Download Ideas to Implementation by Ian Wilkinson

HSC Physics Module 9.4 Summary
1. Increased understanding of cathode rays led to the development of television
Identify that moving charged particles in a magnetic field experience a force
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Recall that a current carrying conductor experiences a force in a magnetic field, and that
current is the movement of charges
The moving charges in the conductor in the magnetic field cause the conductor to
experience a force
Generally, any charge or collection of charges moving relative to a magnetic field will
experience a force
Identify that charged plates produce an electric field
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Recall that an electric field is a region where charged objects will experience a force, and
that a point charge or collection of charges will produce an electric field
When two charged plates are brought together, they will exert a force on each other, thus
an electric field exists between the plates
See below for diagrams of the electric field between two oppositely-charged plates.
Describe quantitatively the force acting on a charge moving through a magnetic
field
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As mentioned above, a charge moving through a magnetic field will experience a force
The magnitude of the force a moving charge experiences in a magnetic field is given by:
where:
o F = Force [N]
o q = Charge [C] Note: 1 coulomb = 6.24x1018 elementary (electron) charges
o v = velocity [ms-1]
o B = magnetic field strength [Tesla]
o Θ = angle between the velocity and the direction of the magnetic field lines
The term sinΘ is included because only the component of the charge’s velocity
perpendicular to the magnetic field is subject to a force
The direction of the force can either be given by either the right-hand-palm rule or general
right-hand rule
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For the right-hand-palm rule, point the thumb in the component of velocity of a
POSITIVE charge perpendicular to the magnetic field (opposite direction for a
negative charge), and the fingers in the direction of the magnetic field. The palm will
point in the direction of the force.
For the right-hand rule, point the fingers in the component of velocity of a positive
charge perpendicular to the magnetic field, and curl fingers in the direction of the
magnetic field. The thumb will point in the direction of the force.
Discuss qualitatively the electric field strength due to a point charge, positive and
negative charges and oppositely charged parallel plates
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The strength and direction of electric fields can be represented by flux lines.
The strength of an electric field is the relative magnitude of force a charged particle would
experience in the field
The direction of an electric field is defined as the direction a positive charge would
experience a force in the field
o Thus a negative charge would move in the opposite direction to direction of the flux
lines if it were placed in an electric field
A number of rules apply to the interpretation of electric field diagrams using flux lines:
o Flux lines begin on positive charges and end on negative charges
o Flux lines never cross
o Flux lines enter and exit at right angles only
o Flux lines that are close together represent strong fields
o Flux lines that are well-separated represent weak fields
o A negative charge placed in the field will experience a force in the direction opposite
to the arrow of the flux lines
FIELDS OF POINT CHARGES
The field strength of a point charge obeys the inverse square law, thus the field strength
decreases proportionally to the inverse square of the distance to the charge
o Thus we can consider the electric field around a point charge to be radial
Point charges with a stronger charge produce stronger electric fields, which are represented
by closer field lines in electric field diagrams
When multiple charges are placed closed together, the resultant field is the superposition of
the fields of each charge
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The diagram below shows the electric fields of point charges of equal/opposite charge
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The diagram below shows the superposition of electric fields of charges of different
magnitudes of charge
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FIELDS OF OPPOSITELY-CHARGED PARALLEL PLATES
Oppositely-charged parallel plates, also known as parallel-plate capacitors, produce uniform
electric fields
If the area of the parallel plates is significantly greater than the distance between the plates,
the resulting electric field will be uniform, except at the edges where it slightly bulges
When drawing the electric field between two parallel plates, remember the following:
o The flux lines must be evenly spaced
o The flux lines should bulge at the edge of the plates
o The flux lines should go from the positively-charged plate to the negatively-charged
plate
Below is a diagram of the field between two oppositely-charged parallel plates
Describe quantitatively the electric field due to oppositely charged parallel plates
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Recall that the magnitude of an electric field is equal to the force per unit charge at a point
in the field, given by
OR
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Where
o E = electric field strength [NC-1]
o q = charge [C]
o F = force on the charged object [N]
Recall that voltage is the change in potential energy or work done on charge per unit charge,
given by
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OR
But work also equals force multiplied by distance. Therefore…
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Equating the two expressions for work, we get
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Dividing be q and rearranging we get
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Where
o E = electric field strength in between the plates [Vm-1]
o V = potential difference between the plates [V]
o d = distance between the plates [m]
Thus the electric field between two oppositely-charged parallel plates can be calculated by
considering the potential difference and distance between the two plates
When using the above formula, quote the units as Vm-1 rather than NC-1. Whilst both units
are equal, quote the units for field strength according to the equation used.
Thus we can see from the above formula that the electric field strength is…
o Proportional to the potential difference between the plates
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Inversely proportional to the distance between the plates
Equal at all points between the plates
Perpendicular to the plates everywhere in the region between the plates
Solve problems and analyse information using:
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ALWAYS specify the direction of any vectors, including force, electric field, and magnetic
field
Ensure all calculations include dimensions and are dimensionally correct
IMPORTANT NOTES
Use your right-hand when working out the direction
Crosses mean that the field is INTO the page, points mean OUT of page
When calculating the force an electron experiences in either a magnetic field, reverse the
direction of velocity when working out the direction of force
When calculating the force an electron experiences in an electric field, ensure the force is in
the direction opposite to a magnetic field
Explain why the apparent inconsistent behaviour of cathode rays caused debate as
to whether they were charged particles or electromagnetic waves
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Cathode rays were observed during the 19th century in vacuum tubes with two electrodes
inside and a voltage applied. Their presence was detected by the glowing glass opposite of
the cathode (negative electrode).
Scientists determined that the glow was due to a ray emitted from the cathode (hence their
name cathode rays), but their properties were inconsistent with both particle and wave
motions.
The conflicting observations led physicists to become divided on whether cathode rays were
particles or electromagnetic waves
o For example, Crookes demonstrated that cathode rays were deflected by magnetic
fields (which supports the particle model), but Hertz showed that cathode rays
weren’t deflected by electric fields (which supports the wave model)
o Hertz’s experiment was later shown to be flawed however, as Thomson
demonstrated the deflection of cathode rays due to an electric field by using a more
complete vacuum than Hertz. At higher gas pressures, the cathode rays ionised the
gas, which were attracted to the oppositely charged plates, and neutralised the
charge on the plates, thus the rays weren’t deflected in Hertz’s experiment.
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The following observations supported the wave model:
o Cathode rays travelled in straight lines
o If an opaque object (such as a Maltese cross) was placed in their path, a shadow of
that object appeared
o They could pass through thin metal foils without damaging them
The following observations supported the particle model:
o The rays left the cathode at right angles to the surface
o They were deflected by magnetic fields
o Small paddlewheels turned when placed in the path of the rays, showing they had
momentum and thus mass
o They travelled considerably slower than light.
