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IDEAS TO IMPLEMENTION
QUESTIONS & PROBLEMS
SOLUTION QUIDELINES

How to answer a question: problem solving (t0_372.pdf)
p1.23
Waves transfer energy without any material being transformed. At first cathode rays were
though to be waves like X-rays which has been recently discovered.
Cathode rays were deflected both by electric and magnetic fields indicating that cathode rays
were a stream of negatively charged particles and J J Thompson measured the qe/me ratio
providing compelling evidence that cathode rays were a beam of negatively charged electrons.
p1.28
E
m = 10-30 kg
q = +6×10-12 C
F  ma
a = 7.0×1021 m.s-2.
F  Eq
30
21
F m a 10  7  10 
E 

N.C-1  1.2  103 N.C-1
12
q
q
6

10


+q
+I
F
out of page
B
right hand palm rule
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p1.30
force on a moving charge in a magnetic field
F  B q v sin 
F
B
q
v

force on charge particle [N]
strength of uniform magnetic field [T tesla]
magnetic flux density
charge of particle [C coulomb]
velocity of charged particle [m.s-1]
angle between magnetic field lines and current direction
 = 0  F = 0 current direction & magnetic field
direction parallel
 = 90o  F = Fmax current direction perpendicular to
magnetic field direction
magnitude only
magnitude only
magnitude only
magnitude only
magnitude only
p1.34
I
v
B
right hand screw rule:
magnetic field into
page
F
eright hand palm rule:
motion of electron is
directly towards the wire
p1.44
B = 1.2310-3 T
d = 22.5 m = 2.2510-3 m
accelerating voltage Va = 10.5 kV = 10.5103 V
(a)
v = ? m.s-1
accelerating voltage does work on the electron increasing its kinetic energy
2 eVa
eVa  12 m v 2 v 
 6.08  107 m.s-1
m
(b)
right hand screw rule – magnetic force directed up the page
FB  B e v  1.20  1014 N
(c)
magnetic force & electric force must cancel
– electric force directed down the page
FB  FE  1.20  1014 N
(d)
Electric field between the plates – same direction as force on a positive charge
– up the page
F
FE  e E E  E  7.47  104 V.m-1
e
(e)
voltage across the plates
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2
– bottom plate is positive (electron attracted to bottom plate)
E
V
d
V  E d  1.68  103 V
(f)
J.J Thompson’s e/m experiment
e
V2

m 2 d 2 Va B 2
From measurement of E (or V and d), B and Va the e/m ratio can be found.
Alternatively: Thomson adjusted the electric field strength and magnetic field strength to
make the electrons pass straight through. This allowed him to calculate the velocity v of the
electrons (cathode rays). He then passed the electrons through the magnetic field B alone, and
from the radius R of their path calculated the q/m ratio
e
v

m BR
p1.65
(a)
striations
(b)
right hand side is the cathode – striations are nearer than anode
(c)
See notes Electrical discharges
(d)
Pattern will not significantly change but molecules of different gases emit radiation of
different frequencies in such transitions. Therefore, the glow of the positive column
has a characteristic colour for each gas.
(e)
The pressure of the gas is mainly responsible for the observed patterns.
p1.68
right hand screw rule – force on positive charge is directed towards the right
The magnetic force acting on the positive charge will always act towards the centre of a circle
– the electron will move in a circular path.
p1.71
(a) (b) See notes – spectral tubes
(b)
The tubes have different pressures inside them.
(c)
highest to lowest pressure B D A C
p1.75
force on a moving charge in a magnetic field
F  B q v sin 
The force only depends on the component of the velocity which is at right angles to the
magnetic field. it does not dependent on the parallel component.
