Transcript The Electron - DCS Physics

The Electron
By a Gentleman
Insulators and Conductors
Conduction
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All conduction is due to the movement
of free electrons.
I’m
free
+
In a Semiconductor the electrons are
fixed until they receive a little energy
-
The Silicon, Si, Atom
Silicon has a valency
of 4 i.e. 4 electrons in
its outer shell
Each silicon atom
shares its 4 outer
electrons with 4
neighbouring atoms
These shared electrons
– bonds – are shown as
horizontal and vertical
lines between the
atoms
This picture shows
the shared electrons
Intrinsic Semiconductors
Conduction half way between a conductor
I’m
and an insulator
free
 Crystals of Silica
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A photon releases an electron that now
can carry current
Intrinsic Semiconductors
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A photon releases an electron that now can
carry current
Heating Silicon
We have seen that,
in silicon, heat
releases electrons
from their bonds…
This creates
electron-hole pairs
which are then
available for
conduction
Intrinsic Conduction
If more heat is
applies the process
continues…
More heat…
More current…
Less resistance…
The silicon is acting
as a thermistor
Its resistance decreases
with temperature
Slide 8
The Thermistor
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Thermistors are used to
measure temperature
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They are used to turn
devices on, or off, as
temperature changes
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They are also used in
fire-warning or frostwarning circuits
Thermistor
Symbol
Light Dependent Resistor (LDR)
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The LDR is very similar to the
thermistor – but uses light
energy instead of heat energy
When dark its resistance is
high
As light falls on it, the energy
releases electron-hole pairs
They are then free for
conduction
Thus, its resistance is
LDR Symbol
reduced
Two semiconductor devices
1) Light dependant
resistor – resistance
DECREASES when light
intensity INCREASES
Resistance
2) Thermistor –
resistance
DECREASES when
temperature
INCREASES
Resistance
Amount of light
Temperature
THE VARIATION OF THE RESISTANCE OF
A THERMISTOR WITH TEMPERATURE
10°C
Digital
thermometer
Ω
Water
Thermistor
Glycerol
Heat source
Method
1.Set up the apparatus as shown.
2.
Use the thermometer to note the
temperature of the glycerol and thermistor.
3.
Record the resistance of the thermistor
using the ohmmeter.
4.
Heat the beaker.
5.
For each 10 C rise in temperature, record
the resistance and the temperature using the
ohmmeter and the thermometer.
6.
Plot a graph of resistance against
temperature and join the points in a smooth,
continuous curve.
Precautions
Heat the water slowly so temperature
does not rise at end of experiment
 Wait until glycerol is the same
temperature as water before taking a
reading.
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Extrinsic Semiconductors
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Doping is adding an element of different
valency to increase conductivity of
semiconductor
Extrinsic Semiconductors
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P-type have more holes (Add Group3)
The Boron Atom
Boron is number 5
in the periodic table
It has 5 protons and
5 electrons – 3 of
these electrons are
in its outer shell
Extrinsic Semiconductors
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N-type have more electrons (Add
Group5)
The Phosphorus Atom
Phosphorus is
number 15 in the
periodic table
It has 15 protons and
15 electrons – 5 of
these electrons are in
its outer shell
Extrinsic Conduction – p-type silicon
A current will
flow – this time
carried by positive
holes
Note:
The positive holes
move towards the
negative terminal
Junction Diode
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Two types grown on the same crystal
P-type
N-type
Junction Diode
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Near the junction some electrons from the
‘N’ fill the holes in the ‘P’ crystal.
P-type
N-type
Junction Diode
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This creates area in the middle where
there are no carriers so no conduction
P-type
N-type
This barrier is called the
DEPLETION LAYER
Junction Diode
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When the diode is in FORWARD BIAS the
depletion layer disappears. The diode
conducts.
+
P-type
N-type
-
Junction Diode
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When the diode is in REVERSE BIAS the
depletion layer increases. The diode acts
as a barrier or insulator.
-
Ptype
Ntype
+
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2009 Question 12 (b) [Higher Level]
A semiconductor diode is formed when small
quantities of phosphorus and boron are added to
adjacent layers of a crystal of silicon to increase
its conduction.
Explain how the presence of phosphorus and boron
makes the silicon a better conductor.
What happens at the boundary of the two
adjacent layers?
Describe what happens at the boundary when the
semiconductor diode is forward biased
Describe what happens at the boundary when the
semiconductor diode is reverse biased.
Give a use of a semiconductor diode.
