Upload
others
View
12
Download
4
Embed Size (px)
Citation preview
PHYSICSFORM 5
Cikgu DesikanCompiled by
Analysis of Past Year Questions
Learning Objectives :
Dear students,
1. Understanding the uses of the
Cathode Ray Oscilloscope (C.R.O.)
2. Understanding semiconductor diodes
3. Understanding transistors
4. Analysing logic gates
Look at the sky. We are not alone. The whole universe is friendly
to us and conspires only to give the best to those who dream
and work.
- Dr. Abdul Kalam
Chapter 9
Electronics
FO
RM
5 P
HY
SIC
S
2016
2007 2008 2009 2010 2011 2012 2013 2014 2015
P1 4 3 4 1 4 4 4 5
P2
A - 2 1 1 1 1 1 -
B 1 - - - - - - -
C - - - - - - 1 -
P3A - - - - - - - -
B - - - - - - - -
Concept Map
Dear brothers and sisters,
STOP searching for strength and willpower.
START creating it !!!
Electronics
Cathode ray
Oscilloscope
Semiconductor
diodes
Thermionic
emissionp-type and n-type
semiconductor
diodes
Transistors Logic gates
AND
OR
NOT
NAND
NOR
Truth Table
Controlling systems
Applications
Current
amplifier
Electrical
power
Half-wave
rectification
Half-wave
rectification
C.R.O
Structure
Applications
Chapter 9
Electronics
9.1 Cathode Ray Oscilloscope
Thermionic emission
How does thermionic emission occur?
1. Metal consists of a large number of electrons which are free to
move.
2. At room temperature, the electrons are free to move but
remain inside the metal.
3. The electrons cannot escape at the surface because they are
held back by the attractive forces of the atomic nucleus.
4. If the metal is heated at a high temperature, some of the free
electrons may gain sufficient energy to escape from the metal.
Cathode rays
Cathode Ray
Oscilloscope
4
e
e
e
e
Heated filament
cathode
- +Beam of
electrons
Fluorescent
Screen
Vacuum
Anode
6 V a.c
Cathode
EHT
Process of ______________ of electrons from a heated metal’s surface
1. Thermionic emissions can be used to produce a continuous flow of electrons in a cathode ray
tube.
2. When the cathode is connected to the anode by an extra high tension (EHT) voltage supply,
a narrow beam of fast electrons will move to the anode.
3. The beam of electrons moving from the cathode to the anode is called cathode rays.
4. Cathode rays can be used in picture tube of a television, a cathode ray oscilloscope and the
visual display on a radar screen.
Properties Of Cathode Rays
1.
2.
3.
4.
5.
5
Cathode rays
Investigate the properties of electron streams in a Maltese cross cathode ray tube
ProcedureObservation on the
fluorescent screen Explanation
Connect only
the 6.3 V power
supply to the
filament
A dark shadow of the Maltese
Cross is formed on the screen
When the 6.3 V power supply is switched on,
the filament is heated. The Maltese cross
shadow is formed on the screen due to the
light from the filament.
6
- +
Maltese
cross
Fluorescent
Screen
VacuumAnode
6 V a.c
Cathode
EHT
7
Connect the
6.3 V and EHT
to the
electrodes
A darker shadow of the
Maltese Cross is seen
on the screen. The
shadow is surrounded
by green light
1. When the EHT power supply is switched on, a
high voltage is applied between the cathode
and anode causing electrons to accelerate at
high speeds from cathode to anode. These
electrons are the cathode rays .
2. The cathode rays blocked by the Maltese Cross
causing a shadow to form on the screen. The
cathode rays travel in straight lines.
3. The green screen formed around the shadow
when the EHT power supply is switched on
shows that the kinetic energy of the electron is
converted into light energy when the electrons
hit the fluorescent screen.
Bring a pole of
a bar magnet
near to the
neck of the
tube
Two shadow are seen
on the screen. The light
shadow remains at the
centre of screen while
the dark one is shifted
1. When a strong magnet is placed at the side of
the Maltese Cross tube, the shadow formed is
moved and distorted.
2. This shows that cathode rays are deflected by a
magnetic field.
