Download Ideas To Implementation

Physics:
From Ideas to
Implementation
1
Cathode Ray Debate
1.1
Explain why the apparent inconsistent behaviour of cathode rays caused debate as to
whether they were charged particles or electromagnetic waves:
Observations that fit the Wave model:
 Travelled in straight lines
 A shadow created when an opaque object was in its path
Observations that fit the Particle model:
 Rays left the cathode at right angles to the surface
 Deflected by magnetic fields
 Did not appear to be deflected by electric fields
 Small paddle wheels turned when placed in their path
 Travelled considerably slower than light
1.2
Explain that cathode ray tubes allowed the manipulation of a stream of charged particles:
The Cathode Ray Tube:
 Highly evacuated glass tube (to reduce obstruction/collisions) containing two electrodes
 High voltage applied across the electrodes
 Cathode rays (streams of electrons) flow from the cathode towards the anode
Manipulation of cathode rays:
Structures built into or around cathode ray tube, manipulating the rays:
 Further electrodes built into cathode ray tube creating an electric field, changing the path of
the cathode rays
 Magnetic fields applied to cathode rays through the glass from outside the tube
 Solid objects placed inside tube to block the path of the rays (e.g. Maltese Cross)
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1.3
Outline Thompson's Experiment to measure the charge to mass ratio of an electron:
Determining the velocity:
 Varied the magnetic and electric fields
acting on the rays until they were equal
 This allowed the rays to pass through the
fields unaffected
 He equated the electric and magnetic force
equations, using the resulting formula to
calculate the velocity of the rays, finding it
was much lower than the speed of light
Determining the radius of the circular path:
 By manipulating the magnetic field only, he
determined the radius of circular path
travelled by particles in the magnetic field
Charge to mass ratio:
 Combined these results to obtain the magnitude of the charge to mass ratio
 Given by:
𝑞
𝐸
= 2
𝑚 𝑟𝐵

2
From these results, he could conclude that:
o The charge on cathode rays was 1800 times larger than charge on a hydrogen ion; or
o The mass of cathode ray particles was 1800 times less than that of a hydrogen ion
Forces on Moving Charges
2.1
Identify that moving charged particles in a magnetic field experience a force & describe
quantitatively the force acting on a charge moving through a magnetic field:
Magnetic fields on moving charges:
 Magnetic fields exert forces on electric currents (moving charged particles)
 If the particle is travelling parallel to the field lines, there is no force experienced
 Maximum force is experienced when travelling at 90 degrees to the field lines
 The right-hand-palm-rule (RHPR) is used to determine the direction of the force (thumb is in
the direction of a positive charge movement)
Formula:
F = qvBsinθ
where F = force on the moving charge (N)
q = charge (C)
v = velocity (ms-1)
B = magnetic field strength (T)
θ = angle between magnetic field and charge
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3
Electric Fields
3.1
Identify that charged plates produce an electric field & describe quantitatively the electric
field due to oppositely charged parallel plates:
 Plates with a potential difference between them produce an electric field running between
them - the field lines are parallel and run from the positive plate to the negative plate
 Electric fields exist in any region in which an electrically charged object experiences a force
 Charged plates exert a force on other charged objects indicating an electric field exists
Formulae:
 Electric field strength:
𝐸=