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p1.76
(a)
CRO – cathode ray tube
(b)
Cathode ray tubes using in televisions and cathode ray tubes
(c)
A – low voltage to provide current to heat cathode
B – hot cathode – electron gun
C – accelerating voltage – large used to accelerate electrons to give them large kinetic
energies
D – accelerating voltage plates
E – electric deflecting plates – to change the path of the electron beam
F – fluorescent screen – when an electron hits the screen a flash of light is emitted
(d)
v  0.60 c
Ek  12 mev 2  qeV
V
mev 2
 9.2  104 V
2qe
(e)
d  18 mm  18  103 m
V  254 V
V
 1.41  104 V .m 1
d
F  qe E  2.3  1015 N
F
a
 2.5  1015 m.s 2
me
E
(f)
(g)
p1.79
right hand palm rule  electron travels in a circular orbit
v = 2.0107 m.s-1 |q| = 1.60210-19 C m = 9.1110-31 kg
R = ? m T = ? s f = ? Hz
Magnetic force = centripetal force
m v2
mv
Bqv 
R
 1.38  104
R
Bq
circumference
f 
2 R  v T
T
B = 0.010 T
m
2 R
 4.34  1011 s
v
1
 2.31  1010 Hz
T
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p1.93
C plates must be conductors
E
V
d
plates must be conductors
p1.94
(a)
The shadow of the cross showed that the cathode rays travelled in straight lines.
Without the cross there is no shadow.
(b)
The glass fluoresces because the energy carried by the electrons are absorbed by the
glass molecules. The molecules of the glass become and excited and then loss energy
by emitting photons of visible light.
(c)
The cathode rays are negatively charged electrons – negative charged particles
moving in a magnetic field experience a force – electron beam deflected causing a
distortion of the shadow of the cross.
(d)
Electrons are constituent of atoms, investigate properties of electrons, measure the
qe/me ratio.
(e)
Need a high voltage source such as an induction coil
*******************************************************************
p2.05
high
voltage
source
spark
gaps
induction coil
receiver
transmitter
Sparks across the gap of the transmitter produced the induction coil emitted electromagnetic
radiation which included radio waves and UV radiation. When a sheet of glass was placed
between the transmitter and receiver, the spark in the receiver was weaker due to the
absorption of the UV by the glass. Hertz noted the observation but did not follow it up. The
UV absorbed by the metal receiver enabled electrons to be mitted more easily giving a bigger
spark – photoelectric effect.
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p2.06
Planck
energy of atomic oscillators is quantized E = h f
Einstein
particle model for light – light consisted of photons – energy of each photon hf
An electron emitted immediately from the surface by the interaction with a photon
Applied the Law of Conservation of Energy
Energy of incident photon = Energy to release electron from surface
+ KE of ejected electron
2
2
h f = ½ m v +  = ½ m vmax + W
W is the min energy needed to remove an electron from the surface – work function
For electron to be released h f  W
threshold frequency h fc = W
p2.08
Stopping voltage
Threshold frequency
Threshold wavelength
Voltage required to reduce photocurrent to zero – eVs is the
measurement of the maximum kinetic energy a photoelectron
Minimum frequency for the release of electrons from the surface of
the material
Maximum wavelength for the release of electrons form the surface
of the material
h f  12 mvmax 2  W  eVstopping  W
Vstopping 
h
W
f 
e
e
2
Stopping Voltage Vs (V)
1.5
1
0.5
0
-0.5
-1
-1.5
-2
0
slope
intercept
Work function
Planck’s constant
Threshold frequency
Threshold wavelength
2
4
6
frequency f (Hz)
8
10
x 10
14
m = 3.7510-15 (V.Hz-1)
b = -1.67 V
W = 1.7 eV = 2.710-19 J
h = 6.010-34 J.s
note – answer low than accepted value
fc = 4.51014 Hz
c = 6.710-7 m = 670 nm
The two lines have the same slope = h / e.
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p2.13
Max Planck was the first person to give a qualitative explanation of the curves for the
radiation emitted by a blackbody. He assumed that energy is quantized. The change in energy
of an oscillator is a nhf where n is an integer.
The total area under the curves represents to energy emitted by the blackbody. The higher the
temperature, the more energy radiated.
Wien’s Displacement Law
peak T = constant
The higher the temperature, the shorter the wavelength of the em radiation emitted by the
blackbody.
p2.33
(a)
Blackbody - object which gives the maximum amount of energy radiated from its
surface at any temperature and wavelength and the absorbs all the radiation that falls
on it.