Reverse the
pole of the bar
magnet
The light shadow
remains at the centre of
screen while the dark
one is shifted to the
opposite direction
Investigate the properties of cathode rays in an electric field
8
-
-
+
+
Plates
VacuumAnode
6 V a.c
Cathode
EHT 1
EHT 2
No voltage connected to the
deflecting plates
Top plate is connected to
EHT (+) and lower plate is
connected to EHT (-)
Top plate is connected to EHT
(-) and lower plate is
connected to EHT (+)
Summary of Investigation
1.
2.
1. 2. 3.
Cathode Ray Oscilloscope
9
- +EHT
Accelerating
Anode
6 V a.c
Cathode
Focusing
Anode
Filament
Control grid Y-plate X-plate
Graphite
coating
Electron
beam
Fluorescent
Screen Vacuum
Bright spot
a. Uses a cathode ray tube that converts electronic and electrical signals to a visual display.
b. The graph produced consist of a horizontal axis which is normally a function of time, and a
vertical axis which is a function of the input voltage.
c. The components in a cathode ray tube consists of a vacuum glass tube with
i. an electron gun,
ii. a deflection system for deflecting the electron beam and
iii. a fluorescent coated screen.
10
Part Function
Filament Is heated when current flows through it. It is used to heat up the
cathode.
Cathode Heated cathode emits electrons through the process of thermionic
emissions.
Control grid Control the number of electrons in the electron beams.
The more negative the grid, the fewer the electrons are emitted from the
electron gun and the less the brightness of the bright spot on the
screen.
Focusing anode To focus the electrons into a beam and to attract electrons from the area
of the control grid.
Accelerating anode To accelerate the electron beam towards the screen.
Electron Gun
The electron gun is used to produce a narrow beam of electrons.
No input voltage.
The electron beam does not deflect
and the bright spot is at the centre
+ve voltage is applied.
The electron beam deflect upward.
The bright spot moves to the top.
Deflection System
1. The deflection system allows the electron beam to be deflected from its straight-line path when
it leaves the electron gun.
2. Y-plates is to move the electron beam vertically up and down the screen when an input voltage
is applied across it.
Y-plate X-plate
Electron
beam
Bright spot
Screen
No input voltage
is applied
+V
Electron
beam
A positive voltage
is applied Bright spot
Screen
-ve voltage is applied.
The electron beam deflect
downward.
The bright spot moves to the
bottom.
a.c voltage is applied.
The electron beam deflects up and
down.
The bright spot moves up and
down to form a bright vertical
trace on the screen
The function of the X-plates is to sweep the electron beam across the screen
horizontally from left to right at a steady speed. 12
Electron
beam
A positive voltage
is applied
Bright spot
-VScreen
Screen
A.C.voltage is
applied
Electron
beam
13
Fluorescent Screen
1. The fluorescent screen is coated on the inside surface with some fluorescent material such as
phosphor or zinc sulfide.
2. When electron beam strikes the screen, the material becomes glows. This enables a bright spot
to appear whenever an electron beam strikes the screen.
3. The moving electrons have kinetic energy. When this electrons strikes the screen, the
fluorescent coating on the screen converts the kinetic energy of the electrons into light energy.
Application of CRO
Working principle of the cathode ray oscilloscope, CRO
Y-Gain
Time base
Control knob Function
Power switch 1. Control the power supply
Focus 1. Control the sharpness of the bright spot
2. Connected to the focusing anode
3. The sharpness of the bright spot is also affected by the brightness
Brightness
1. To control brightness or intensity of the bright spot
2. Connected to the control grid
3. Brightness level should be set as low as possible to obtain a clear
and sharp trace
X-shift 1. To adjust the horizontal position of the bright spot on the screen
2. Connected to the X-plates
Y-shift 1. To adjust the vertical position of the bright spot or the trace displayed
2. Connected to the Y-plates
Y gain
(volts / div)
1. To control the magnitude of the vertical deflection of the bright spot or
the trace displayed on the screen by adjusting amplitude
2. Connected to the Y-plates
Time-base
(time/div)
1. To control the magnitude of the horizontal deflection of the bright spot
or the trace displayed on the screen by adjusting the frequency
2. Connected to the X-plates
15
Control knob Function
X-input 1. A terminal to connect the voltage to the X-plates
Y-input 1. A terminal to connect the voltage to the Y-plates
AC/DC switch
1. To select the type of input received
2. When the switch is at DC position, the a.c and d.c voltages will be
displayed
3. When the switch is at AC position, only the a.c voltage will be
displayed. Any signals of d.c voltage will be blocked by a capacitor in
the CRO
Earth 1. To disconnect the input voltage at the Y-plates and to earth the input
terminal
Life affords no higher pleasure than that of surmounting
difficulties, passing from one step of success to another,
forming new wishes, and seeing them gratified. He that labors
in any great or laudable undertaking has his fatigues first
supported by hope, and afterwards rewarded by joy...