𝑉
𝑑
where E = electric field strength (NC-1)
V = voltage/potential difference (V)
d = distance between plates (m)
Force on a moving charge:
𝐹 = 𝑞𝐸
where F = electric force (N)
q = electric charge (C)
E = electric field strength (NC-1)
3.2
Discuss qualitatively the electric field strength due to a point of charge, positive and
negative charges and oppositely charged parallel plates:
 The strength of an electric field at any point is defined as the size of the force acting per unit
of charge
 The direction of the electric field at any point is defined as the direction of the force a
positive charge would experience placed at this point
Point of Positively Charged:
 Diminishes with distance from particle
 Direction points radially away from particle
Point of Negatively Charged:
 Diminishes with distance from particle
 Direction points radially towards particle
Between Positive and negative charges:
Between oppositely charged parallel plates:
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4
TV Displays and Oscilloscopes
4.1
Outline the role of electrodes in the electron gun, the deflection plates or coils, the
fluorescent screen in the cathode ray tube of conventional TV displays and oscilloscopes:
Electron gun:
 Heated cathode releases electrons by thermionic emission
 Positively charged anode develops strong electric field exerting a force
 The electron gun produces a narrow beam of electrons - the electrodes (cathode and anode)
in the gun accelerate the electrons
Deflection plates/coils:
 The deflection plates (in CRO) or coils (in TV) establish an electric field that controls the
deflection of the electron beam from side to side and up and down
Fluorescent screen:
 The fluorescent screen is coated with a material that emits light when struck by electrons in
the cathode ray
 This allows the position of the beam to be seen where it strikes the screen
5
Hertz and the Photoelectric Effect
5.1
Describe Hertz's observation of the effect of a radio wave on a receiver and the
photoelectric effect he produced but failed to investigate:
The Photoelectric Effect: when light of an appropriate frequency is shone onto a metal surface,
electrons are emitted from that surface.
 Hertz observed that the spark between the gap in the transmitter loop caused an electrical
disturbance between the gaps in the detecting loop
 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 (the photoelectric effect), thus allowing the
discharge (spark) to occur across a wider gap
5.2
Outline qualitatively Hertz's experiments in measuring the speed of radio waves and how
they relate to light waves:
 Hertz set up an induction coil connected to the transmitter loop
 As sparks were generated across a small gap they induced
sparks in a detecting loop a small distance away
 This spark was evidence for electromagnetic waves travelling
through space from the induction coil to the detecting loop
 Hertz was able to calculate the velocity of the waves by reflecting the generated waves off a
metal sheet and measuring the wavelength of the standing wave set up by interference
 Substituting this wavelength and the known frequency of the wave generator into the
general wave equation (v = f λ), Hertz calculated the wave speed at 3 x 108 ms-1
 This value was very close to the values for the speed of light earlier estimated by Maxwell
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6
Photoelectric Effect Applications
6.1
Identify data sources, gather, process and present information to summarize the use of
the photoelectric effect in solar cells and photocells:
Solar cells:
 Uses the photoelectric effect to convert the energy from sunlight into electrical energy
 Uses silicon semiconductor – when light (photons) hits the top layer of a solar cell, some of
the electrons in the n-type silicon semiconductor are set free
 The electrons flow out of the n-type silicon, into the external circuitry (e.g. a light bulb), and
return to the cell via the p-type silicon
Photocells (photodetectors):
 Uses the photoelectric effect directly to produce an electric current in the circuit attached
 Some utilize photoelectric effect to detect presence of light or radiation – used in automatic
switching devices (e.g. used to switch street lights on/off)
 Also used in burglar alarms, automatic opening doors, entrance buzzers (when entering into
a shop), breathalysers, etc.
7
Social and Political Forces
7.1
Process information to discuss Einstein and Planck's differing views about whether science
research is removed from social and political forces:
Background:
 Application of scientific study to ‘the war effort’ was seen as a problem by many scientists
 Two groups emerged:
o Those against turning their ideas into weapons of destruction
o Those agreeing with decisions on the use of scientific study – left to politicians to decide
Planck's View:
 Plank was a nationalist, believed in and worked for his country
 Loyal to whatever government decided
 One of first German intellectuals to sign the document supporting the war effort
 Then devoted his work and research to the war effort
Einstein's View:
 Supported no government, had no particular political views
 Refused to sign document supporting the war effort, signed peace document instead
 Pacifist (peace-keeper); did not want his work contributing to killing others
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8
Black Body Radiation
8.1
Identify Planck's hypothesis that radiation emitted and absorbed by the walls of a black
body cavity is quantised:
Black body:
 An idealized object that absorbs all electromagnetic radiation that falls on it
 No electromagnetic radiation passes through it and none is reflected