(b)
JWS Rayleigh & James Jeans
Math model described blackbody radiation curves at long wavelengths accurately but
not at shorter wavelengths in the UV, radiation from cavity absorbed and re-emitted
with ever smaller and smaller , energy emitted from cavity becoming infinite – UV
catastrophe
(c)
Max Planck was the first person to give a qualitative explanation of the curves for the
radiation emitted by a blackbody. He assumed that energy is quantized. The change in
energy of an oscillator is a nhf where n is an integer.
p2.50
Light – wave model – interference effect
Light – particle model - Photoelectric Effect
Einstein – special relativity
speed of light is constant independent of motion of source or observe
equivalence of mass and energy E = m c2
Einstein – light stream of particles called photons
energy of photon E = hf
photon energy absorbed by electron and given sufficient energy
to escape the surface of a metal
photons have mass m = E/c2
Photoelectric Effect
The photoelectric effect - release of an electron from the surface of a metal exposed
to electromagnetic radiation. Classical physics was unable to explain - the threshold
frequency, which is the frequency of light at which electrons are emitted from the surface of a
metal. It predicted that energy from light would be absorbed over time until an electron had
sufficient energy to leave the surface of the metal. Einstein used Planck’s theory and the
concept that photons carried energy to explain the photoelectric effect. Einstein assumed that
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light existed as photons, each with an energy equal to E = h f and that the number of photons
determined the light intensity. Photons with the highest energy correspond to the highest
frequency, and an electron would not be emitted from a metal surface unless the photons
possessed energy equal to or greater than the energy needed to overcome the energy holding
the electron on the metal surface. The energy required to release the electron from the surface
was called the work function, W. Einstein was able to combine the photon energy with the
work function and the KE of the emitted electrons in the equation h f = EKmax + W
p2.61
E = ? eV = ? J f = ? Hz
 = 550 nm = 55010-9 m
c=f E=hf=hc/
f = c /  = 5.51014 Hz
E = 3.610-19 J = 2.3 eV
visible
p2.62
(a)
Using a reverse voltage to reduce the photocurrent to zero  stopping voltage.
2
1
2 mvmax  eVstopping
(b)
h f  W  12 m v 2
hf  Wmin  12 m vmax 2  Wmin  eVs
Vs 
(c)
h
W
f  min
e
e
see notes
Plot Vs against f  slope = h/e  h
p2.66
violet 1 = 400 nm = 40010-9 m f1 = ? Hz E1 = ? J = ? eV
green 2 = 400 nm = 40010-9 m f2 = ? Hz E2 = ? J = ? eV
red
3 = 715 nm = 71510-9 m f3 = ? Hz E3 = ? J = ? eV
c
c f  f 
E  h f 1 eV  1.602  1019 J

14
f1 = 7.510 Hz E1 = 5.010-19 J = 3.1 eV
f2 = 5.51014 Hz E2 = 3.610-19 J = 2.2 eV
f3 = 4.21014 Hz E3 = 2.810-19 J = 1.7 eV
Violet light photons have 0.9 eV more energy  released electrons will have 0.9 eV extra
kinetic energy.
The energy of the red photons is less than the work function of the surface.
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p2.70
(a)
high
voltage
source
spark
gaps
induction coil
receiver
transmitter
(b)
In order of increasing energy
radio microwave IR visible UV X-rays -rays
(c)
Oscillating charges  oscillating electric fields  oscillating magnetic fields
 production of radio wave
Radio wave – oscillating electric & magnetic fields  oscillating electric force on
free electrons in wires – detection of radio waves
(d)
The glass in transparent to radio waves.
(e)
UV does not pass through glass.
(f)
When glass plate blocks UV, spark shorter because less energy absorbed by electrons
– less easily removed  more difficult to create the spark.
(g)
Photoelectric effect – release of electrons from a surface by absorption of incident em
radiation.
p2.72
Photoelectric effect – release of electrons from a surface by absorption of incident em
radiation. The predictions of classical physics could not explain the experimental
observations made. Einstein used Planck’s idea of energy quantization to explain the
photoelectric effect by applying the law of conservation of energy where light behaves as a
stream of particles called photons and the energy of each photon is given by E = hf. This
explanation also supported the radical idea introduced by Planck. The impact of his
explanation was great and was the starting point for modern physics where the ideas of
classical physics were not valid.
p2.90
In classical physics, the energy of a system could has a continuous range of values, but
Planck introduced for the first time the concept of discrete changes in energy of a system in
his explanation of blackbody radiation. This was a radical idea at the time and one of the first
steps in developing a new way to describe the world in terms of the ideas in modern physics.