The apparent inconsistencies of the behaviour of cathode rays were due to the inadequacies
of experimental design and the current state of knowledge about the nature of atoms.
o The atom was later shown to be predominately empty space, so small electrons
could pass through metal foils without causing damage
Thomson showed that the rays were deflected towards the positively charged plate, thus
demonstrating that they were negatively charged particles
Further experiments showed that cathode rays were a stream of electrons
The resolution of the inconsistencies of the behaviour of cathode rays is an example of the
scientific method, i.e. observations from experiments are interpreted and a hypothesis
developed to explain what is thought to be happening. Opposing models are then resolved
through improved experimentation, allowing us to gain a greater understanding of the
nature of cathode rays.
Explain that cathode ray tubes allowed the manipulation of a stream of charged
particles
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A cathode ray tube is a highly-evacuated, sealed glass tube containing two electrodes.
o The negatively-charged electrode is called the cathode, whilst the positively-charged
electrode is called the anode
o Remember that cations (positive ions) are attracted to the cathode, and anions
(negative ions) are attracted to the anode
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Applying a high voltage across the tube causes cathode rays to be produced, which are
streams of negatively-charged particles (electrons) to flow from the cathode to the anode,
with little obstruction from collisions with remaining gas particles
As cathode rays are negatively-charged, they can be deflected by applying an external
electric or magnetic field
In addition, placing solid or otherwise objects in the path modifies the path of the beams
For example…
o Applying an external electric field deflects cathode rays to the positive plate,
demonstrating that they are negatively charged (parabolic deflection)
o Applying an external magnetic field deflects the cathode rays perpendicularly to the
magnetic field (circular deflection)
o Placing solid objects in the cathode ray tube inhibits the movements of cathode rays
 A Maltese cross produces a shadow
 A paddlewheel rotates when placed in between the anode and cathode
Outline Thomson’s experiment to measure the charge/mass ratio of an electron
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Many of Thomson’s experiments centred on his study of cathode rays, such as his
demonstration of the deflection of cathode rays due to an electric field
Thomson was also able to determine the charge to mass ratio of an electron through the
analysis of cathode rays
His experimental set-up is shown below:
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The glass tube is sealed and at reduced pressure
The cathode rays are emitted from the cathode (negatively-charged electrode)
The anodes consist of charged plates with thin slits => they act as anode collimators,
as the cathode ray accelerates towards the plates, passes through the slit, and
enters the main tube as fine and well-defined beam
o The charged plates produce an electric field (in exams, mark which one is positive,
which is negative, and the direction of the field)
o The coils act as electromagnets, and produce a magnetic field
o The fluorescent screen allows the cathode rays to be detected
His experiment consisted of two steps:
o He first varied the magnetic and electric fields until their opposing forces cancelled,
leaving the cathode ray undeflected. By equating the magnetic and electric force
equations, Thomson was able to determine the velocity of the cathode-ray particles.
o
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He then applied the same strength magnetic field (alone), and determined the
radius of the circle path travelled by the charged particles in the magnetic field
As v had already been calculated, and r and B can be measured (r by careful observation),
Thomson was able to calculate the charge-to-mass ratio of the particles in the cathode ray
His experiment demonstrated that cathode rays were particles, because if the cathode ray
has a charge-to-mass ratio, the cathode-ray particles must have a measurable mass
He calculated that all cathode-ray particles (electrons) had a charge-to-mass ratio of
1.76x1011Ckg-1, regardless of the cathode material, gases, or other conditions
o This indicated that the cathode-ray particles (electrons) were common to all
materials, which was one piece of evidence indicating that atoms were made of
subatomic particles
The calculated charge-to-mass ratio was over a thousand times higher than that of a
hydrogen ion (H+), suggesting that the particles were either very light or very highly charged.
o Such results contributed to the development of Thomson’s plum-pudding model of
the atom
Outline the role of:
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electrodes in the electron gun
the deflection plates or coils
fluorescent screen
in the cathode ray tube of conventional TV displays and oscilloscopes
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Below is a diagram of the cathode ray tube used in oscilloscopes, which is similar to that
used in TV displays with a few differences (see below)
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The three primary components of the cathode ray tube used are the electron gun, the
deflection plates or coils, and the fluorescent screen.
ELECTRON GUN
The electron gun consists of the cathode, anode collimator, and heater
The cathode emits the cathode rays, which are accelerated towards to the multiple anodes,
and then travel into the deflection part of the tube as a fine, well-defined beam
An electrode grid in between the cathode and electrode is used to control the number of
electrons reaching the anode, as the grid can be made more positive or negative. This
controls the brightness of the display
The heater heats up the cathode, which releases many free electrons that can be easily
accelerated towards the cathode => this is called thermionic emission
DEFLECTION PLATES/COILS
The deflection plates or coils produce a unidirectional electric or magnetic field respectively
to deflect electrons vertically or horizontally to produce a useful display on the screen
o Oscilloscopes use plates to produce an electric field, whilst televisions use coils to
produce a magnetic field
There are two sets of parallel plates/coils, each set perpendicularly to each other, so that
they can deflect the beam both in the vertical and horizontal direction on the screen
according to the voltage applied to the plates/coils
Thus the deflection plates/coils allow the cathode ray to be deflected to any position on the
screen
FLUORESCENT SCREEN
The fluorescent screen is coated with layers of fluorescent material, which emits light when
high energy electrons strike it
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This allows the position of the beam to be seen where it strikes the screen, and allows a
useful image to be formed.
Application to the oscilloscope
The CRO (Cathode-Ray Oscilloscope) is a diagnostic tool that allows voltage to be plotted
against time
The horizontally-deflecting plates (X-plates) supply a time-based voltage, so the beam
sweeps horizontally across the screen in time intervals that can be controlled as desired
The vertically-deflecting plates (Y-plates) are connected to the input voltage (which is
amplified as necessary), so their deflection allows the voltage of the input to be measured
against time
Application to TV displays
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A colour TV display contains three electron guns, each corresponding to the different colours
of red, blue and green
The image sent by the signal to the TV is reconstituted on the screen by an additive process
involving three coloured phosphors corresponding to red, blue and green.