The amount of energy radiated reaches a maximum at a wavelength that depends on the
temperature of the black body
Planck’s hypothesis:
 Emission and absorption of electromagnetic radiation for a black body cavity is quantised
 Energy is not continuous, can only be radiated or absorbed in small, discrete amounts or
packets called quantum
8.2
Identify and assess Einstein's contribution to quantum theory and its relation to black
body radiation:
Einstein's Explanation of Planck's work:
 The energy associated with the radiation from a black body is concentrated in packets of
energy that he called photons
 He extended Planck’s quanta idea to light and also explained that wave and particle
behaviour can coexist in light – dual nature
Photon:
 Is the smallest amount of radiation energy possible at a particular frequency
 Photon can only transfer all or none of its energy, not part of it
 Amount of energy carried by photon is proportional to its frequency (by E=hf)
 Einstein determined that the intensity of light is proportional to number of photons
Relation to Black Body Radiation:
 The higher the frequency the greater the total energy radiated (E=hf)
 Shown by the black body radiation curves (in above graph)
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9
Particle Model of Light
9.1
Explain the particle model of light in terms of photons with particular energy and
frequency:
Particle Model:
 Instead of being emitted as continuous waves, light travels in packets of energy called
photons
 A photon carries an amount of energy that is proportional to the frequency of the
radiation/light
 All photons of light of a particular frequency have precisely the same amount of energy
 The higher the frequency of the light, the more energy the photon possesses, thus photons
of ultraviolet light have higher energy than those of blue light, which in turn have higher
energy than photons of red light
 All photons, regardless of their frequency, have zero rest mass and travel at 3x108ms-1 in a
vacuum
9.2
Identify the relationships between photon energy, frequency, speed of light and
wavelength:
Note: convert to J when using eV


E = hf
where E = energy of the photon (J) or (eV – electron volts)
h = Planck’s constant (6.626x10-34 Js-1)
f = frequency of the light (Hz)
c=fλ
where c = speed of light (3.0x108 ms-1)
f = frequency of the wave (Hz)
λ = wavelength of the wave (m)
10
Electrons in Solids
10.1


Identify that some electrons in solids are shared between atoms and move freely:
In some solids, the outer electrons are very loosely bound to particular atoms
These electrons can therefore move across the entirety of the solid
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11
Conductors vs Insulators
11.1 Describe the difference between conductors, insulators and semiconductors in terms of
band structures and relative electrical resistance:
Conductors:
 Valence band is only partially filled, thus the conduction and valence bands overlap
 Valence electrons easily move along conduction band
 Material has low electrical resistance
Insulators:
 Valence band is completely filled, thus there is a large forbidden energy band/gap
 Difficult for valence electrons to move into the conduction band
 Material has high electrical resistance
Semiconductors:
 Valence band is almost filled, thus there is small, not to wide energy band/gap
 At higher temperatures, electrons in valence band gain sufficient energy to cross gap
 This reduces electrical resistance of the material
11.2 Compare qualitatively the relative number of free electrons that can drift from atom to
atom in conductors, semi-conductors and insulators:
 Conductors contain high number of free electrons in the conduction band
 Under normal conditions, insulators and semi-conductors have far less free electrons
 Raising the temperature, using certain lighting conditions or applying a potential difference,
can induce electrons in some semiconductors to move into the conduction band
12
Electrons and Holes
12.1 Identify absences of electrons in a nearly full band as holes, and recognise that both
electrons and holes help to carry current:
 When an electron in a semiconductor moves into the conduction band it leaves a “hole”,
i.e. an atom with one less valence electron than normal
 An electron from a nearby atom in the valence band can move and fill the hole – this then
creates another hole, and so on
 The creation of holes and the movement of electrons to fill them is equivalent to an electric
current in the semiconductor
 Electrons flow in one direction – the apparent movement of holes in the opposite direction
can be considered as a flow of positive charge
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13
Solid State vs. Thermionic Devices
13.1 Describe differences between solid state and thermionic devices and discuss why solid
state devices replaced thermionic devices:
Thermionic Devices:
 Contains a cathode that emits electrons only when heated to a high temperature
 It requires a separate heating circuit to heat the cathode, which takes time to heat up
 Requires a near vacuum to allow electrons to flow between the electrodes, thus they are
commonly packaged in an evacuated glass tube
 Thermionic devices are larger in size
Solid State Devices:
 Uses semiconductors to generate electron flow - no heated circuit required
 Functions immediately
 Operates at normal pressures
 Solid-state devices are smaller in size
Reasons for replacement:
 The combined advantages of smaller size, simpler and cheaper construction, higher
reliability, less fragility, lower power requirements and speed of operation make solid state
devices more attractive to electronics manufacturers than equivalent thermionic devices
14
Semiconductors
14.1

Describe how 'doping' a semi-conductor can change its electrical properties
Doping is the addition of an impurity (such as gallium or arsenic) to a semiconductor to
change its conductivity
The atoms of the doping element need to fit reasonably well into the semi-conductor lattice
structure so as not to distort it and impede electron flow
The doping element needs to have either one more or one less valence electron than the
semi-conductor material itself
Doping increases the potential conductivity of the semiconductor – extra electrons or holes
to act as charge carriers