Homework
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2004 HL Q12(d)
The p-n Junction – no potential
0.6 V
As the p-type has
gained electrons – it
is left with an overall
negative charge…
As the n-type has
lost electrons – it is
left with an overall
positive charge…
Therefore there is a voltage across the junction – the junction
voltage – for silicon this is approximately 0.6 V
The Reverse Biased P-N Junction
Take a p-n junction
Apply a voltage
across it with the
p-type negative
n-type positive
Close the switch
The voltage sets
up an electric
field throughout
the junction
The junction is said to be reverse – biased
The Reverse Biased P-N Junction
Negative electrons
in the n-type feel
an attractive force
which pulls them
away from the
depletion layer
Positive holes in
the p-type also
experience an
attractive force
which pulls them
away from the
depletion layer
Thus, the depletion layer ( INSULATOR ) is
widened and no current flows through the
p-n junction
The Forward Biased P-N Junction
Take a p-n junction
Apply a voltage
across it with the
p-type postitive
n-type negative
Close the switch
The voltage sets
up an electric
field throughout
the junction
The junction is said to be
forward – biased
The Forward Biased P-N Junction
Negative electrons
in the n-type feel a
repulsive force
which pushes
them into the
depletion layer
Positive holes in
the p-type also
experience a
repulsive force
which pushes them
into the depletion
layer
Therefore, the depletion layer is eliminated
and a current flows through the p-n junction
The Forward Biased P-N Junction
At the junction
electrons fill holes
Both disappear
as they are no
longer free for
conduction
They are
replenished by the
external cell and
current flows
This continues as long as the external voltage
is greater than the junction voltage i.e. 0.6 V
The Forward Biased P-N Junction
If we apply a
higher voltage…
The electrons feel
a greater force
and move faster
The current will
be greater and
will look like
this….
The p-n junction is called a DIODE
and is represented by the symbol…
The arrow shows the
direction in which it
conducts current
Diode as Valve
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Only allows current in one direction
Forward Bias
Reverse Bias
LED

An LED (Light Emitting Diode) works in the same
way. We use it for pin lights.
Forward Bias
Reverse Bias
Characteristic Curve - Diode
I/A
In
reverse
Bias
No
conduction
V/v
Junction Voltage (0.6V)
Must be Overcome
before Conduction starts
VARIATION OF CURRENT (I) WITH
P.D. (V)
mA
+
6V
-
V
Diode in
forward
bias
VARIATION OF CURRENT (I) WITH
P.D. (V)
A
+
6V
-
V
Diode in
Reverse
bias
Rectifier
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Uses this to turn AC to DC
Mains
Resistor
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This is called half wave rectification
Rectifier
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We use a capacitor to smooth the signal to
get something more like DC
Amplification
On 16 December 1947
William Shockley, John
Bardeen and Walter
Brattain built the first
practical transistor at
Bell Labs
 Despite hardly talking
to each other.
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Transistors
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Small changes in the input signal greatly
changes the size of the depletion layer
The current increases if the D.P. is small
3A
1A
10mA
30mA
Signal Amplification
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So small changes in input signal
create large charges in output.
Thermionic Emission
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Electrons (as named by G. Stoney)
leaving the surface of a hot metal
e-
e-
e-
e-
Hot Metal
e-
Cathode Rays (Really Electrons)
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First we heat the cathode to make the
electrons jump off by Thermionic Emission
We can use a high voltage to accelerate
the electrons to form a stream
C
A
T
H
O
D
E
e-
e-
High Voltage
A
N
O
D
E
Electron Energy Units
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We calculate the energy of each electron
first in electron volts. The energy gained
when an electron crosses a potential
difference of 1Volt.
Energy Gained = 1 eV
C
A
T
H
O
D
E
e-
e-
1v
A
N
O
D
E
Electron Energy
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We calculate the energy of each electron
first in electron volts
Energy Gained = 2000eV
C
A
T
H
O
D
E
e-
e-
2000v
A
N
O
D
E
Electron Energy
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Then we convert this to joules ( Charge on the
electron = e = 1.6x10-19 C)
Energy Gained = e.V = 1.6x10-19 . 2000
= 3.2x10-16 JoulesC
C
A
T
H
O
D
E
e-
e-
2000v
A
N
O
D
E
Electron Velocity
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All the energy on an electron must be kinetic
energy.
Energy Gained = 3.2x10-16 = 0.5mv2
electron mass = 9.1 × 10-31 kg
C
A
T
H
O
D
E
e-
e-
2000v
A
N
O
D
E
Electron Velocity
Energy Gained = 3.2x10-16 = 0.5mv2
electron mass = 9.1 × 10-31 kg
3.2x10-16 = 0.5 (9.1 × 10-31) v2
V2=7x1015
V= 2.6x107 m/s
C
A
T
H
O
D
E
e-
e-
2000v
A
N
O
D
E
CRT and Demo
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2003 Question 9
List two properties of the electron.
Name the Irishman who gave the electron
its name in the nineteenth century.
Give an expression for the force acting on
a charge q moving at a velocity v at right
angles to a magnetic field of flux density B.
An electron is emitted from the cathode
and accelerated through a potential
difference of 4kV in a cathode ray tube
(CRT) as shown in the diagram.
How much energy does the electron gain?
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What is the speed of the electron at the anode?
(Assume that the speed of the electron leaving
the cathode is negligible.)