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p2.95
(a)
Light was thought to be a wave phenomena
(b)
Light could produce as interference pattern a wave behaviour
(c)
Light a stream of particles – photons
(d)
Both the observations made on the photoelectric effect and blackbody radiation could
not be explained in terms of classical physics – radial new ideas had to be introduced
– e.g. energy quantization and light as both a particle and a wave
********************************************************************
p3.13
Boron – valence 3  p type semiconductor
Conduction by the movement of holes to the right
p3.18
Metal – free electrons moving through crystal lattice – collisions between electrons and
lattice ions  resistance
p type semiconductor – holes (+)
n type semiconductor – electrons
p3.22
Small energy band gap between the valence band and the conduction band
p3.23
p type semiconductor – small number impurities atoms of valency 3 (B Al Ga) added to
very pure silicon
p3.24
Early radio communications was based upon valve technology (diodes and triode valves)
Valves – expensive, fragile, large, energy inefficient
With the introduction of semiconductor devices such as the transistor – valves became
obsolete in radio communications
Solid state devices far superior – no warm time, require less energy input, smaller, less fragile,
more robust, cheaper, can construct integrated circuits on a single chip of semiconducting
material
Next development – integrated circuits – completed circuits containing millions of
components on the one silicon chip
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p3.27
(a)
Doping of very pure silicon involves the additions of small amount of impurity atoms
of Group 3 or 5 elements.
n type semiconductor – Group 5 – phosphorus P, arsenic As, antimony Sb
p type semiconductor - Group 3 – aluminium, gallium Ga
(b)
Electrons produced by photoelectric effect where an electron is freed by the
absorption of a photon.
(c)
A potential difference is
established between the different
silicon layers when a photon is
absorbed, the holes migrate
through the p type semiconductor
while free electrons cross the n
type semiconductor and a current
is maintained by the continual
absorption of photons.
n-Si
_
undoped
Si
-
+
p-Si
photon absorbed
creates a free electron & hole
+
electron
flow
p3.30
(a)
At first it was difficult to obtain large quantities of very pure silicon
(b)
Chips made with silicon are much less temperature sensitive than using germanium
Pure silicon was not available when the first transistors were developed. It was then very
difficult to produce silicon of sufficient purity, while it was possible to produce high purity
germanium.
p3.33
See notes
p3.39
Solar cell – current – photoelectric effect – when a incident photon of sufficient energy
(greater than work function of surface) is absorbed by an electron in the semiconductor layer
it creates a free electron and a hole The electrons acts as charge carriers in the n type
semiconductor material and holes act as charge carriers in the p type semiconductor material
 potential difference between the n and p semiconductor layers – external circuit connected
 current. As long as light falls on the photocell, the electrons and holes are mobilized to
give a current through the light globe
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p3.44
Valves
thermionic devices - evacuated glass containers (gas at low pressure)
electrons released from hot filament – thermionic emission
large (bulky)
hot – used lots of energy
diodes – rectification
triodes – amplifiers, switches
Solid state devices - miniaturisation
diodes (pn junctions) transistors (npn, pnp)
IC - integrated circuits
consume little power
Superconductor – zero resistance below critical temperature
Applications – solid state devices
switching
amplification
Applications – superconductors
power transmission
magnetic levitation
electron energy
p3.52
insulator
n type semiconductor – a donor impurity atom has a 5th valence electron that does
not participate in the covalent bonding and is very loosely bound. It is relatively easy
for an electron to be excited from the donor level into the conduction band where it
acts as a charge carrier – charge carriers are mainly negative electrons in the
conduction band.
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semiconductor
conductor
p type semiconductor – acceptor impurity atom has only 3 valence electrons, so it can
borrow an electron from a neighbouring atom. An electron can move from the valance
band to the acceptor level creating a hole in the valance band. The resulting hole is free to
move about the crystal – positive charge carriers moving in valence band.