14.2 Identify differences in p and n-type semiconductors in terms of relative number of
electrons and holes:
P-type Semiconductors:
 There are more positive holes than negative charge carriers (electrons)
 Elements such as aluminium and gallium (3 valence electrons – group 3 elements) are used
as doping agents with silicon to produce p-type semiconductors
N-type Semiconductors:
 There are more negative charge carriers (electrons) than positive holes
 Elements, such as arsenic and phosphorus (5 valence electrons – group 5 elements) are used
as doping agents with silicon to produce n-type semiconductors
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15
Transistors
15.1 Identify that the use of germanium in early transistors is related to lack of ability to
produce other materials of suitable purity:
 At first, germanium was widely used as a semi-conductor because it was easier to purify than
other known semi-conductors, such as silicon
 Silicon eventually replaced the germanium as semi-conducting material of choice in
transistors because:
o it is the second most abundant element on earth by weight, which means it is
relatively cheap
o it retains its semiconducting properties at relatively high temperatures (when
compared to germanium)
o it can handle higher electric currents before overheating (which destroys its
semiconductor properties)
o processing techniques were developed to produce very pure, single-crystal forms
o in single-crystal form (very pure silicon), the molecular structure of the material is
uniform, thus ensuring consistency of properties
15.2 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:
 Problem in the early days of the radio was amplification – the received signal was extremely
weak and could not produce a loud sound without being amplified
 Achieved amplification by using vacuum tubes (thermionic devices), which were very fragile,
large, expensive and required high power consumption
 The invention of the transistor solved these significant shortcomings:
o Contained 3 semiconductors – NPN transistors and PNP transistors
o It was smaller, longer lasting, more reliable, less expensive and required less power
o It quickly replaced the vacuum tube in many electronic devices
15.3 Assess the impact of the invention of transistors on society with particular reference to
their use in microchips and microprocessors:
 The discovery of the transistor led to the development of integrated circuits in which many
thousands of transistors could be constructed on a tiny microchip
 This lead to complex circuitry development which was incorporated into appliances to make
many tasks formerly done manually, now done electronically
 For society, this meant that fewer manual labour jobs were available, but at the same time,
more jobs became available to control and manipulate the new machinery and devices
 The development of the microchip made lead to computers and their high accessibility,
where in today’s age, every home has at least one computer
 Other developments due to microchips and microprocessors contributed to medical
diagnosis and treatment as well as entertainment, communications, and much more –
affecting societies higher living standards
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16
Crystal Structures
16.1



Outline the methods used by the Braggs to determine crystal structure:
Produced x-rays by allowing high energy cathode rays to strike a metal anode
Concentrated a fine beam of x-rays into a crystal of a metal salt
Produced scattered/diffracted x-rays – an interference pattern which was recorded
photographic film
This enabled them to mathematically analyse the crystal structure and determine the
internal structure of the crystal

16.2



Identify that metals possess a crystal lattice structure:
The atoms in a crystal are in a regular repeating pattern called the crystal lattice
A crystal lattice is defined by a repeated three-dimensional unit
Atoms of a pure metal arrange themselves in an ordered, geometrical pattern that is
repeated over and over again producing a crystalline structure
17
Metal Conductors
17.1

Describe conduction in metals as a free movement of electrons unimpeded by lattice:
In a metal, the valence electrons are thought of as being shared by all the positive ions therefore, the electrons are free to move throughout the crystal lattice
Metals have many electrons that are free to move, thus they are good conductors of
electricity

18
Superconductors
18.1 Identify that resistance in metals is increased by the presence of impurities and scattering
of electrons by lattice vibrations:
 Chemical impurities disrupt the lattice integrity by distorting its shape - in turn, impeding the
free movement of electrons and therefore increasing resistance
 Lattice vibrations also impede the flow of free electrons;
o As temperature increases, more lattice vibrations occur
o The vibrating lattice collides with free moving electrons, thus deflecting or scattering
them from their linear progress through the crystal
o In effect, increasing resistance
18.2 Describe the occurrence in superconductors below their critical temperature of a
population of electron pairs:
 At room temperatures, the metallic bonds (the lattice) holding the conductor together
vibrates and interferes with electron movement through the conductor
 Along with chemical impurities, these factors are responsible for resistance effects
 Superconductivity describes the state reached in a conductor when the resistance to
electron movement in a conductor drops to zero
 Superconductivity occurs at & below a certain temperature – called the ‘critical temperature’
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