A shadow mask is used on the screen to ensure the beam from each colour gun only hits the
corresponding spot on in each pixel, thus forming the correct image
The TV display use deflection coils (which produce magnetic fields) to deflect the beam
The horizontally- and vertically-deflecting coils are connected to a time-based voltage that
scans each line of pixels on the display 50 times a second
The phosphors glow for a short time, so no flickering is observable to the retina
The colour of each pixel is controlled by the intensity of the beam striking its corresponding
phosphor, thus an image is formed
Perform an investigation and gather first-hand information to observe the
occurrence of different striation patterns for different pressures in discharge tubes
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METHOD
An induction coil was connected to a power supply to supply the high voltages required.
Discharge tubes of different pressures were connected to the induction coil, one at a time,
and the striation patterns that formed were observed and recorded
SAFETY:
Take care when handling the discharge tubes, because the low pressures can cause them to
easily IMPLODE (not explode)
The sparks produced emit X-rays, which are potentially dangerous for prolonged exposure.
Stand at least 2m away from the coil whilst it is turned on (consider the inverse square law),
and only turn the coil on for short periods of time (no more than 5 seconds) for observation
The sparks also produce ozone, which can aggravate respiratory problems => conduct the
experiment in a well-ventilated area, and again only turn the induction coil on for short
periods of time
RESULTS
Pressure (mmHg) Striation pattern observed
40
 Flashes of purple at anode and cathode
 Black in the middle
10
 Bright purple at anode and cathode
 Pink stream of light in the body
6
 Purple light at anode and cathode
 One dark gap near cathode
 Pink-purple body
3
 Pink anode
 Purple flashes at cathode
 Pink-orange body with striations
0.14
 Pink-purple glow at anode and cathode
 Purple body
 Less striations
 Glass fluorescing
0.03
 Tube is dark except for the purple fluorescence behind the
anode on the glass
The glass fluoresced when there was low pressure because there were almost no gaseous
molecules for the cathode ray to collide with, so it continued moving up to the glass where it
was able to fluoresce it
ACCURACY/RELIABILITY/VALIDITY
The dot point only asks for observations, so accuracy, reliability and validity are not
particularly relevant beyond usual comments
Perform an investigation to demonstrate and identify properties of cathode rays
using discharge tubes:
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containing a maltese cross
containing electric plates
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with a fluorescent display screen
containing a glass wheel
analyse the information gathered to determine the sign of the charge on the
cathode rays
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METHOD
An induction coil was connected to a power supply to supply the high voltages required.
Four discharge tubes, one containing a Maltese cross, one containing electric plates, one
containing a fluorescent display screen, and one containing a glass wheel were connected to
the induction coil, one at a time. For the one containing the fluorescent screen, bar magnets
were placed around the cathode ray tube, and any observations were noted
SAFETY: See practical above
RESULTS
Discharge tube
Observation
Maltese cross
A shadow of the Maltese cross was projected at the
glass near the anode
Electric plates
The cathode ray was deflected towards the positive
plate, detected by the position of the fluorescence
near the anode
Fluorescent screen and bar magnets The screen fluoresced when the power supply was
turned on, leaving a clear trace of itself. The ray was
deflected by the presence of bar magnets, as
detected on the fluorescent screen
Glass wheel
The wheel began to spin when the power supply was
turned on
The shadow of the Maltese cross indicated that cathode rays travel in straight lines
The deflection by the magnetic field and the rotation of the glass wheel indicated that
cathode rays have momentum, and thus have mass
The deflection of the cathode rays towards the positively-charge plate indicated that
cathode rays are negatively-charged
ACCURACY/RELIABILITY/VALIDITY
The cathode ray tubes had very low pressure, thus the deflections observed were accurate
and valid
The induction coil was turned on and off several times for each tube, and the results
matched expected results, so the results were reliable
Validity could have been improved by using different discharge tubes, and showing the same
effects
2. The reconceptualisation of the model of light led to an understanding of the
photoelectric effect and black body radiation
Outline qualitatively Hertz’s experiments in measuring the speed of radio waves
and how they relate to light waves
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Maxwell’s developed his theory of electromagnetic waves, in which he proposed that visible
light propagated by oscillating electric and magnetic fields without a medium. His equations
and calculations implied the following predictions:
o Electromagnetic waves could exist with many different frequencies
o All electromagnetic waves propagate through space at the speed of light
(3.11x108ms-1)
Hertz conducted a series of experiments to test and verify these predictions. He produced
radio waves by connecting an induction coil to a spark gap and applied a high voltage, which
caused current to oscillate across the gap and the rods, thus producing electromagnetic
waves.
He observed that when a small length of wire, bent into a loop with a small gap, was placed
near the sparking induction coil, a spark would synchronously jump across that coil too.
Since the loop was placed several metres away from sparking induction coil, he concluded
that radio waves caused the sparking in the second coil, thus he was able to use this as a
detector of radio waves
Hertz determined the frequency of the radio waves from the frequency of oscillation of the
electric current, as these frequencies were equal
He was able to determine the wavelength of the radio wave by reflecting the waves off a
large flat zinc plate to produce a standing wave. A spark in the detector was produced at the
anti-nodes of the standing wave, but not at the nodes, so he was able to determine its
wavelength
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As v=fλ, he was able to determine the velocity of the radio wave using the wavelength and
pre-determined frequency. He calculated the speed of light to be 3x108ms-1, which was close
to Maxwell’s prediction (3.11x108ms-1) and the experimentally measured speed of light
Hertz also showed that radio waves could be refracted by passing them through a large
asphalt prism. He further showed that they could be polarised, as the detector loop sparked
whilst parallel to the detector, but did not when perpendicular to the detector.
Thus he showed that radio waves behaved similarly to light waves in terms of velocity,
reflection, refraction, interference and polarisation, and concluded that light and radio
waves were parts of the broad electromagnetic spectrum
Describe Hertz’s observation of the effect of a radio wave on a receiver and the
photoelectric effect he produced but failed to investigate
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During his investigation into the properties of radio waves, Hertz observed that the sparks
were easier to produce in the detector loop when it was illuminated with ultraviolet light
When he placed a glass panel between the transmitter and receiver, the maximum spark
length of the detector loop reduced, as glass absorbs UV light produced by the spark. When
he placed quartz there instead, the spark length was unchanged, as quartz does not absorb
UV light.
He also placed the entire apparatus in a darkened box, and noted that the maximum spark
length decreased.
His observations were caused by the photoelectric effect, which is the ejection of electrons
from the surface of a polished metal surface when light is shone on it.
The UV light caused electrons to be ejected to the surface, which made it easier for them to
jump across the spark gap when subject to the radio waves. Hertz did not pursue
investigation of this effect however, nor did he make any attempt at explaining how the
phenomenon was brought about.
Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a
black body cavity is quantised
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A black body is an idealised body that absorbs all incident electromagnetic radiation, and reemits the radiation in a spectrum characteristic to its temperature
A black body curve of the radiation emitted is shown below:
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According the laws of thermodynamics in classical physics, as the wavelength of the black
body radiation decreased, the radiance would increase to an infinite amount. This was
named the ‘ultraviolet catastrophe’ as it could not be reconciled with the laws of
conservation of energy, thus an explanation had to be found.
To reconcile this problem, Planck proposed that the radiation absorbed and emitted from a
black body is not continuous as waves, but is emitted as discrete packets of energy called
quanta (now known as photons). The size of each quantum of energy is characteristic of the
frequency of light emitted.
Identify Einstein’s contribution to quantum theory and its relation to black body
radiation
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Einstein extended the concept of quantised energy to light waves, reasoning that the energy
from light waves may be treated as a stream of discontinuous quanta, called photons.
He proposed that black body radiation was emitted as discrete quanta of energy, not as
continuous streams of energy under the classical wave model, but explained that wave and
particle properties of light can coexist
He proposed the following properties of photons:
o The amount of energy carried by a photon is proportional to its frequency
o A photon cannot transfer part of its energy; it can only transfer all of its energy, or
none of it.
o The intensity of light is proportional to the number of photons
Explain the particle model of light in terms of photons with a particular energy and
frequency
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Many properties of light are best explained if light is considered to consist of a stream of
massless particles, or discrete bundles of energy, called photons
The energy of a photon is proportional to its frequency => the shorter the wavelength of a
photon, the higher the frequency, thus the greater the total energy
The intensity of light is the number of photons per unit area, regardless of the energy of
each photon
All photons travel at the speed of light (3x108ms-1) in a vacuum
The success of the particle model of light can be seen when considering the photoelectric
effect
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PHOTOELECTRIC EFFECT AND PHOTONS
Consider the apparatus below
As explained above, electrons are released from the polished surface of the cathode when
light is shone at it due to the photoelectric effect. When electrons are released, they
complete the circuit, thus a reading on the voltmeter and ammeter can be read.
Many aspects of the photoelectric effect were inconsistent with the classical wave model of
light:
o No current was registered below a certain frequency of incident light on the
cathode. This contrasted to classical theory, which predicted electrons can gain
enough energy given enough time from any frequency of EMR to complete the
circuit. The cut-off frequency (also known as threshold frequency) depended on the
material of the cathode.
o
The kinetic energy of the electrons increases as the frequency of incident light
increases, but there is no change if frequency is constant and intensity is increased.
Under the classical wave model, an increase in intensity should increase the energy
of the electrons
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There is no time delay between the time the light is shone on the surface and the
time the electrons are emitted, regardless of the intensity of light. Under the
classical model, electrons would require a length of time to gain enough energy from
a low-intensity light source in order to escape the surface of the metal
Einstein was able to explain the above observations using the fact that a photon’s energy is
proportional to its frequency (E=hf, h is constant), and a photon can only transfer all of its
energy or none of it. The energy of a photon is transferred entirely to the electron.
The minimum energy required to remove an electron from the surface of a metal is called
the work function (W), and depends on the type of metal. The kinetic energy of a
photoelectron can thus be expressed as the following:
From that, we can see that below a certain frequency depending on the work function,
kinetic energy will be a negative value, thus no current will flow => this explains the
observation of a cut-off frequency
As the work function is constant for a particular metal, the final kinetic energy of an electron
depends on the frequency of incident light, regardless of its intensity.
Thus the particle model of light successfully explains the above observations that were
inconsistent with the classical wave model
Identify the relationships between photon energy, frequency, speed of light and
wavelength:
and
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As stated above, the frequency of light is proportional to its frequency. With the constant of
proportionality h, called Planck’s constant, we get:
Where:
o E = Energy of photon [J]
o H = Planck’s constant = 6.626x10-34Js
o f = Frequency [Hz]
Recall that the velocity of a wave is given by v=fλ. As the speed of photons is constant, the
following relationship holds:
Where:
o c = Speed of light = 3.00x108ms-1
o f = Frequency [Hz}
o λ = Wavelength [m]
Solve problems and analyse information using:
and
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Remember to always use the correct units before using the above equations. The energy of
electrons is commonly given in electron volts (eV), and the wavelength of EMR is often given
in nanometres (nm). The conversion from electron volts to joules is given in the data sheet
If given data on the kinetic energy of photoelectrons and incident light and asked to
calculate frequency, use the wavelength in calculations, as the work function generally
needs be taken into account when using the energy of photoelectrons
Identify data sources, gather, process and analyse information and use available
evidence to assess Einstein’s contribution to quantum theory and its relation to
black body radiation
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Einstein’s quantisation of the model of light allowed for the explanation of the photoelectric
effect, particularly the ‘all-or-nothing principle and E=hf.
This also explained the work function of metals (the minimum energy a photoelectron
requires to escape the surface of a metal) and how this was related to the threshold
frequency
Using the particle model of light, Einstein was able to successfully explain the why black
body radiation intensity reached a peak and then continued to decrease, rather than
increase to infinity as predicted by classical physics
Thus Einstein’s contribution to quantum theory, in explaining the photoelectric effect and
the quantum nature of light and black body radiation, expanded our understanding of
quantum mechanics, and opened up the world of modern physics
Identify data sources, gather, process and present information to summarise the
use of the photoelectric effect in photocells
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A photocell is a device that converts energy from sunlight into electrical energy, such as
solar/photovoltaic cells or photoconductive tubes. The operation of solar/photovoltaic cells
will be covered in the next section.
The diagram below shows the structure of a phototube
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The surface of the cathode is made of a high-resistance semiconductor material, such as
CdS, PbS and PbTe. Light shines upon the cathode, which causes the material to release
electrons from the structure into the conduction band due to the photoelectric effect.
These free electrons can conduct electricity quite easily from the cathode to the anode,
which significantly reduces the resistance of the material. As a result, a current starts to flow
in the circuit, which can trigger another functional system.
Photocells can be used as switches that turn on and off depending on the amount of light
available. For example:
o Street lights can be programmed to turn on when there is less natural light.
o Sensors in automatic doors emit infra-red light, which is reflected from an
approaching object and triggers the photocell to control opening the doors
Photocells are also used in light meters, which measure the amount of light present. An
increase in the intensity of light increases the number of photons above the threshold
frequency, which increases the number of photoelectrons. The reading on a galvanometer
connected to the phototube can be interpreted as a reading of light intensity.