12
p3.55
conduction band
( empty)
conduction band
(nearly empty)
Egap
valance band (full)
valance band (full)
semiconductor
diamond
conduction band
electrons move in conduction band
-
hf
valance band
+
holes move in valance band
Semiconductor
 = ? m Egap = 0.8 eV = (0.8) qe J
qe = 1.60210-19 C
h = 6.62610-34 J.s
min energy of photon and max wavelength of photon
c = 3.0 m.s-1
E = h f = h c / = Egap = 1.2810-19 C
 = h c / Egap = 1.5510-6 m = 1.55 m infrared
Diamond
 = ? m Egap = 5.0 eV = (5.0) qe J qe = 1.60210-19 C
min energy of photon and max wavelength of photon
c = 3.0 m.s-1
E = h f = h c / = Egap = 8.0110-19 C
 = h c / Egap = 2.4810-6 m = 248 nm ultraviolet
p3.57
As the temperature is increased, lattice vibrations increase and some electrons gain sufficient
energy to be excited across the energy gap form the valance band to the conduction band.
More electrons can take part in conduction lowering the resistance of the semiconductor. For
metals, the increased lattice vibrations causes energy to be lost from electrons by collisions
with the lattice, increasing the resistance.
The conductivity of a material depends upon the number of charge carriers available. In a
metal there are energy levels available for electrons to gain energy and take part in
conduction since the band gap is zero - approximately one electron per atom is free to act as a
conduction electron. In a insulator, the band gap is very large and at room temperatures there
are almost zero free electrons in the conduction band and hence the high resistance. In a
semiconductor there is a smaller band gap are there are a few electrons in the conduction
band at room temperatures and the resistance is less than than of an insulator.
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p3.77
Changed society greatly
electronics revolution – small scale circuits – energy efficient compared with values integrated circuits
computer revolution – microprocessors, robots
information revolution – world wide web - GPS
p3.90
Diagram shows acceptor level  p type semiconductor  valence 3 elements  B, Al, Ga
p3.92
right hand screw rule – charges moving in a magnetic field
Holes(+) move up
Electrons (-) move down
p3.95
(a) (b) (d)
10
9
increased intensity
photo current I (uA)
8
7
6
5
4
3
2
1
0
0.5
(c)
1
1.5
photon energy E (eV)
2
2.5
Work function W = 1.6 eV
c = ? m
W = 1.6 eV = (1.6) qe J
qe = 1.60210-19 C
c = 3.0 m.s-1
W = h fc = h c /c = 2.56310-19 C
 = h c / W = 7.810-7 m = 780 nm
(e)
IR
For photon energy less than the threshold frequency, no electrons are released from
the surface irrespective of the intensity. For energies above the threshold, as the
energy is increased the number of electrons released increases as shown by the
increase in the current – the greater the intensity more photons – more electrons
released  greater current. When the energy becomes sufficiently high, almost all the
available electrons are released from the surface and there is only a very small
increase in current for increased energy of the photons– saturation effect.
**********************************************************************
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p4.04
(a)
interference of waves – constructive and destructive
(b)
By making measurements on the interference pattern produced by impacting X-ray
beams onto crystalline structures, the spacing between atoms could be estimated. This
lead to the important field of X-ray crystallography which is used to determine the
structure of materials.
p4.14
(a) (b)
Magnetic Leviation - Meissner Effect
When material cooled below the transition temperature in the
presence of a magnetic field, a specimen becomes perfectly
diamagnetic (a diamagnetic object is repelled by a permanent
magnet) due to expulsion of the external magnetic field from
the interior of the superconductor by the cancelling the
magnetic flux inside.
If a small magnet is brought near a superconductor, it will be
repelled because induced super-currents around the surface will
produce mirror images of each pole. If a small permanent magnet is
placed above a superconductor, it can be levitated by this repulsive
force.
Trains – magnetic levitation (Meissner Effect)
Use powerful superconducting electromagnets mounted on
bottom of train and normal electromagnets on the tracks repel the
superconducting magnets and give rise to the forces for
propulsion. Benefits –reduced resistance to the motion
Limitations- need very strong magnets – produced by
superconducting electromagnets but need very low temperatures
for superconducting state – expensive.
p4.20
(a)
A material becomes superconducting (resistance = 0 and expels external magnetic
fields) when its temperature drops below a certain temperature known as the critical
temperature.
(b)
-257 oC -181 oC -140 oC
(c)
High temperature superconductors – goal to find a material that would be
superconducting at room temperature
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p4.44
BSC theory – named after Bardeen, Cooper, Schrieffer who proposed it.