After leaving the anode, the electron travels at a
constant speed and enters a magnetic field at
right angles, where it is deflected. The flux
density of the magnetic field is 5 × 10–2 T.
Calculate the force acting on the electron.
Calculate the radius of the circular path followed
by the electron, in the magnetic field.
What happens to the energy of the electron when
it hits the screen of the CRT?
mass of electron = 9.1 × 10–31 kg; charge on
electron = 1.6 × 10–19 C
H/W
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2005 OL Q10
High Tension
Voltage
+-
X-Rays
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Electrons jump
from the
surface of a hot
metal cathode–
Thermionic
Emission
Accelerated by high voltage they smash into
tungsten target anode to produce x-rays
Most of the electron energy is lost as heat.-about
90%
X-rays very penetrating, fog film, not effected by
fields.
Photons
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Bohr first suggested a model for the
atom based on many orbits at
different energy levels
E2
E1
Photons
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If the electron in E1 is excited it
can only jump to E2.
E2
E1
Photons
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Then the electron falls back. The
gap is fixed so the energy it gives
out is always the same
A small amount
of energy in the
form of an e-m
wave is produced
E1
E2
Photons
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So Max Planck said all energy must come in
these packets called photons.
He came up with a formula for the
frequency
E2 –E1 = h.f
E2
E1
Where f=frequency
h= Planck’s constant
Internet demo
X-ray Tube
Energy Gained = e.V = hf = hc/λ
1.6x10-19 . 2000 = (6.6 x 10-33)(3x108)/λ
2006 Question 12 (d) [Higher Level]
 The first Nobel Prize in Physics was awarded in
1901 for the discovery of X-rays.
 What are X-rays?
 Who discovered them?
 In an X-ray tube electrons are emitted from a
metal cathode and accelerated across the tube to
hit a metal anode.
 How are the electrons emitted from the cathode?
 How are the electrons accelerated?
 Calculate the kinetic energy gained by an electron
when it is accelerated through a potential
difference of 50 kV in an X-ray tube.
 Calculate the minimum wavelength of an X-ray
emitted from the anode.
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H/W
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HL 2010 Q9
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Now show them the spectra of different lights using linear
disperser
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Demo Light Emission
Albert Einstein
Uncle Albert was already a published
scientist but the relativity stuff had
not set the world alight.
 He set his career in real motion when
he solved a problem and started the
science of Quantum Mechanics that
the old world Jew in him could never
come to terms with.
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The Problem
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If you shine light on the surface of
metals electrons jump off
e
e
e
e
e
Polished Sodium Metal
• Electrons emitted
• This is The PHOTOELECTRIC
EFFECT
A charged Zinc plate
is attached to an
Electroscope
When a U.V. lamp is
shone on the plate
the leaf collapses as
all the electrons
leave the surface of
the zinc
We can also prove this with the
experiment below
Vary intensity by moving lamp
back and forth
The Photoelectric Effect
The more intensity you gave it the
more electrical current was
produced
Light
Intensity
(# of
photons)
Current
(# of electrons)
Use of photocell
Light meter
 Burglar alarms
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The Photoelectric Effect
However something strange happened
when you looked at frequency
Electron
Energy
Newtonian Physics
could not explain this
Frequency of
light
So we define the Photoelectric effect as:Electrons being ejected from the surface of a
metal by incident e-m radiation of a suitable
frequency.
Albert used Planck’s theory that as energy came in
packets each packet gives energy to 1 electron only
A small packet would not give the electron enough
energy to leave
Low frequency light had too small a parcel of energy
to get the electron free.
Energy of each
photon = h.f
Electron
Energy
Einstein’s Law
f0=Threshold
Frequency
Frequency of
light
Energy of incident photon =
h.f = h. f0+ KE of electron
Work Function,
Energy to release Electron
Energy
left over
turned
into
velocity
Einstein's Explanation
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Waves come in packets called photons
Energy of a photon only depends on it’s
frequency
One photon gives all it’s energy to one
electron
If the energy is greater than the work
function the electron escapes
Incident Photon must be above a threshold
frequency (Greater energy than work
function)
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2004 Question 9 [Higher Level]
Distinguish between photoelectric emission and thermionic
emission.
A freshly cleaned piece of zinc metal is placed on the cap of
a negatively charged gold leaf electroscope and illuminated
with ultraviolet radiation.
Explain why the leaves of the electroscope collapse.
Explain why the leaves do not collapse when the zinc is
covered by a piece of ordinary glass.
Explain why the leaves do not collapse when the zinc is
illuminated with green light.
Explain why the leaves do not collapse when the electroscope
is charged positively.
The zinc metal is illuminated with ultraviolet light of
wavelength 240 nm. The work function of zinc is 4.3 eV.
Calculate the threshold frequency of zinc.
Calculate the maximum kinetic energy of an emitted
electron.
H/W
2003 HL Q 9
 2005 HL 12(d)
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Lets do Homework –oh goody
2004 HL Q12(d)
 2005 OL Q10
 2010 HL Q9
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