Process information to discuss Einstein’s and Planck’s differing views about
whether science research is removed from social and political forces
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Both Planck and Einstein lived in Germany during the early 20th century, and their work was
subject to social and political forces under the Nazi regime
Planck remained in Berlin to continue research into physics, as he supported the German
cause. Planck worked for the war effort in Germany, even though his teachings were
sometimes ridiculed.
On the other hand, Einstein was a pacifist, and believed the science should be completely
removed from social and political forces. He emigrated to the U.S. due to the increasing
political discrimination of Jews and the ‘Jewish science’ of relativity. He was prompted by
the Nazi atrocities however to encourage Roosevelt to pursue the development of the
nuclear bomb despite his pacifist values, which was a decision he later regretted.
Perform an investigation to demonstrate the production and reception of radio
waves
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METHOD
A DC induction coil was attached to a power supply, and adjusted to give a strong spark
between the electrodes (around 2cm). A radio was set to a static frequency and placed 50cm
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from the induction coil. The induction coil was turned on and off repeatedly, and any
observed sounds were noted. The receiver was then placed in different areas of the room no
further than 5m away, and three different static frequencies were tested.
SAFETY:
The sparks produced emit X-rays, which are potentially dangerous for prolonged exposure.
Stand at least 2m away from the coil whilst it is turned on (consider the inverse square law),
and only turn the coil on for short periods of time (no more than 5 seconds) for observation
The sparks also produce ozone, which can aggravate respiratory problems => conduct the
experiment in a well-ventilated area, and again only turn the induction coil on for short
periods of time
RESULTS
When the induction coil was turned on, the volume of the static crackling increased
significantly, and returned to its normal volume when the coil was turned off
The further away the radio was from the induction coil, the smaller the change in observed
sound
The increase in sound was more pronounced for lower frequencies
The crackling sound heard was due to interference of the radio waves produced by the
induction coil being received by the radio’s antenna
ACCURACY/RELIABILITY/VALIDITY
Only qualitative observations were recorded, so accuracy is irrelevant
The induction coil was turned on and off several times and produced the same results
A control was used, as the radio was kept on whilst the induction coil was turned off, thus
the increased static sound can be attributed to the production of radio waves by the
induction coil
Repeating the procedure with different radio frequencies showed the production of a range
of radio waves
3. Limitations of past technologies and increased research into the structure of
the atom resulted in the invention of transistors
Identify that some electrons in solids are shared between atoms and move freely
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In some solids, the outer electrons (also known as the valence electrons) are very loosely
bound to particular atoms
Solid metals exist as a lattice structure containing an orderly array of positive metal ions.
Valence electrons are delocalised and free to move throughout the lattice to maintain
stability
These electrons can move freely across the entirety of the solid in a random pattern. If an
electric field is applied across the solid, the electrons move single direction, thus causing
electricity to flow
In insulators however, the atoms are held together by covalent bonds, which means that the
molecules have full valence shells. No electrons are free to move, so these solids are poor
conductors of electricity
Describe the difference between conductors, insulators and semiconductors in
terms of band structures and relative electrical resistance
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In an atom, negatively-charged electrons orbit a positively-charged nucleus in orbitals which
have defined energy (hence the angular momentum of electrons is quantised)
In isolated atoms (such as gases), the energy levels for each corresponding orbital of that
element are the same
According to Pauli’s exclusion principle, no two electrons in the same atom may share the
same quantum state, such as orbital energy. Thus if two atoms are brought closely enough
together, their electron’s energy levels split slightly so that they have different orbital
energies
The splitting of energy levels results in two distinct levels close to one another. In solids,
billions of atoms are close together (such as in crystalline solids), the energy levels of atoms
form continuous but discrete bands rather than discrete value
As the number of discrete energy levels becomes large, the gap between the allowed energy
bands becomes extremely small. There remain some sections of energy level in which no
orbital energy exists: these are called band gaps or forbidden energy levels
Two energy bands of particular concern are the valence band and the conduction band
o
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The valence band is the highest energy level occupied by electrons in an atom or
molecule in its ground state
o The conduction band is the energy level occupied by electrons that are free to move
and conduct electricity
The difference between the conductivity of conductors, insulators, and semiconductors lies
in the energy difference between the valence and conduction bands
o In a conductor, the valence band is only partially filled, and the valence and
conduction bands overlap. Thus valence electrons are free to move throughout the
solid, and allow current to flow when a potential difference is applied across them
o In an insulator, the valence band is completely filled, and gap between the valence
and conduction band is very large. Valence electrons are resistant to motion when
the solid is subjected to an electric field, thus the electrical resistance of insulators
are very high.
o In a semiconductor (such as silicon or germanium), the gap between the valence and
conduction bands is not too large. Under certain conditions, such as external heat or
light, electrons in the valence band can gain sufficient energy to cross the gap and
enter the conduction band. Thus in semiconductors, electrical resistance decreases
markedly with increasing temperature
Property
Valence band
Conduction band
Insulators
Completely filled
Empty
Forbidden gap
Resistance
Very large
High
Semiconductors
Almost filled
Empty at absolute zero,
partially occupied at high
temperatures
Small
Reduced, particularly at
high temperatures
Conductors
Partly filled
Overlaps with the valence
band, so electrons are free to
move between them
None
Very low
Compare qualitatively the relative number of free electrons that can drift from
atom to atom in conductors, semiconductors and insulators
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Insulators have very few electrons in the conduction band under normal conditions, so the
resistance of insulators is very high
Conductors have many electrons in the conduction band, so they easily conduct electricity
Semiconductors have few electrons in the conduction band, though the number increases
with the supply of heat or light energy
Identify absences of electrons in a nearly full band as holes, and recognise that both
electrons and holes help to carry current
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When an electron is excited to the conduction band, it leaves a vacancy in the valence band.
The absence of an electron in the valence band is called a hole.
When a solid is conducting electricity, other electrons can jump from the valence band of
neighbouring atoms to fill a hole, subsequently creating a hole in their original position
As the temperature of a semiconductor increases, the number of electrons excited to the
conduction band increases, thus the number of holes increases
The movement of holes can be considered the flow of positive charges, and they move in
the opposite direction to electrons when a material is conducting electricity. Thus both
electrons and holes help to carry current.
Describe how ‘doping’ a semiconductor can change its electrical properties
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Silicon and germanium are called intrinsic (pure) semiconductors, as their semiconducting
properties occur naturally due to the 4 electrons in the valence shell.
Doping involves adding specific impurity atoms (dopants) to a semiconductor to change its
semiconducting properties. Doped semiconductors are also known as extrinsic
semiconductors
If the dopant atom has a different number of valence electrons from the atom of the
semiconductor, the number of free electrons or holes changes, and hence electrical
conductivity is increased
Identify differences in p and n-type semiconductors in terms of the relative number
of negative charge carriers and positive holes
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p-type conductors are formed when an element with three electrons in the valence band
(Group 3 elements) such as boron or gallium is substituted into the lattice of a
semiconductor through doping.