When the temperature drops below the critical temperature the behaviour of the conduction
electrons suddenly changes – electrons pair up forming Cooper pairs which have opposite
spin and linear momentum and all pairs move cooperatively – this state reduces the total
energy of the system well below all other states – The conduction electrons acting as a
collection of Cooper pairs are in the ground state. There is a large energy gap to the next
higher energy states which would destroy the Cooper pairs– the conduction electrons can no
longer interact with the lattice and exchange energy with the lattice which is the mechanism
that produces resistance, therefore the material is superconducting – the conduction electrons
flow through the material with zero loss in energy.
p4.49
The superconductor excludes magnetic fields at very low temperatures.
see p4.14
p4.50
Metals at room temperature – movement of free or conduction electrons – electrons in the
conduction band.
Superconductor – collective flow of Cooper pairs – state of lowest energy – zero resistance to
the flow of electrons.
p4.60
Tin - superconducting
 (10-11 .m)
20
tin
silver
10
0
0
p4.75
Power transmission – see notes
Magnetic levitation – trains – see notes
10
TC = 3.722 K
20
T (K)
p4.78
Above the critical temperature the magnetic field
can penetrate and pass through the material. Below
the critical temperature the material becomes
superconducting and expels the magnet field from
its interior – the magnetic field lines can not pass
through it.
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above critical
temperature
below critical
temperature
16
p4.80
The capacity of microprocessors and the efficiency of their operation is in apart limited by
the heating effect in the devices they operate in. Having zero resistance circuits would allow
their capacity and speed of operation to be increased substantially.
The use of superconductors in transistors (Josephson junction) and superconducting
connection films also allow processing of signals to be greatly speeded up.
p4.88
(a)
A material becomes superconducting (resistance = 0 and expels external magnetic
fields) when its temperature drops below a certain temperature known as the critical
temperature.
(b)
If a small magnet is brought near a superconductor, it will be repelled because
induced super-currents around the surface will produce mirror images of each pole. If
a small permanent magnet is placed above a superconductor, it can be levitated by this
repulsive force.
(c)
Meissner Effect
(d)
Magnetic levitation – trains, motors – reduction in frictional forces
**********************************************************************
8
Max KE of photo-electrons KE (eV)
m008
(a)
(b) Reliability – repeating an
experiment a number of times - use
mean value – graphing the results.
Ignore the data point that is obviously
wrong
(c)
No change to graph.
(d)
Critical frequency – min
frequency below which photo-current
is zero.
fc = 51014 Hz
(e)
Work function – min energy to
remove an electron from the surface.
W = 3.010-19 J
x 10
-19
6
4
2
0
-2
0
5
10
frequency f (Hz)
15
x 10
14
m013
When the temperature of a metal is decreased its electrical resistance decreases because
conduction electrons loss less energy in collisions with the lattice ions whose vibrations
decrease by lowing the temperature.
m020
Power transmission – see notes
Magnetic levitation – trains – see notes
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m045
f = 102.8 MHz = 102.8106 Hz
E = ? J = ? eV  = ? m
qe = 1.60210-19 C
E = hf
c = 3.0 m.s-1
E(eV) = E(J)/qe
c=f
h = 6.62610-34 J.s
 = c/f
E = 6.810-26 J = 4.310-7 eV
very small photon energy since in radio part of em spectrum
 = 2.9 m
long wavelength because of small frequencies
m047
(a)
Cathode rays were deflected by electric and magnetic fields  negatively charged
particles. J.J. Thompson was able to measure qe/me ratio  electrons
(b)
To produce an electrical discharge – need very high voltages > 10 000 V
(c)
make sure that your do not come into contact with the high voltage source
m049
proton q = 1.610-19 C
m = ? kg
v = 6.00 m.s-1
B = 1.82 T
R = 0.350 m
m0 = ? kg
Magnetic force = Centripetal force
mv 2
qv B 
R
p  mv  q B R
qBR
m
 1.70  1027 kg
v
m0
m
v2
1 2
c
m0  m 1 
v2
 1.67  1027 kg
2
c
m050
Paddle wheel spins when struck by cathode ray beam  beam carries momentum
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m065
CRO
Transistor circuit
step-up transformer
step-down transformer
- - - - -
E
+ + + + +
m072
m = 9.610-6 kg
electric field E 
V = 49 V
d = 2.0 cm = 2.010-2 m
E = ? V.m-1
q=?C
V
 2.5  103 V.m -1
d
electric force (up) = gravitational force (down) charge must be positive
qE  mg
mg
q
 3.8  108 C
E
m076
diagram of an induction coil with a spark gap ~ 5 mm
diagram of a radio not tuned to a radio station
The sparking across the terminals of the induction coil produced radio wave. These radio
waves were detected by a radio not tuned to a radio station. A static or crackling noise was
heard and this was heard when tuner was scanned over the frequency range for the AM radio
stations.