To strive with difficulties, and to conquer them, is the highest
human felicity.
Samuel Johnson
16
Display wave forms and measuring voltage from a DC source using a CRO
17
Type of power
supply
connected to
Y-input of CRO
Time-base
switched off
Time-base
switched on
No input
DC power
supply
AC power suppy
a.c d.cY-input
CRO
Battery
Measuring Potential Difference using the CRO
The selected range of
the Y-gain control
Displacement of the bright
spot from the zero position X
time-base off time-base on
DC voltage =
What is the value of the dc voltage in
figure (a) and (b) if the Y-gain control is
1 V/div ?
a) b)
18
a.c d.cY-input
a.c d.cY-input
Measuring Potential Difference using the CRO
The selected range
of the Y-gain control
Height of vertical trace from
the zero position XPeak ac voltage =
Y-gain = 2 V/div
Height of vertical trace from zero position =
Peak ac voltage =
19
a.c d.cY-input
a.c d.cY-input
Measure short time intervals
1. The time-base is set to 1 ms/div
2. It means I div = 0.001 s
3. The number of div is counted between
two crests of a wave
4. The short time interval between pulses =
Multiplying the number of division by the
time-base .
Length between 2 signals = ____ div
Time base is set = 10 ms/div
Time taken, t =
Solve problems based on the CRO display
Example 1
Diagram 1 shows a trace produced by an ac
power supply which is connected to Y-input of an
CRO setting at 20 V/div and 5 ms/div.
Calculate:
(a) Period
(b) Frequency
(c) Peak voltage
20
a.c d.cY-input
Example 3
Diagram 3 shows a wave produced by an audio generator displayed
the screen of a CRO. The length between the two crests is 3 cm.
(Given 1 division = 1cm)
(a) What is the period of the wave?
(b) If the time-base is set to 5 ms/div, find the frequency.
(c) When the frequency of the wave is double, what is the length
between the two crests?
Example 2
21
3 cm
Diagram 2 shows a trace produced by an a.c power supply connected to a
CRO with the time base is switched of. The Y-gain is set to 20 V/div. Find
the peak voltage.
4.2 Semiconductor Diodes
Metal Insulators Semiconductors
Good conductors of
electricity because
they have free
electrons that can
move easily between
atoms
The resistance of
metals is generally
very low.
Poor conductors of
electricity because
they have too few
free electrons to
move about.
The resistance of
insulators is very
high.
1. A material that has an electrical conductivity that
is between that of a conductor and an insulator.
2. The resistance of semiconductors is between
that of conductors and insulators.
3. Semiconductors can be pure element such as
silicon or germanium.
4. At 0 Kelvin it behaves as an insulator. When the
temperature increases, the conductivity of the
electricity will increase because its resistance
will be lowered.
Semiconductors in terms of resistance
Charge Carriers
Electricity conductivity in semiconductors occurs because there is two type of charge carriers:
1. _______________ which is negatively charged
2. _____________which is positively charged.
22
Characteristics of a silicon atom
1. There are four electrons in the outermost shell of a
silicon atom and they are shared between four other
neighbouring atoms to form four covalent bonds.
2. Each of the covalent bonds has a pair of electrons.
Every atoms shares one electron with each of its
neighbours.