There are two types of superconductors:
o (Type 1) Pure metals – superconductivity occurs at temperatures close to absolute
zero
o (Type 2) Complex alloys of metals and metal oxides – superconductivity occurs at
higher temperatures, up to 130K
At temperatures below the critical temperature of a superconducting material, lattice effects
that impede the electron movement change dramatically from impeding to assisting
electron flow
That assistance comes about by an effect that pairs electrons and assists them to move
freely through the conductor - the BCS theory
18.3 Discuss the BCS theory:
At temperatures below the critical temperature for the superconducting material:
 The interaction of the electron with the lattice causes distortion of the lattice
 This causes vibrations called ‘phonons’
 A nearby electron – which is attracted by the positive charge density of the lattice – absorbs
the energy (phonon)
 The phonon is changed into momentum
 Some momentum is exchanged between the electrons
 A ‘Cooper pair’ is formed
 The Cooper pair travels through the lattice unimpeded
18.4 Identify some metals, metal alloys and compounds identified as exhibiting the property of
superconductivity and their critical temperatures:
Type I superconductors:
Type II superconductors:
Superconducting
Critical Temperature
Critical Temperature
Element
Material
(K)
(K)
Aluminium
1.2
YBa2Cu3O7
90
Lead
7
Tl2Ba2Cu2O5
80-125
Mercury
4
HgBa2Ca2Cu3O8
133K
Tin
4
Titanium
0.4
Tungsten
0.015
Zinc
0.85
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18.5 Discuss the advantages of using superconductors and identify limitations to their use:
Advantages:
 Carry large currents with no heat loss – highly efficient
 Can generate very strong magnetic fields
 Makes many applications possible, including:
o Magnetic Levitation (Maglev) trains
o Extremely fast computers
o More efficient, smaller generators
Limitations:
 Difficulties of achieving and reliably sustaining the extremely low temperatures required to
achieve superconductivity
 Expensive due to cooling to critical temperatures:
o Liquid He (Type I superconductors)
o Liquid N2 (Type II superconductors)
 The materials of which they are made of are often brittle and hard to manufacture (type II)
19
Applications of Superconductors
19.1 Describe how superconductors and the effects of magnetic fields have been applied to
develop a maglev train:
 Maglev = MAGnetic LEVitation
 Allows frictionless contact with the track – appropriate for high speed trains
 The guiderail under the train contains conventional electromagnets
 On board, helium-chilled superconducting electromagnets produce powerful magnetic fields
 The fields in the rail and the train repel each other so that the entire train is levitated slightly
above the track
 Propulsion and braking is achieved by the fields in front and behind the train attracting and
repelling
19.2 Discuss possible applications of superconductivity and the effects of those applications on
computers, generators, motors and transmission of electricity:
Computers:
 Speed and further minimization of computer chips is limited by heat due to resistance
 Using superconducting materials to construct connecting wires and microchips can increase
the speeds of processors and reduce energy loss in computers
Motors and Generators:
 Generators lose energy by resistance heating in the coils needed to produce magnetic fields,
and are limited in the strength of the fields they can produce
 Superconducting coils would allow generators to be much more powerful, smaller & efficient
Transmission of Electricity:
 A great deal of energy is wasted in the transmission of electricity through conversion of
current to heat due to the resistance of transmission wires
 By using superconducting wires, energy losses through the electricity grid will be eliminated,
resulting in greater efficiency and thus reduced power costs
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20
Magnetic Levitation
20.1 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:
 A superconductor has the property of preventing magnetic field penetration
 This effect – ‘The Meissnor Effect’ – comes about by the generation of an equal and opposite
magnetic field in the superconductor;
o The external magnetic field causes eddy currents to flow in the superconductor
o By Lenz’s law, the magnetic field created by these opposes the field of the external
magnetic field
 The induced field repels any outside field so a light magnet will ‘levitate’ on this field
Page | 14
Practicals to cover:
(1.) Perform an investigation and gather first-hand information to observe the occurrence of
different striation patterns for different pressures in discharge tubes
(1.) Perform an investigation to demonstrate and identify properties of cathode rays using
discharge tubes containing a maltese cross, containing electric plates, with a fluorescent display
screen, containing a glass wheel and analyse the information gathered to determine the sign of
the charge on cathode rays
(2.) Perform an investigation to demonstrate the production and reception of radio waves
(3.) 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
(4.) Perform an investigation to demonstrate magnetic levitation
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