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m080
V = 1234 V
FB
+
+q
v = 2.67104 m.s-1
magnetic field into B page
d = 1210-3 m
-
FE
E = ? V.m-1
electric field E down
B=?T
Electric field E = V / d = 1.03105 V.m-1
For beam of positive charges to move un-deflected, magnitudes of the electric and magnetic
forces must be equal.
FE = FB
qE=qvB
B = E / v = 3.85 T
m082
Need to link the work done by Einstein and Planck to outside factors influencing their work.
Einstein
special relativity
photoelectric effect – light as a stream of particles - photons
energy quantized E = h f
Planck
explanation of blackbody radiation
energy of oscillators quantized E = h f
World War II
Einstein
left Germany
pacifist
Planck
continued working and supported the German governement
m082
work function W = 8.7210-19 J = (8.7210-19) / (1.60210-19) eV = 5.44 eV
min energy of photon to produce photo-electrons
h = 6.62610-34 J.s
c = 3.00108 m.s-1
hf=hc/=W
f = W / h = 1.321015 Hz
 = h c / W = 2.2810-7 m
UV
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m095
electrons – free electrons that can move due to an applied electric field – electrons acquired
sufficient energy to be in the conduction band.
holes – removal of an electron from a bond, holes as if a positive charge due to an applied
electric field – holes located in valance band.
m111
incident radiation
 = 187 nm = 18710-9 m
c=f
c = 3.00108 m.s-1
f = c /  = 1.601015 Hz
max KE
EKmax = 2.5 eV = (1.60210-9)(2.5) J = 4.0010-19 J
Photoelectric Effect
h f = EKmax + W
W = h fc
Need to find threshold frequency fc
f c = f – EKmax / h = 1.01015 Hz

Al
m115
When mercury is cooled below its critical temperature of 4.2 K is becomes superconducting,
that is the conductor has zero resistance.
Mercury is a metal and is a good conductor and conduction is due to the movement of the
free electrons in the conduction band at room temperatures (20 oC). See energy band diagram.
Below the critical temperature, mercury is a superconductor, the conduction is due to the
cooperative movement of Cooper pairs through the lattice.
Conduction in a P type semiconductor is primarily due to the movement of holes in the
valence band See energy band diagram. The resistance of a semiconductor is higher than that
of a metal such as mercury.
m120
The frequency of the incident light was less than the threshold frequency.
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m200
Max Planck (19 Oct 1900) - hypothesis – radiation emitted & absorbed by walls of a
blackbody cavity – radiation quantized
change in energy
| E2 – E1 | = n h f
n = 1, 2 3, …
His assumption of energy levels quantized enable him to obtain an equation to successfully
describe the blackbody radiation curve: Planck radiation law – agreed with experiment.
Each atom behaved as a small antenna (electromagnet oscillator) – energy of an oscillator n
h f (n = 0, 1, 2, ….) - energy of oscillator was quantized. Change in energy of oscillator
(emission & absorption) quantized
E = integer  h f

atomic oscillators do not radiate or absorb energy in continuously variable amounts
Birth of Quantum Physics - Planck did not believe that atomic oscillators behaved this way used his model to simply account for the shape of the blackbody curve
Curve shows the radiation emitted from a blackbody at a fixed temperature. The temperature
determines the peak in the curve
Wien’s Displacement Law
peak T = constant
The total area under the curve represents the rate of the total energy emitted by the blackbody.
m400
Understanding of crystal structures – many advances in science – understanding of
conduction led to the development of the semiconductor industry – developments in
electronics, computers, information technology.