3. Figure on the left shows the outer electrons in a silicon
crystal which all are involved in perfect covalent bonds,
leaving no free electrons to conduct electricity.
4. At very low temperature, pure silicon crystal is an
insulator and has a high resistance to current flow.
5. As the temperature of pure silicon crystal increases, the energy of the vibrating atoms in the
silicon crystal causes some electrons to break free.
6. For every electron that is broken free, there is a hole in the bonding structure between the
atoms of the crystal. (atom X)
7. These holes are said to be carriers of positive charge
8. One outer electron from the neighboring atom (Y) will fill the hole and at the same time will
produce a hole at Y.
9. When the valence/outer electron moves to the left, the hole ‘move’ to the right
10. This is the physical origin of the increase in the electrical conductivity of semiconductors with
temperature
23
Valence electron
Covalent
bond
Pure semiconductor at 0 Kelvin
Doping process
Doping is a
n-type semiconductor
1. n-type doping is to produce an abundance of electrons in the semiconductor
2. A silicon atom has four valence / outer electrons which each electron is covalently bonded with
one of four adjacent silicon atoms
24
Hole
X Y
Hole
Hole ( + )
Electron ( - )
3. If atoms with five valence electrons (pentavalent atoms) are doped into the pure
semiconductor, then each of the pentavalent atoms will have four covalent bonds and one extra
electrons.
4. It takes only a very small quantity of the impurity to create enough free electrons to allow
electric current to flow through silicon.
5. The free electrons are the majority carriers and the holes are the minority carriers
6. Since the pentavalent atom donates an extra electron it is therefore called the donor atom.
7. Example: phosphorus, arsenic, or antimony
Phosphorus atom
25
Phosphorus
atom
1. p-type doping is to create an abundance of holes in the material.
2. If atoms of three valence electrons (trivalent atoms) are doped into the pure semiconductor, one
electron is missing from one of the four covalent bonds. The deficiencies of valence electrons
are called holes.
3. When current passes, a ‘hole’ is filled by an electron from a neighbouring atom. In this way the
hole moves from one atom to another.
4. The holes are the majority carriers and the free electrons are the minority carriers.
5. Since the trivalent atom accepts an electron, it is therefore called the acceptor atom.
6. Examples: boron, aluminium, gallium
Electronic Structure of
Aluminium (Al)
p-type semiconductor
26
Aluminium
(Al)
Comparison between the n-type and p-type semiconductor
Aspect n-type
Semiconductor
p-type
Semiconductor
Pure
semiconductor
Dopants material
Function of the
dopants material
Valens electrons
of the dopant
material
Majority charge
carriers
Minority charge
carriers
27
Semiconductor diodes
1. The simplest semiconductor device is a diode.
2. A diode is made by joining a p-type and n-type
semiconductors
3. A diode is a device that allows current to flow in one
direction only but blocks it in the opposite directions.
p-n junction
1. A p-n junction is formed when a n-type and p-
type semiconductors are joined together.
2. The boundary between the p-type and n-type
regions is called the ________________.
3. At the p-n junction, electrons from the n-side
move to the p-side and recombine with the
holes.
4. Holes from the p-side similarly move into the n-
side, where they recombine with electrons.
5. As a result of this flow, the n-side has a net
positive charge, and the p-side has a net
negative charge.
28
p n
Structure
Symbol
Cathode (-)Anode (+)
Junction electronhole
p n
Depletion Layer
1. The region around the junction is left with
neither holes nor free electrons.
2. This neutral region which has no charge carriers
is called the ______________________.
3. This layer which has no charge carrier is a poor
conductor of electricity.
Forward bias
1. The p-type of the diode is connected to the
positive terminal and the n-type is connected to
the negative terminal of a battery.
2. The diode conducts current because the holes
from the p-type material and electrons from the
n-type material are able to cross over the
junction.
3. A light bulb will _______________.
29
--
++
Junction electronhole
p n
Depletion layer
Narrow depletion layer
What is reversed bias?
1. The n-type is connected to the positive terminal and
the p-type is connected to the negative terminal of
the battery.
2. The reversed polarity causes a very small current to
flow as both electrons and holes are pulled _______
from the junction.
