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5.1
Content standard
Electron
5.2
Semiconductor Diode
5.3
Transistor
Learning standard
• Describe thermionic emission and cathode rays
• Describe effects of electric and magnetic fields on cathode rays
• Determine velocity of an electron in cathode ray tube
• Describe the function of semiconductor diode
• Communicate about the function of semiconductor diode and
capacitor as a rectifier
• Explain the function and use of a transistor as a current amplifier
• Describe circuits that consist of a transistor as an automatic
switch
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1
5.1 Electron
Thermionic emission:
the emission of free electrons from a heated metal surface.
High temperature
tungsten filament
e
e
e
e
e
e
e
e
glass vacuum tube
free electrons gain
enough
kinetic
energy to leave the
metal surface
How to increase rate of thermionic emission?
1. Increase temperature
2. Larger surface area of metal
3. Coat metal with metal oxide
• electrons
accelerate
towards
anode without colliding with air
molecules
e
e
e
e
• no energy loss
• electrons move with maximum
mA
-
E.H.T
+
Cathode rays: beams of electrons moving at high speed in a vacuum
e
-
e
e
+
e
cathode
velocity
•
•
•
•
negatively charged (electron)
deflected by electric and magnetic fields
produces fluorescent effect (emit light)
can be stopped by thin metal
anode
• electrons emitted from the cathode will be attracted to the anode
• E.H.T. power supply circuit is completed
• milliammeter reading will show current flow
2
Effects of Electric Field and Magnetic Field on Cathode Rays
Deflection tube
e
e
No deflection
e
Deflected towards positive plate
Deflected towards positive plate
Maltese cross tube
(thin metal strip shaped like a cross – to produce shadow)
N
•
Light is blocked by metal
•
•
•
•
Cathode ray is blocked by metal
Light and cathode ray “overlap”
on the fluorescent screen
Lights up because electron has
momentum and kinetic energy
•
When there is magnetic field,
the shadow produced by
cathode ray shows deflection
Cathode ray can be deflected
by magnetic field
How to determine deflection of the shadow?
electron
e
current
N
S
N
• Electron flows towards observer
• Current flows away from observer
(into paper)
N
• Current is into paper
• Magnetic field from N to S
• Force downwards
3
Exercise
1.
Will the miliammeter show reading in the circuits below?
a.
b.
+
2.
-
-
E.H.T
+
E.H.T
Sketch the pattern of deflection when magnet is positioned as shown below.
a.
b.
S
N
c.
d.
S
N
4
Velocity of an Electron in a Cathode Ray Tube
e
-
E = eV
e
+
e
𝐸=
1
𝑚𝑣 2
2 𝑚𝑎𝑥
e
E = electrical potential energy
E
= charge of an electron (1.6 × 10–19 C)
e = charge of an electron (1.6 × 10–19 C)
V
= potential difference
V = potential difference
M
= mass of an electron (9.11 × 10–31 kg)
Vmax = maximum velocity of an electron
Principle of conservation of energy
• Energy before = Energy after
• Electrical potential energy = Maximum kinetic energy
eV =
1
2
2
𝑚𝑣𝑚𝑎𝑥
Exercise
1. Calculate the electrical potential energy of an electron between cathode and anode with 500 V
potential difference across them.
2. An accelerating electron beam is moving between cathode and anode that has a potential
difference of 300 V.
(a) Calculate the electrical potential energy of an electron.
(b) What is the kinetic energy of an electron when it reaches the anode?
(c) Calculate the maximum velocity of an electron when it reaches the anode.
5
5.2 Semiconductor Diode
electronic component which allows electric current to flow in one direction only
Symbol
(+)
Anode
Tips to remember
(-)
Cathode
Pause button
(cannot pass through)
Play button
(can pass through)
Exercise
State whether the bulbs in these circuits will light up or not.
1.
2.
3.
4.
Forward and reverse biased circuit
Forward biased circuit
Reverse biased circuit
V
p
(+)
n
Holes
(-)
(+)
Electron
Holes
Depletion layer
• Holes will move towards the n-type, electrons
move towards p-type semiconductor.
p
V
(-)
n
Electron
Depletion layer
• Holes and electrons move away from
depletion layer.
• Depletion layer becomes thinner.
• Depletion layer becomes thicker.
• Junction voltage, V across depletion layer
• Junction voltage, V across depletion layer
decreases.
increases until it reaches V of battery.
• Resistance of the diode becomes very small.
• Resistance of the diode becomes very high.
• Current flows through diode.
• Current does not flow through diode.
• Bulb lights up.
• Bulb does not light up.
6
Rectification of Alternating Current
Rectification: Process of converting an alternating current into a direct current.