Group 4 elements, such as silicon or germanium, have 4 valence electrons, thus four
electrons are required to bond with these atoms. The Group 3 atom has only three valence
electrons, so a hole is incorporated into the crystal lattice as it is one electron short per
dopant atom.
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Electrons from the valence shells of neighbouring atoms can jump into the hole, causing
another hole, and the subsequent movement of the hole through the semiconductor, hence
improving electrical conductivity
Dopants that produce a charge vacancy or a hole are called acceptor impurities
The diagram below shows the lattice of a p-type semiconductor
n-type semiconductors are formed when a Group 5 impurity atom (such as phosphorous or
arsenic) is substituted into the lattice of the semiconductor
Group 5 atoms have 5 valence electrons, but it can only bond with 4 atoms in the silicon
crystal lattice. Thus one electron of the impurity is not strongly bonded to the dopant, and
can easily be moved into the conduction band, which improves electrical conductivity by
lowering the energy required for valence charges to jump into the conduction band
Impurities that produce unbonded electrons are called donor impurities.
The diagram below shows the lattice of an n-type semiconductor
Describe differences between solid state and thermionic devices and discuss why
solid state devices replaced thermionic devices
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Many electrical appliances need to control the direction of current flow, rectify AC into DC,
switch current flow on or off, or amplify current
Early electronics used thermionic devices to control current flow. Thermionic devices rely on
the thermionic emission of electrons from heated filaments and terminals set in glass
vacuum tubes
Below is a picture of a thermionic diode
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The heated filament is connected to the external circuit, and only allows current to flow
from the cathode to the anode
In a thermionic triode, an electrode grid is added to the vacuum diode, which amplifies the
current
Solid state devices on the other hand use semiconductor materials to control current flow
Below is an example of a solid state diode, which operates using the junction between a ptype and an n-type semiconductor
The extra free electrons in the n-type semiconductor move across the junction to fill the
extra holes in the p-type semiconductor
The zone adjacent to the junction (the depletion zone) resists the movement of any more
electrons or holes due to the build-up of electrostatic charge, which effectively means that
current can flow in one direction only
Whilst thermionic and solid state devices both are able to rectify, amplify and switch
current, solid state devices have replaced thermionic devices for the following reasons:
o Thermionic devices produce large quantities of heat, which can damage the
surrounding electronics
o Thermionic devices are require separate heating circuits to heat the cathode, which
takes time to operate. Solid state devices are more than 100 000 faster to operate
and load
o High voltages are required to correctly bias thermionic diodes and triodes, whilst
silicon transistors use 0.6V to achieve the same effect
o Thermionic devices consume much more power than solid state devices and are
more expensive to produce, so are much less economical to operate
o Thermionic devices are bulky, so they cannot be used in portable devices
o
The vacuum tube and heating circuits in thermionic devices means that they are
very fragile and have very short lifetime
Gather, process and present secondary information to discuss how shortcomings in
available communication technology lead to an increased knowledge of the
properties of materials with particular reference to the invention of the transistor
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Many communication technologies require the amplification of electrical signals, such as
radios, telephones and televisions. Early communication technologies used thermionic
devices such as the triode to amplify signals.
The use of thermionic devices in communication had many problems however, as they were
bulky, consumed a lot of power, had limited response times, and were very fragile. As such,
thermionic devices had short lifetimes and were uneconomical, particularly with the growth
of telephone networks during the 1940s and 1950s
The shortcomings of thermionic devices led scientists to research into solid state
replacements that were compact, power-efficient, mechanically durable, and able to
operate high switching speeds => this led to the development of the transistor
The transistor consists of either a p-n-p junction or an n-p-n junction. An n-p-n transistor is
shown below
Electrons initially flow from the collector and emitter to the base. If a potential difference is
applied across the base and across the emitter, a small current flows from the base and a
large current flows from the emitter, thus the transistor amplifies the electrical signal
Transistors can also be used to switch current on and off, which allows it to replicate a digital
code as an electrical signal
The invention of the transistor led to increased research into materials that could be used in
transistors, leading to a greater understanding of the properties of semiconductors such as
silicon and germanium
With further understanding of semiconductors, doping was invented, increasing the
versatility of transistors and the scope of applications for which they can be used
Identify that the use of germanium in early transistors is related to a lack of
ability to produce other materials of suitable purity
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Both germanium and silicon are group 4 elements, thus were considered suitable in the
development of solid state devices using semiconductors
Germanium was used initially because the industrial techniques were available at the time
to purify germanium were not available for other semiconducting materials such as silicon
Once the technology to purify silicon to required level became available, silicon largely
replaced germanium transistors
Silicon is preferred over germanium because:
o Germanium becomes a relatively good conductor when it heats up, which can
damage electronic equipment by allowing too much current to flow, which can
cause transistors to fail
o Silicon is more abundant in Earth’s crust and thus cheaper to produce
o Silicon can curry a higher current for longer periods of time
Identify data sources, gather, process, analyse information and use available
evidence to assess the impact of the invention of transistors on society with
particular reference to their use in microchips and microprocessors
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The invention of the transistor led to the development of integrated circuits, which are tiny
electronic circuits used to perform a specific electronic function
Modern integrated circuits have millions of circuit elements on a centimetre square of
silicon, allowing for smaller microchips to be developed
The use of microchips and microprocessors in everyday devices such as cars, games,
communications technology, and music players have significantly changed the way people
live
IMPACT OF TRANSISTORS ON SOCIETY
Electronic devices have become much cheaper and more commonplace, allowing more
people to access labour-saving and recreational electronic devices
Communication is much faster, easier, and cheaper through electronic technologies such as
mobile phones and the Internet
The development of the Internet and word processing has changed the way people interact
and obtain information
Credit and debit cards have changed the way people spend money
Computerised medical technology has increased life expectancy, but life has become more
sedentary due to the widespread use of labour-saving technology
Reductions in unskilled jobs due to the computerisation of industry and manufacturing has
led to a growth in unemployment
Identify data sources, gather, process and present information to summarise the
effect of light on semiconductors in solar cells
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Solar cells use a p-n junction to convert light energy directly into electrical energy
Below is a diagram of a typical solar cell
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The region around the n-type and p-type junction is called the depletion zone. The donor
electrons from the n-type semiconductor migrate to the p-type conductor, and holes
migrate in the opposite direction. The migration of charges produces an electric field that
resists further migration of charges
When a photon of light is incident on the p-type semiconductor causes electrons to be
excited to the conduction band. These electrons move across the junction to the n-type
semiconductor due to the electric field, and holes move in the opposite direction
These electrons have a much higher lifetime before recombining with a hole. Electrodes
placed on the ends of the semiconductor collect the current, which provides electrical power
as the electrons return to the p-type conductor via an external circuit
Solar cells are manufactured to be thin enough so that most of the incoming light is
transmitted to the p-type semiconductor. An anti-reflection coating on the surface also
helps to maximise the number of electrons that are excited by incident photons.