3. When the potential difference due to the widen
depletion region equals the voltage of the battery,
the current will cease. Therefore the bulb ________
light up.
30
Wide depletion layer
Half-wave rectification
Half-wave rectification by using one diode
1. When a diode is connected in series with the resistor, any current that passes through the
resistor must also pass through the diode.
2. Since diode can only allow current to flow in one direction, therefore the current will only flow in
the first half-cycle when the diode in forward bias.
3. The current is blocked in the second half-cycle when the diode is in reverse bias.
Output
(varying dc)
31
Input
ac current
Diodes as rectifiers
1. A rectifier is an electrical device that converts alternating current to direct current.
2. Rectification is a process to convert an alternating current into a direct current by using a diode.
3. Two type of rectification:
a. ___________________________________________
b. ___________________________________________
Full-wave rectification
1. A process where both halves of every cycle of an alternating current is made to flow in the
same direction.
2. In the first half, the current flows from A to P to TU to R to B
3. In the second half, the current flows from B to S to TU to Q to A.
4. The direction of the ac current passing through the resistor for each half cycle is the same ie T
to U.
Full-wave rectification by using four diodes
Input
ac current
Output
(varying dc)
32
Use of a capacitor to smooth out output current and output voltage in a rectifier circuit
1. When the current pass through the resistor and capacitor, the capacitor is charged and stores
energy.
2. When there is no current pass through the resistor and capacitor, the capacitor discharge and
the energy from it is used to produce voltage across the resistor. As a result it produces a
smooth dc output.
33
Circuit Diagram
Output ( V – t Graph )
Input
ac current
9.3 Transistor
1. A transistor has three leads connected to the emitter, base and
collector.
2. The emitter emits or sends charge carriers through the thin base
layer to be collected by the collector.
3. There is two-type of transistor: npn transistor and pnp transistor.
4. In an npn transistor the emitter sends negative electrons to the
collector.
5. In an pnp transistor, the p-type emitter sends positive holes to the
collector.
6. In both cases, the arrow on the emitter shows the direction of
current flow.
7. The output current, of a transistor flows between the emitter and
the collector.
8. The current in the collector lead is called collector current, IC.
9. The base current, IB is used to control the collector current
through the transistor. The base current can be used to switch the
collector current on or off.
34
npn transistor
pnp transistor
n
p
n
p
n
p
B
Base current is too small compared to the
collector current. The unit of base current
is μA while the unit for the collector
current is mA.
Emitter current, IE is equal the sum of
base current and collector current :
Therefore,
If there is no current flow in the base circuit, then there is also no current flow in the collector
circuit.
IB = 0 then IC = 0 ►
IB ≠ 0 then IC ≠ 0 ►35
R1
R2
E1
E2B
C
E
IB
IC
Transistor as a current amplifier
Transistor as an automatic switch
A small change in the base current, results in a big change in the collector current.
Transistor as an automatic switch
1. Choose a suitable resistor R1 and a variable resistor R2. The voltage at base terminal can be
adjusted to switch the transistor on or off.
2. If the variable resistor = 0, base voltage = 0 and the transistor remains off.
3. If the variable resistor is increased, the base voltage increases.
4. When the base voltage reaches certain minimum value, the base current switches the
transistor on.
5. The large collector current flows through the transistor causing the bulb to light up.
Potential divider circuit
If the variable resistor in the
transistor is replaced by a
device such as light
dependent resistor (LDR), a
thermistor or a microphone,
the transistor can be used as
an automatic switch
controlled by light, heat or
sound.
36
IB
10 kΩ
R2
6 V
10 kΩ
R1
IC
IE
Battery
Voltage
Base
Voltage
Transistor as a Current Amplifier
1. A transistor functions as a current amplifier by allowing a small current to control a larger
current.
2. The magnitude of the ________________________, IC is primarily determined by the _______
_____________, IB.
3. A __________ change in the base current, IB will cause a ________ change in the collector
current, IC.
4. The current amplification can be calculated as follows :
37
Current Amplification =
1. The LDR has a very resistance in darkness and a resistance in light. R is a
fixed resistor.