Half-wave rectification
Full-wave rectification
Diode and capacitor
Half-wave rectification
Input
input
output
+
• positive half cycle
• forward biased
• allows current to flow
+
-
-
• negative half cycle
• reverse biased
• no current flow
Output
+
+
Full-wave rectification
A
B
Y
C
D
+
+
-
input
X
• first half cycle
• current flow from X to Y
• X→B→C→Y
Input
bridge rectifier
-
output
Output
+
-
+
• second half cycle
• current flow from Y to X
• Y→D→A→X
-
Capacitors in Smoothing Direct Current
Half-wave rectification + capacitor
output
capacitor
input
Output without capacitor
+
Output with capacitor
+
+
+
Full-wave rectification + capacitor
input
Output without capacitor
-
+
-
+
-
+
-
output
capacitor
+
Output with capacitor
7
Exercise
1. In the space given, draw a diode
that will allow the bulb to light up.
2. State whether the bulb in the diagram below will
light up?
a.
b.
3. Diagram shows a circuit with four diodes in a specific
arrangement. Sketch the voltage output of the circuit if;
A
B
C
D
output
all the diodes are working properly.
diode A is not working.
diode A and C are not working.
input
a.
b.
c.
4. Explain working principle of capacitor that allows it to smooth the output current.
8
5.3 Transistor
pnp transistor
npn transistor
N
P B
N
E
E
C
C
Nak Pi maNa?
(arrow going out)
B
P
N
P
freepik.com
B
C
freepik.com
C
B
E
E
Pulanglah Nak Pulang.
(arrow going in)
1
: supplies charge carriers to the collector.
Emitter, E
: control the flow of charge carriers from emitter to the
Base, B
Collector, C : collector. receives charge carriers from the emitter.
Transistor circuits
(Observe the direction of current and terminal)
npn transistor circuit
RB is large to limit IB and
protect transistor from
becoming too hot and burn.
pnp transistor circuit
Switch is closed
• current flow in base circuit
• bulb L1 is dim
• resistance of RB is high, IB is very small
• small IB can produce a base voltage, VBE
• when VBE reaches a certain value, collector
circuit can turn on.
• bulb L2 lights up very brightly
• IC > IB.
1. Transisto at English Wikipedia, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons
9
A Transistor Functions as a Current Amplifier
Small change in base current will produce large change in current in collector circuit
Amplification factor of the transistor, β
𝑚=
50 − 0
0.5 − 0
𝑚 = 100
𝛽 = 100
Change of 1 mA in IB will cause a change of 100 mA in IC.
Potential divider
“divide the potential difference”
• To function, transistor requires potential
difference, VBE that is higher than the
minimum voltage
• However, the voltage supplied to the
circuit is fixed
• To get VBE needed, potential divider circuit
can be used
• Different value of R can produce different
value of potential difference across it
based on the formula below
𝑉𝑜𝑢𝑡 =
𝑅1
× 𝑉𝑖𝑛
𝑅1 + 𝑅2
E X A M P L E
The bulb will light up when the resistance
of a thermistor is 40 kΩ. Calculate the
potential difference
across the fixed
resistor.
10 V
𝑉𝐵 =
10
× 10
10 + 40
= 2𝑉
10
The Use of a Transistor as an Automatic Switch
light-controlled switch : switch on and off based on the brightness of the surrounding
heat-controlled switch : switch on and off based on the temperature of the surrounding
Light-dependent resistor in a light-controlled switch
•
•
•
•
When you like something, you don’t resist it.
LDR “likes” light
When there is light (bright), resistance is low (doesn’t resist)
When there is no light (dark), resistance is high (resist)
Bright, R↓
LDR
Dark, R↑
When the surrounding is dark
• Resistance of LDR is very high.
• VLDR increase
• When VLDR exceeds the minimum voltage
across B and E, IB will flow
• Transistor can turn on
• High IC flow in the collector circuit
• Bulb will light up
Thermistor in a heat-controlled switch
•
•
•
•
Thermistor
When you like something, you don’t resist it.
Thermistor “likes” heat
When there is high heat (hot), resistance is low (doesn’t resist)
When there is low heat (cold), resistance is high (resist)
Hot, R↓
Cold, R↑
When the surrounding is hot
• Resistance of thermistor is very low.
• Vthermistor decrease, VR increase
• VR exceeds the minimum voltage across B and E
• IB will flow
• Transistor can turn on
• High IC flow in the collector circuit
• Bulb will light up
11
Exercise
1.
2.
Diagram shows a graph for a transistor circuit.
a. “Amplification factor of a transistor is 100”. What does that
statement means?
b. Calculate the amplification factor represented by the graph.
The bulb will light up when surrounding
temperature is high. Explain what will happen if
the surrounding temperature is low.
3.
Diagram shows a circuit with LDR.
a.
15 V
b.
c.
What will happen to the bulb during the
day? Explain.
If the base voltage needed to switch on the
transistor is 10 V, what is the resistance of
the thermistor?
Describe the modification you can make to
the circuit to create a circuit that can alert
occupant in a house when temperature is
high.
i. Draw a diagram of the modified circuit
ii. State the modifications based on the
following aspects;
• Electronic components to be
replaced
• Positions of the components
• Additional component
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