X ray diffraction – X ray beam incident on the material – X rays diffracted from atoms that
make up the crystalline structure to produce an interference pattern. By examining the
distance between maxima in the interference pattern Bragg were able to determine the
distance between parallel planes of atoms.
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m550
Accelerating voltages increases KE of electrons
qe V  12 me v 2
Electron in a uniform magnetic field experiences a magnetic force that is always directed
towards the centre of a circle – centripetal force
m v2
B qe v  e
R
m v
B e
qe R
me v  2 me qe V
B
2 me qe V
qe R
qe = 1.60210-19 C
me = 9.10910-31 kg
R = 557 mm = 5.5710-3 m
V = 5.00 kV = 5.00103 V
B=?T
B = 0.043 T
m680
(a)
UV radiation on metal – photoelectric effect – electrons more easily removed – larger
spark.
(b)
Increasing the gap between the terminals of the receiver decreases the strength of the
spark across the terminals.
(c)
A spark produced by the transmitter emits some UV – placing the glass plate reduced
the UV from reaching the receiver – decrease in the strength of the spark.
m690
The kinetic energy of the emitted electrons was proportional to the frequency of the incident
radiation and independent of the intensity of the light.
No electrons were emitted unless the incident light was above a certain threshold frequency.
For frequencies above the threshold frequency, electrons were released immediately from the
surface even for very low intensity incident light.
m700
It was Heinrich Hertz who first observed the photoelectric effect while performing his
experiments on the production and reception of radio waves. He noticed that when light
produced by the spark produced by the spark of his transmitter loop fell directly onto the
receiving loop, the spark at the receiver was stronger (more easily produced). He noticed that
something to do with the light falling on the receiver was allowing the discharge to be more
easily achieved, but he did not investigate further.
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m820
Wien’s Displacement Law
(a)
peak T = 2.910-3 m.K
peak = 550 nm = 550 10-9 m
2.9  103
T
 5.3  103 K
 peak
(b)
T=?K
blue < red  Tblue > Tred
m950
Magnetic force on charged particle is directed towards centre of a circle  circular motion
electron charge
e = q = 1.60210-19 C
electron mass
me = m = 9.1110-11 kg
electron velocity
v = 4.56106 m.s-1
magnetic field
B = 3.2110-2 T
electron moving at right angles to B field  = 90o sin = 1
Force on charge moving in magnetic field
F  B q v sin 
Magnetic force = Centripetal force Fc = m v2 / R
R = m v / B q = 8.0810-4 m
m955
Cathode rays travel in straight lines
m960
Magnetic field directed into page
Right hand rule
Cathode beam deflected down the page
South pole brought near beam
Magnetic field directed out of page
Right hand rule
Cathode beam deflected up the page
m964
See notes
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m965
Einstein – Special Relativity
Speed of light is constant independent of the motion of the source or an observer
(independent of the frame of reference in which it is measured)
Inertial frames of references – time and space not absolute – length contraction, time
dilation, mass / energy E = mc2, mass dependent upon velocity
Einstein – Quantum Theory
Explanation of photoelectric Effect (Nobel Prize)
Particle nature of light – photons E = hf
Emission of photoelectron depends upon frequency of incident radiation
The contributions made by Einstein were instrumental in the start and development of
Modern Physics where ideas were completely at odds with classical physics of the times.
m970
Observation – light on a metal surface  photo-electrons
Classical physics – light as wave – could not explain observations
Radical new theory – light as a stream of particle – photons E = h f
New theory could explain the observations
m 990
Planck and Einstein good friends – supported each other scientific research but their views on
the role of science and scientific research were very different.
Einstein
Activist for world peace and was only one of four scientists to sign a partition for
peace at the start of WW1.
Maintained that scientific research should be removed from social and political forces
and devoted to the pursuit of knowledge and understanding.
Did support the development of the atomic bomb.
after WWII, totally opposed the research, development and use of nuclear weapons –
warned of the devastating energy that could be released.
Planck
Staunch patriot – support the German government and the rise of the Nazi regime.
Work for the the Nazi during WWII.
Science to support a social and political agenda.
Rapid progress in German science as a benefit to all Germans and a clear indication
of their superior abilities.
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