Download Astrophysics by Jonathan Chan

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Ideas and implementations
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:
The rays:
 Travelled in straight lines
 Created a shadow when an opaque object was in its
path
 Passed through thin metal foils without damage.
Observations that fit the Particle model:
The 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
It was not until Thompson efficiently evacuated his tubes and was able to calculate the velocity, and to deflect the ray.
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. (–ve to +ve)
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)
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, there is no force
experienced
 Maximum force is experienced when travel ling at
90 degrees to the field lines

The right-hand-palm-rule (RHPR) is used to determine
the direction of the force.
o v = thumb
o T= fingers direction
o F= direction of palm
o velocity = thumb
o If negative flip hand
o Put paper up to determine if force is into or
out of page.
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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Ideas and implementations
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:
where
𝑉
𝐸=
𝑑

Force on a moving charge:
𝐹 = 𝑞𝐸
where
F = electric force (N)
q = electric charge (C)
E = electric field strength (NC-1)
E = electric field strength (NC-1)
V = voltage/potential difference (V)
d = distance between plates (m)
outline Thomson’s experiment to measure the charge/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

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
𝑚𝑣 2
= 𝑏𝑞𝑣
𝑟
Fe=fb
Eq= Bqv
𝐸
V=
𝐵
Centripetal = magnetic force
Substitute V=
𝐸
𝐵
𝑞
𝐸
=
𝑚 𝑟𝐵2
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Ideas and implementations
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
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
Part
Electron gun
Deflection
plates/coils
Fluorescent
screen
Role in oscilloscopes and TV
 Heated cathode releases electrons and produces a fast-moving stream of electrons.
 The electron gun produces a narrow beam of electrons - the electrodes (cathode and anode) in
the gun accelerate the electrons.
 IN TV’s 3 electron guns are used to strike red, blue and green filters.
 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.
 The fluorescent screen is coated with a material that emits light when struck by electrons.
 The screen is scanned 50 times a second.
 The screen glows for a short time so the screen always shows a picture.
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
Ideas and implementations
solve problem and analyse information using: 𝑭 = 𝑄𝑣𝐵𝑆𝑖𝑛 𝜃 , 𝑭 = 𝑞𝐸 and 𝑬 =
𝑉
𝑑
1.
A proton travelling at 5.0 x 104 m s-1 enters a magnetic field of strength 1.0 Tesla at 90°. Determine the magnitude of the force
experienced by the proton.
2.
The path of a helium nucleus, travelling at 3.0 x 103 m s-1, makes an angle of 90° to a magnetic field. The electron experiences
a force of 1.2 x 10-15 N while in the field. Calculate the strength of the field.
3.
Two parallel plates, placed 0.1 m apart, are connected to a 6-volt battery. Determine the electric field strength between
the plates.


E = 6 V ÷ 0.1m
E = 60 V m-1
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Ideas and implementations
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
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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Ideas and implementations
identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantized
Black body:
 An idealized object that absorbs all electromagnetic
radiation that falls on it, increases the temperature
and then radiates it out as the temperature falls.

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 and occurs when
an atom changes level from one quanta level to
another.



Energy is not continuous, can only be radiated or absorbed in small, discrete amounts or packets called quantum, now
known as photons. There cannot be 1.5 quanta.
The size of each quantum of energy depends on the frequency of the light.
hc
Mathematically, 𝐸 = hf =
, where:
λ
o E= energy of a photoelectron (J)
o h = Planck’s constant = 6.626 x 10-34
o c = 3 x 108m/s
o f = frequency (hz)
o λ =wavelength (m)
1eV = 1.6 x 10-19 joules.
E joules = Eev x 1.6 x 10-19
V=fλ
identify 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.
 Einstein combined the particle model of light and Planck’s hypothesis to explain the photoelectric effect.
 He extended Planck’s quanta idea to light and also explained that wave and particle behaviour can coexist in light – dual
nature.
 Einstein made 5 major conclusions:
o the intensity of light is proportional to number of photons ( each with the same amount of energy)
o The energy of a single photon is E= hf, which is used to break electrons away
o Excess energy gives the photoelectron its kinetic energy (KE = hf-W) ,
 W= work function (the minimum energy to free electrons from the metal surface. This is different for
every metal.