Perform an investigation to model the behaviour of semiconductors, including the
creation of a hole or positive charge on the atom that has lost the electron and the
movement of electrons and holes in opposite directions when an electric field is
applied across the semiconductor
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METHOD
Ten marbles were placed in a row on a table, and two random marbles were removed. The
remaining marbles were then moved towards the left by sequentially moving a marble
adjacent to a hole
Use a diagram to show the marbles (electrons) and gaps (holes) resting on the table (valence
band) and lifted (conduction band)
RESULTS
The original ten marbles resting on the table represent the electrons in the valence band of a
semiconductor
Removing the two marbles represents the excitation of electrons to the conduction band,
and the gap left by the marbles represent holes or positive charges
The movement of electrons to the left represents the movement of electrons in the opposite
direction to an electric field. Both the marbles and the holes were observed to move in
opposite directions
NOTE: If asked a question on this practical, make sure to show how holes/electrons are able
to move with energy input across the forbidden energy level from the valence band to the
conduction band, and that both electrons and holes are able to move, but in opposite
directions
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ACCURACY/RELAIBLITY/VALIDITY
As this was a qualitative model of semiconductors, accuracy and reliability are irrelevant
Validity was established as the model was carefully thought out, and symbols corresponded
to physical principles
The validity of the model was undermined by the oversimplification of conduction in
semiconductors
4. Investigations into the electrical properties of particular metals at different
temperatures led to the identification of superconductivity and the
exploration of possible applications
Outline the methods used by the Braggs to determine crystal structure
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Sir William and Lawrence Bragg studied the structure of crystals analysing the interference
pattern of scattered X-rays produced from the diffraction of rays incident on a crystalline
solid
The X-rays were produced from cathode rays hitting a metal anode and directed at a metal
salt in a 40kV X-ray tube. These rays were collimated so they remained parallel upon
reflection from the crystalline solid
The diagram below shows parallel X-rays striking a crystal. Note that whilst X-rays are
scattered, the model below uses reflection to simplify the description
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Bragg’s equation can be used to calculate the distance between the planes:
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The Braggs used an ionisation chamber to determine the position and intensity of the
diffracted X-rays, and photographic film to provide an image of the interference pattern
Using mathematical analysis and calculations based on the spacing between maxima in the
X-ray interference pattern , the Braggs were able to confirm the crystal lattice of materials –
parallel layers of orderly arranged atoms
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Identify that metals possess a crystal lattice structure
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Metals are often described as a crystal lattice of positive ions surrounded by a cloud of
delocalised electrons
Metals are considered crystalline as the structure is uniform and not amorphous (i.e. no
distinct shape)
Some examples of the structure of metallic crystals are shown below (body-centred cubic,
face-centred cubic and hexagonal close-packed)
Describe conduction in metals as a free movement of electrons unimpeded by the
lattice
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As mentioned above, metallic crystals contain a cloud of delocalised electrons that move in
random directions throughout the lattice
When an electric field is applied, the delocalised electrons experience a force; hence
electrons experience a net movement in one direction when conducting electricity
As the electrons are delocalised, their movement during conduction is generally unimpeded
by the lattice
Identify that resistance in metals is increased by the presence of impurities and
scattering of electrons by lattice vibrations
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Electrons generally experience resistance when conducting, as the kinetic energy of the
electrons is converted to heat and other forms of energy
Recall from the preliminary course that resistance depends on the following factors:
o Cross-sectional area
o Temperature
o Length of conductor
o Resistivity of conducting material
The temperature of a metal can be considered as a measure of the vibration of lattice ions
due to their energy => the more the ions vibrate, the higher the temperature
The lattice vibrations cause electrons to scatter as they collide with the lattice ions. The
scattering of electrons decreases their kinetic energy and increases resistance of the
conductor.
The presence of impurities and imperfections in the conductor also causes obstructions to
electron movement and increase resistance, as they are not necessarily part of the lattice
Describe the occurrence in superconductors below their critical temperature of a
population of electron pairs unaffected by electrical resistance
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Superconductivity is the phenomenon in which a conductor exhibits negligible resistance
when cooled past its critical temperature (Tc)
The graph below shows the resistance of a superconductor and a normal conductor as a
function of temperature
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As mentioned above, lattice vibrations of a conductor inhibit the movement of electrons,
which increases resistance
On the other hand, below the critical temperature of superconductors, the lattice actually
assists electrons flow, and electron pairs are unaffected by electrical resistance (see below
for further detail)
There are two types of semiconductors, type I and type II
o Type I superconductors are pure metals, and have a critical temperature of up to
23K
o Type II semiconductors are ceramics and metal alloys, and have a critical
temperature of up to 120K
Process information to identify some of the metals, metal alloys and compounds
that have been identified as exhibiting the property of superconductivity and their
critical temperatures
Element/alloy/compound
Aluminium
Tin
Mercury
Lead
Technetium
Niobium-nitride
Vanadium-silicon
Tin-Niobium alloy
Ni-Al-Ge Alloy
MgB2
Alkali-doped fullerene (RbCs2C60)
YBa2Cu3O7 (YBCO)
Bi2Sr2Ca2Cu3O10
HgBa2Ca2Cu3O8
Tc (K)
1.2
3.73
4.12
7.22
11.2
16
17.5
18
21
39
33
92
110
133
Tc (°C)
-271.95
-269.42
-269.03
-265.93
-261.95
-257.15
-255.65
-255.15
-252.15
-234.15
-240.15
-181.15
-163.15
-140.15
Discuss the BCS theory
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BCS theory (named after physicists Bardeen, Cooper, and Schrieffer) is used to explain
superconductivity in type I superconductors, but cannot explain its occurrence in type II
superconductors
According to BCS theory, when a conductor is cooled to below its critical temperature, the
very low thermal energy (i.e. reduced random lattice vibrations) leads to a quantum state
where an electron can lead to a distortion in the lattice of positive ions
This distortion, also called a phonon, gives rise to a localised positive region, which attracts a
second electron with opposite spin in to combine with the first as a ‘Cooper pair’. The
electron pairing is favoured as it puts electrons into a lower energy state
The Cooper pair interacts with the lattice to produce destructive interference with the
distortion, so the phonon’s lifetime is too short to propagate through the lattice like a wave.
