F = qE
&
F = qVB sinθ
1.4
Identify that charged plates produce an electric field.
Electric fields exist when an electrically charged object experiences a force. When two charged plates
interact they experience a force, thus producing an electric field. The field between them is uniform, except
at the edges where it bulges.
1.5
Describe quantitatively the force acting on a charge moving through a magnetic field F=qvBsin θ
F = The force acting on a charge moving through a magnetic field. (N)
q = The charge on an electron, –1.602 x 10–19 C
v = velocity of charge (m/s)
B = Magnetic Field Strength (T)
θ = Angle of charge to field.
1.6
Discuss qualitatively the electric field strength due to a point charge, positive and negative charges
and oppositely charged parallel plates.
Point charge has strongest electric strength where field lines are closest together, i.e. closer to the charge.
Negative acts towards the charge, positive acts away from the charge. Oppositely charged plates produce
uniform electric field travelling from positive to negative, bulging at the sides.
1.7
Describe quantitatively the electric field due to oppositely charged parallel plates.
The strength of an electric field, E, produced by charged parallel plates
separated by a distance, d, and charged by a voltage, V.
1.8
Outline Thomson’s experiment to measure the charge/mass ratio of an electron.
Cathode rays passed between charged plates. The direction of the rays moved towards the positive plate.
Showing the rays were negatively charged. Applying same strength magnetic field and determining the
radius of the circular path of the electron he could figure out the charge to mass ratio.
1.9
Outline the role of - in the cathode ray tube of conventional TV displays and oscilloscopes.
-electrodes in the electron gun. The electron gun produces a narrow beam of electrons. The electrodes in the
gun accelerate the electrons.
-the deflection plates or coils. The deflection plates or coils establish an electric field that controls the
deflection of the electron beam from side to side and up and down
-the fluorescent screen Covered in substance that emits light when high energy electrons strike it
2.
The reconceptualisation of the model of light led to an understanding of the photoelectric effect
and black body radiation.
2.1
Describe Hertz’s observation of the effect of a radio wave on a receiver and the photoelectric effect
9.4 From Ideas to Implementation
he produced but failed to investigate.
Hertz set up an induction coil which sparked across a loop, a second coil was placed a small distance away
and the spark was seen to appear in this one too. Hertz hypothesized that the sparks set up changing electric
and magnetic fields that propagated as an EM wave. Hertz observed that the gap in the detector could be
made larger and still generate sparks when the radiation from the transmitting spark shone directly into the
gap in the detecting loop. Hertz did not recognise that the UV component in the transmitter spark removed
free electrons from the surface of the metal, thus allowing the discharge (spark) to occur across a wider gap.
2.2
Outline qualitatively Hertz’s experiments in measuring the speed of radio waves and how they relate
to light waves.
Hertz measured the time taken for the spark in the detector to appear in the induction coil, using a
determined frequency from an oscillating circuit and a measured wavelength, as determined by interference,
and found it was the speed of light.
2.3
Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity
is quantised.
His theory said that an oscillating charge can only have discrete values for frequency. The
temperature-wavelength relationship forms the basis of quantum theory. The small packets of energy
were called photons and contributes to the particle theory of light. When the frequency is limited to
discrete amount, it is referred to as being "quantised".
2.4
Identify Einstein’s contribution to quantum theory and its relation to black body radiation.
Einstein said energy associated with the radiation from a black body is concentrated in packets of energy
called photons. A photon is the smallest amount of radiation energy possible at a particular frequency. A
photon cannot transfer part of its energy, it can only transfer all of its energy or none of it. The amount of
energy carried by a photon is proportional to its frequency. The intensity of light is proportional to the
number of photons. The energy possessed by a photon is proportional to its frequency, hence the observation,
in relation to black body radiation, that the shorter the wavelength (thus the higher the frequency) the greater
the total energy radiated (for a given temperature).
2.5
Explain the particle model of light in terms of photons with particular energy and frequency.
The particle model of light explains that light is composed of photons. These photons can be
considered as discrete packets of energy. They have a particular energy and frequency related by the
relationship. E = hf
2.6
Identify the relationships between photon energy, frequency, speed of light and wavelength: E = hf
and c = f λ
The energy of a photon is given by the relationship E = hf, where: E is the energy of the photon in joules (or
electron volts), h is Plank's constant: 6.6 X 10-34 J s, f is the frequency of the light in hertz (seconds-1).
The speed of light is given by the relationship c = f λ, where c is the speed of light: 3 x 108 m s-1 , f is the
frequency of the wave, λ is the wavelength of the wave.
By combining the two equations,
3.
Limitations of past technologies and increased research into the structure of the atom resulted
in the invention of transistors.
3.1
Identify that some electrons in solids are shared between atoms and move freely.
Solids which are conductors have metallic bonds in which the valence electrons are readily shared.
3.2
Describe the difference between conductors, insulators and semiconductors in terms of band
structures and relative electrical resistance.
Conductors have no gap between valence and conduction bands, therefore a low resistance.
9.4 From Ideas to Implementation
Semi-conductors have small energy gaps between bands, conduction increases with heat.