o
o
threshold frequency- minimum frequency the light must have to cause the photoelectric effect in a
metal.
As the intensity of light approaches an appropriate frequency, photoemissions increase as more photons are
emitted.
Photons of light below the threshold frequency do not carry enough energy, so no photoelectrons can be
emitted/released.
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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Ideas and implementations
Graphical analysis
 gradient = Hertz’s constant
𝑊
 Threshold =
𝐻

Equation = hf-W
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 3x10 8ms-1 in a vacuum
identify the relationships between photon energy, frequency, speed of light and wavelength: E  hf and c  f

E = hf






Note: convert to J when using eV
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
 c = speed of light (3.0x108 ms-1)
 f = frequency of the wave (Hz)
 λ = wavelength of the wave (m)
identify data sources, gather, process and present information to summarise the use of the photoelectric effect in photocells
and solar cell
Solar (photovoltaic) 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
Photocells (photodetectors):
 Photocells are electric devices with resistances that
alter in the presence of light.

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
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
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
Ideas and implementations
devices (e.g. used to switch street lights on/off)
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

Also used in burglar alarms, automatic opening
doors, entrance buzzers (when entering into a shop),
breathalysers, etc.
solve problems and analyze information using: 𝐸 = ℎ𝑓and c  f

c  f
Calculate the wavelength and the energy of a photon of light with frequency equal to 1.984 x 10 14 Hz.

x
= 1.51 x 10-6
Calculating the energy of the photon: E = hf
E = 6.628 x 10-34 x 1.984 x 1014 = 1.31 x 10-19 J

Two of the lines in the emission spectrum of mercury represent violet light of wavelength 4.05 x 10 -9 m and red light
of wavelength 6.90 x 10-9 m. Which of these wavelengths would have the more energetic photons?
Photons of the violet light would have more energy as violet light has a shorter wavelength than red light, its frequency is
higher.
process information to discuss Einstein’s 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
Einstein's View:
 Supported no government, had no particular
political views

Loyal to whatever government decided

Refused to sign document supporting the war effort,
signed peace document instead

One of first German intellectuals to sign the
document supporting the war effort

Pacifist (peace-keeper); did not want his work
contributing to killing others.
Then devoted his work and research to the war
effort

He left to Switzerland and made it publically known
that science was for the good of humanity.

identify that some electrons in solids are shared between atoms and move freely


Metallic bonds are interactions through which metal atoms are joined together to
form a lattice structure described as a “sea of electrons”. In most metal 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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Ideas and implementations
describe the difference between conductors, insulators and semiconductors in terms of band structures and relative electrical
resistance
Valence band
Conduction band
Energy gap
Diagram/image
Insulator
Completely filled
Well separated
Large
Type of material
Semi-conductor
Almost filled
Just separated
Small
Conductor
Partially filled
Overlaps
None
identify absences of electrons in a nearly full band as holes, and recognise that both electrons and holes help to carry current


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 force electrons in some
semiconductors to move into the conduction band leaving holes. Electrons keep moving to fill the gap, thus helping to
create a current.
compare qualitatively the relative number of free electrons that can drift from atom to atom in conductors, semiconductors
and insulators



Insulators – no free electrons
Conductors- many free electrons
Semi conductor- one valence electron. 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.
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 semiconductors, 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
o it can handle higher electric currents before overheating
o processing techniques were developed to produce very pure, single-crystal forms
o in single-crystal form, the molecular structure of the material is uniform, thus ensuring consistency of
properties
describe how ‘doping’ a semiconductor 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 ratio of free electrons to holes changes producing an extrinsic semiconductor.
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
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Ideas and implementations
identify differences in p and n-type semiconductors in terms of the relative number of negative charge carriers and positive
holes
P-type Semiconductors:


N-type Semiconductors:
An extra electron
is bound to the
lattice which can
be easily freed.
Elements such as
aluminium and
gallium (3 valence
electrons – group
3 elements) are
used as doping
agents with silicon.