2. The LDR and R form a potential divider in the circuit.
A light controlled switch
Circuit switches on the light at daytime and
switches off the bulb at night automatically
Circuit switches on the bulb at night and
switches off the bulb at day time automatically
38
Z
X
Y
50 kΩ
5 kΩ
LDR6 V
Bulb
Z
X
Y
50 kΩ
5 kΩ
LDR
6 V
Bulb
1. In daylight, the LDR has a very low
resistance as compared to R.
1. In daylight, the LDR has a very low
resistance as compared to R.
“ Your diamonds are not in far distant mountains or in yonder
seas; they are in your own backyard, if you but dig for them.”Russell H. Conwell
39
2. Therefore the base voltage is _______
enough to switch the transistor on and to
light up the bulb.
3. In darkness, the LDR has a very ________
resistance and therefore the base voltage is
too _______ to switch the transistor on. The
bulb light off.
2. Therefore the base voltage is too _______
to switch the transistor on
3. In darkness, the LDR has a very _______
resistance and the base voltage is ______
to switch the transistor on to light on the
bulb.
Heat-Controlled Switch
1. Figure shows a transistor-based circuit that function as a heat controlled switch.
2. A _______________ is a special type of resistor. Its resistance becomes very __________
when it is cold.
3. When the thermistor is heated, its resistance __________ rapidly.
4. At room temperature, the thermistor has a __________ resistance compared to R.
Therefore, the base voltage of the transistor is too low to switch on the transistor.
5. When the thermistor is heated, its __________ drops considerablely compared to R.
6. Therefore, the ______________ , VB is high enough to switch ______ the transistor.
7. When the transistor is switch on, the relay switch is activated and the relay is switched ____.
8. The circuit can also be used in a fire alarm system.
9. Function of a diode in this circuit : protect transistor from being damaged by the large
e.m.f in relay coil when IC drops to zero.40
50 kΩ
40
kΩ
6 V
Loud speaker
10
kΩ
Sound amplifier
Microphone
1. Microphone converts audio (sound) signal into ______________signal (varying current).
2. Capasitor allows the varying current flow it ( and prevent direct current to flow from
the battery to the transistor and the loudspeaker).
3. Base current is changed and causes large change in the collector current, IB.
4. Collector current flows in loudspeaker.
5. Sound waves with higher amplitude is produced.
41
9.4 Logic Gates
Logic gates as switching circuits in electronic systems
1. Security lamps, alarm systems, and washing machines can make some simple decisions.
2. The switching on and off operations are controlled by electronic switches made up of logic
gates.
3. Logic gates work using tiny transistors as switches. They are manufactured as integrated circuit
(IC), with each chip holding several gates.
4. A logic gate is a circuit that has one or more input signals but only one output signal.
5. For each gate, the input or inputs are on the left of the symbol. The output is on the right
6. Each input and output can be either high (logic 1) or low (logic 0).
7. A binary “0” represents 0 V, and a binary “1” represents a non zero voltage.
Truth table
A truth table lists all input possibilities and the corresponding output for each input.
Gates Truth Table Action
AND gate For the input to be ON, both
inputs must be ON.
Output in ON only when both
inputs A and B are ON.
42
A
BX
INPUT OUTPUT
A B X
Gates Truth Table Action
OR gate For the output to be ON at
least one of the inputs must be
ON.
Output Q is ON when input A
or B or both is ON
NOT gate The output is ON when the
input is OFF, and vice versa
43
A
BX
INPUT OUTPUT
A B X
A X
INPUT OUTPUT
A X
“Believe it is possible to solve your problem. Tremendous things happen
to the believer. So believe the answer will come. It will.”Norman Vincent Peale
Gates Truth Table Action
NAND gate It is equivalent to an AND gate
with its output inverted by a
NOT gate.
Output Q is OFF when inputs A
and B are both ON
NOR gate It is equivalent to an OR gate
with its output inverted by a
NOT gate.
Output Q is ON when both
input A and input B are OFF
44
A
BX
INPUT OUTPUT
A B X
A
BX
INPUT OUTPUT
A B X