Insulators have a large energy gap between bands making conduction very low and thus resistance very high.
3.3
Identify absences of electrons in a nearly full band as holes, and recognise that both electrons and
holes help to carry current.
When electrons are absent from the valance bad they are know as holes. Holes act as positive charge carriers
and move in the opposite direction to electron drift. Electrons and holes carry current.
3.4
Compare qualitatively the relative number of free electrons that can drift from atom to atom in
conductors, semiconductors and insulators.
Conductors contain high numbers of free electrons in the conduction band. Under normal conditions,
insulators and semiconductors have far fewer free electrons than conductors.
Raising the temperature, using certain lighting conditions or applying a potential difference, can induce
electrons in some semiconductors to move into the conduction band.
3.5
Identify the use of germanium in early transistors is related to lack of ability to produce other
materials of suitable purity.
Germanium was widely used as a semi-conductor because it was easier to purify than other known semiconductors, such as silicon. Silicon has since replaced the germanium as semi conducting material of choice
in transistors because; it is the second most abundant element on earth by weight, which means it is
relatively cheap, it can handle higher electric currents before overheating, it forms an oxide that can be doped
and made into thin, flat layers processing techniques were developed to produce very pure, single crystal
forms, giving it consistant properties.
Describe how ‘doping’ a semiconductor can change its electrical properties.
Doping is a process of enhancing conductivity of a semiconductor. A tiny amount of an impurity is
placed in a crystal lattice to alter its electrical properties at a ratio of about one part per million. Doping
increases the potential conductivity of the semiconductor as the dopant contains either one more or one less
valence electron, thus giving it extra charge carriers (electrons or holes.)
3.6
3.7
Identify differences in p and n-type semiconductors in terms of the relative number of negative
charge carriers and positive holes.
In n-type semiconductors electrons carry the majority of current and holes are minority carriers as
the dopant has one or more extra valance electrons.. In p-type semiconductors positive holes
conduct the majority of carriers as the dopant has one less or more valance electrons .
3.8
Describe differences between solid state and thermionic devices and discuss why solid state devices
replaced thermionic devices.
Solid state devices are made from semiconductor materials. Thermionic devices relied on the emission
of electrons due to high temperatures. A filament would act as a cathode when it released electrons,
and the high potential difference will accelerate the electrons, eg, valves and early diodes.
The combined advantages of smaller size, simpler and cheaper construction, lower power requirements and
speed of operation make solid state devices much better than equivalent thermionic devices.
4.
Investigations into the electrical properties of particular metals at different temperatures led
to the identification of superconductivity and the exploration of possible applications.
4.1
Outline the methods used by the Braggs to determine crystal structure.
The Braggs experiment utilised x-rays reflected from adjacent atomic planes within the crystal lattice. The
reflected x-rays interfered constructively and destructively. Measurements of the angles allowed the spacing
and arrangement of the crystal to be determined.
4.2
Identify that metals possess a crystal lattice structure.
Metals posses a crystal lattice structure that consists of an ordered array of metallic atoms.
4.3
Describe conduction in metals as a free movement of electrons unimpeded by the lattice.
In metals the outer, valence electrons are easily displaced from their atoms and move randomly between the
9.4 From Ideas to Implementation
atoms. They usually occur as positive ions with these electrons moving from ion to ion within the lattice.
The random motion of the electrons is caused by heat energy in the metal.
4.4
Identify that resistance in metals is increased by the presence of impurities and scattering of
electrons by lattice vibrations.
When electrons collide with the crystal lattice structure, they will lower the amount of current traveling
through the metal. This is because the lattice vibrations often impedes the path of the electrons.
Resistance in metals is increased by impurities.
4.5
Describe the occurrence in superconductors below their critical temperature of a population of
electron pairs unaffected by electrical resistance.
Below their critical temperature, superconductors will have a population of electron pairs, which are
unaffected by electrical resistance. They are known as Cooper pairs; which act like single particles with
properties very different from those of single electrons. These Cooper pairs are utilised in the BCS
theory.
4.6
Discuss the BCS theory.
The BCS theory explains superconductivity in terms of electron pairs and packets of sound waves related to
lattice vibrations (called phonons). Below critical temperature, movement of electrons is enhanced by
phonons which cause electric field effects resulting in electron pairing (by overcoming what would normally
be strong repulsive forces between like charges) and an assisted passage through the lattice with negligible
energy loss.
4.7
Discuss the advantages of using superconductors and identify limitations to their use.
Below critical temperature, cooper pairs stay together. Because resistance is effectively zero, very narrow
wires can carry very large currents. The lower the temperature, the higher that current can be. That current
produces a magnetic field around the conductor. The strength of the magnetic field will reach a point where
it will cause the loss of the superconducting state thus putting an effective limit on the current that can flow
in any particular superconductor. The practical application of superconductors is based on the combination
of critical temperature (Tc the point below which superconductivity occurs), the critical field (Hc the strength
above which superconductivity is stopped) and the current density (Jc above which superconductivity
ceases).