A hole is
produced, as
there are only 3
valence
electrons
available.
Elements, such
as arsenic and
phosphorus (5
valence
electrons –
group 5
elements) are used as doping agents with silicon.

describe differences between solid state and thermionic devices and discuss why solid state devices replaced thermionic
devices
Properties/workings
How it works?
Construction
Size
Heat produced
Start-up time
Reliability
Thermionic Devices
 Contains a cathode that emits
electrons only when heated to a high
temperature
 Fragile
 Large
 Large amount of heat
 Long
 low
Solid State Devices
 Uses semiconductors to generate electron flow no heated circuit required





Robust
Small
Small
Fast
High
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
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



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
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Ideas and implementations
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





The discovery of the transistor led to the development of integrated circuits or microchips in which many thousands of
transistors could be constructed on a tiny microchip. This led to the Microprocessor which are more complicated forms
of microchips that contain logic units.
Many tasks formerly done manually, are now done electronically. E.g. robots and machines can perform work in areas
that dangerous to humans (heavy labour).
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.
An negative impact is the overdependence on technology and a throw away society.
identify data sources, gather, process and present information to summarize the effect of light on semiconductors in solar
cells



When light strikes a semiconductor material a certain portion of
the light is absorbed into the semiconductor material.
The energy of the absorbed light, in the form of photons, is
transferred to the semiconductor resulting in electrons and
positively charged holes moving across the PN-junction in opposite
directions.
An electric field within the photovoltaic cell acts to force the
electrons in a certain direction. A metal grid on either side of the
solar cell allows the electrons to collect and, if connected to an
external circuit, a current will flow.
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
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.
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Ideas and implementations
describe conduction in metals as a free movement of electrons unimpeded by the 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
The outer electrons from adjacent atoms have overlapping orbitals, which makes it easy for electrons to move.
Metals have many electrons that are free to move, thus they are good conductors of electricity
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
describe the occurrence in superconductors below their critical temperature of a population of electron pairs unaffected by
electrical resistance



At room temperatures, the metallic bonds (the lattice) holding the conductor together vibrates and interferes with
electron movement through the conductor
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’

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.
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Ideas and implementations
discuss the BCS theory
At temperatures below the critical temperature for the superconducting material:
 When an electron travels past the positive ion >> lattice distortion
 This causes vibrations are called ‘phonons’. >>> positive trough formation
 A nearby electron – which is attracted by the positive trough of the lattice –
absorbs the energy (phonon) and is able to overcome the repulsion.
 The nearby electron can catch up.
 A ‘Cooper pair’ is formed.
 The Cooper pair travels through the lattice unimpeded
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)
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
Type I superconductors:
Element
Aluminium
Lead
Mercury
Tin
Titanium
Tungsten
Zinc
Critical Temperature (K)
1.2
7
4
4
0.4
0.015
0.85
Type II superconductors:
Superconducting
Material
YBa2Cu3O7
Tl2Ba2Cu2O5
HgBa2Ca2Cu3O8
Critical Temperature (K)
90
80-125
133K
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



The magnet floats due to the meissner effect. A magnet placed near the superconductor will induce a
magnetic field inside it (lenz’s law).
The superconductor will expel the field as it has zero resistance, thus making the magnet float. (Meisnner
effect)
Since the currents in the superconductor meet no resistance, they adjust themselves to maintain the
levitation.
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Ideas and implementations
gather and process information to describe how superconductors and the effects of magnetic fields have been applied to
develop a maglev train
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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