As a result, electrons are able to move through the lattice with no loss of energy, hence a
superconductor in its superconducting state has no electrical resistance
The diagrams below show the occurrence of phonons in superconductors
BCS Theory was highly successful in explaining the microscopic and macroscopic properties
of some semiconductors. It predicted certain properties that were verified later, such as the
Meissner effect and heat capacity
On the other hand, BCS Theory cannot explain the occurrence of high-temperature
superconductivity in ceramics
Analyse information to explain why a magnet is able to hover above a
superconducting material that has reached the temperature at which it is
superconducting
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Below its critical temperature, a superconductor becomes perfectly diamagnetic and cancels
all magnetic flux inside => this is called the Meissner effect
Consider the diagram below of a superconductor in its normal state and superconducting
state
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An external magnetic field is able to penetrate the superconductor in its normal state, but
produces currents in its superconducting state, which cancel all magnetic flux inside the
superconductor
When a magnet is brought near a superconductor in its superconducting state, the induced
eddy currents produce an image of the permanent magnet to cancel all magnetic flux within
the superconductor. This results in a repulsion that balances the weight of the magnet, thus
causing levitation
Discuss the advantages of using superconductors and identify limitations to their
use
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ADVANTAGES
The main advantage of superconductors is that there is no heat lost when passing current
through them, which significantly reduces power losses in the transmission of electricity and
allows induced currents to persist indefinitely.
o This is particularly useful in electromagnets, as its magnetic field persists after the
power supply has been turned off
Superconducting electromagnets are also amongst the most powerful magnets known,
which is useful in maglev trains and medical diagnostic equipment (e.g. MRI)
LIMITATIONS
The main disadvantage of superconductors is the very low temperatures required to
maintain their superconducting state, which significantly reduces their practicality
o Metals require liquid helium to reach their superconducting state, whilst hightemperature ceramic superconductors require liquid nitrogen
Superconductors are also very expensive due to the required low temperatures
High-temperature ceramic superconductors are too brittle to be shaped easily, thus have
limited useful applications
Superconductors are also very sensitive to moving magnetic fields, meaning applications
using AC (e.g. transistors) may need research in order to use DC instead
Process information to discuss possible applications of superconductivity and the
effects of those applications on computers, generators and motors and
transmission of electricity through power grids
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COMPUTERS
The speed and further miniaturisation of computer chips are limited by the generation of
heat, and the speed with which signals can be conducted => these two factors could be
improved by using superconductors in electronics
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One application of superconductors in electronics is the Josephson junction, which consists
of two superconductors separated by an oxide layer, and acts as a superfast switch
By using superconducting film such as the Josephson junction, integrated circuits could be
more densely packed due to the reduction in heat generated => this would allow for far
greater computing power and more energy-efficient electronics
As superconductors have no resistance in their superconducting state, electrical signals can
also be transmitted faster, allowing information to be processed several orders of
magnitude faster
GENERATORS AND MOTORS
The electromagnets used in large-scale generators heat up significantly due to the resistance
of their wires. Electromagnets also require an iron core to strengthen the field, which
increases power required to rotate the magnet due to its heavy mass
Using superconducting electromagnets without iron cores could potentially halve the
amount of power lost in the electromagnets, which would significantly increase the power
output of generators
Superconducting magnets in motors provide a much stronger magnetic field than
permanent magnets, so no iron core would be needed => this would lower the weight of the
rotor and increase energy-efficiency
TRANSMISSION OF ELECTRICITY THROUGH POWER GRIDS
Recall that electricity is transmitted at low currents to minimise power losses in wires (P=I2R)
The lack of resistance in superconductors means that electricity could be transmitted
without loss of energy as heat. This would allow them to carry many times more current
than conventional wires.
Superconducting transmission wires would also be able to transmit electricity in DC, as
voltage would not need to be stepped-up in transformers. This would minimise power losses
in transmission, as the production of eddy currents is a major source of energy loss in AC
transmission, particularly in transformers
NOTE: Remember to consider the impact of each application on society and the environment.
Generally, energy-efficient = reduced cost (society) = less dependence on fossil fuels
(environment)
Gather and process information to describe how superconductors and the effects of
magnetic fields have been applied to develop the maglev train
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Magnetic levitation (maglev) trains have been built that use powerful electromagnets made
from superconductors
Maglev trains require extremely powerful magnets as they are very heavy, so only
superconducting magnets can be used
The diagrams below show the basic structure of maglev trains
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The superconducting magnets are mounted on the train and kept cool with liquid helium
Once the train is levitated using magnetic repulsion from powerful superconductor magnets,
the polarity of alternate magnets on the track is continually changed, which generates a
series of attractions and repulsions to accelerate the train
The superconducting electromagnets rely on conventional ‘like-pole’ repulsion to achieve
levitation, not the Meissner effect
BENEFITS
Magnetic levitation provide a frictionless method of movement as the train is free from any
surface, which allows for high speeds to be achieved
Maglev trains are also quieter than conventional train system, despite reaching much higher
speeds
LIMITATIONS
Extremely large amounts of energy are expended, which impedes wider use due to the
negative economic and environmental impact
The electromagnet that levitate the train are expensive to run over long distances, and the
new lines that have to be constructed also increase the cost of implementing maglev trains
on a wider scale
The strong magnetic field inside the train excludes passengers with pacemakers, and may
damage devices that use magnetic storage such as computers and credit cards
Perform an investigation to demonstrate magnetic levitation
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METHOD
A high-temperature superconductor disc was placed in a Styrofoam cup and immersed with
liquid nitrogen, allowing the disc to cool to below its critical temperature. A neodymium
magnet was then carefully placed, using plastic tweezers, just above the superconducting
disc
SAFETY
Liquid nitrogen can cause severe damage to skin due to its very low temperature => only the
teacher should handle the liquid nitrogen, using safety glasses, cotton gloves and plastic
tweezers to handle the magnet and the superconductor in the Styrofoam cup, which is an
insulator
RESULTS
The neodymium magnet hovered above the disc for about five minutes, then slowly came to
rest on top of the disc as the liquid nitrogen disappeared
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See above for an explanation of magnetic levitation and the Meissner effect
ACCURACY/RELIABILITY/VALIDITY
The experiment was qualitative, so accuracy is irrelevant
Magnetic levitation was demonstrated multiple times, so the experiment was reliable
All the conditions for magnetic levitation were met, such as using a superconductor below it
critical temperature and a neodymium magnet, so the experiment was valid