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╞╡§¥ Physics
Chapter 9: Electronics
SPM 2015
CHAPTER 9: ELECTRONICS 9.1
Cathode Rays
9.1.1 Thermionic Emission Thermionic emission is the emission of electrons from a heated metal surface. Factors that influence the rate of thermionic emission: Temperature (dependent on current) – the hotter the temperature, the higher the rate Surface area – the larger the area, the higher the rate Type of metal – different metals have different rates of emission Metal surface – if coated with a mixture of barium oxide or strontium oxide, the rate is increased
Cathode rays are rays are the beam of electrons which move at high speed from the cathode to cathode to the anode. anode.
9.1.2 Maltese Cross Tube
Situation
Results seen on the fluorescent screen
Explanation
The low voltage is switched on; the extra high voltage is off
Shadow of the Maltese cross caused by the light emitted from the hot filament. No green shadow as there are no cathode rays.
Both low voltage and extra high voltage are switched on
Green shadow of the Maltese cross caused by the electron beams overlap the shadow caused by the light emitted. This proves that cathode rays travel in a straight line.
A magnetic bar is placed near the fluorescent screen
The green shadow of the Maltese cross is deflected. Deflection is downwards if the north pole is placed near the screen. Direction of deflection can be determined by the lefthand Fleming rule.
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╞╡§¥ Physics
SPM 2015
Chapter 9: Electronics
9.1.3 Perrin Tube (Deflection tube)
Situation The extra high voltage is switched off
Results seen on the fluorescent screen
The extra high voltage is switched on
(If P is positive)
(If Q is positive)
9.1.4 Characteristics of Cathode Rays
Movement is in a straight line because it is light and has high high velocity. Has momentum and energy; produces fluorescent effect when connects with with fluorescent items. Can be deflected by magnetic fields (determine using Fleming’s Left Hand Rule) Can be deflected by electric fields (deflected towards positive plates). When colliding with metal targets: kinetic energy → 99% light and 1% X-rays
9.1.5 Cathode Ray Oscilloscope (CRO) Uses: Measure potential difference Measure short time intervals Display wave forms
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╞╡§¥ Physics
Chapter 9: Electronics
SPM 2015
Functions of the components in a cathode ray oscilloscope Part
Component
Function
Electron gun
Filament
Heat up the cathode
Cathode
Release electrons (via thermionic emission)
Control grid
Control number of electrons that flow (controls brightness of the bright spot on the screen)
Focusing anode
Focus the cathode rays
Deflection system
Accelerating anode
Accelerate the cathode rays
X-plates
Deflects the cathode rays horizontally. Connected to the time-base.
Y-plates
Deflects the cathode rays vertically. Connected to the external input.
Fluorescent screen
Converts the kinetic energy of the electrons to light energy Graphite coating
CRO Reading
Traps stray electrons
No input
Direct current (from dry cell)
Alternating current
Time-based switched off
Time-based switched on
9.1.6 Speed of Cathode Rays If potential energy provided by the potential difference = 2 eV and and kinetic energy is ½ mv , the relationship of a cathode ray is:
Note: The time-base is connected to the Xplates and generates a time varying voltage as below:
2
eV = ½ mv
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╞╡§¥ Physics 9.2
Chapter 9: Electronics
SPM 2015
Semiconductors
9.2.1 Doping of Semiconductors
Materials usually used in the electronics industry as semiconductors are silicone and silicone and germanium. germanium. Doping process is the addition of a small quantity of foreign objects into a semiconductor to increase its conductivity. The atom size of the foreign object has to be about the same size as the atom size of the semiconductor.
Type of foreign atoms added Examples Major charge carrier Minor charge carrier
A semiconductor diode is also known as a p-n junction. junction. A diode allows current to flow in one direction only. only.
A diode consists of a combination of an n-type and a p-type semiconductor. semiconductor. junction of these these two semiconductors, semiconductors, the electrons from the n-type At the junction semiconductor will float over to fill up the holes in the p-type semiconductor. This creates a layer known as the depletion layer . The potential difference across the depletion layer is known as junction voltage. This is the minimum voltage that must be supplied before current can flow through the diode. voltages for silicone and germanium are approximately approximately 0.6 V and 0.1 Junction voltages V respectively.
Forward Bias
Current can flow in forward bias connection because the depletion layer is thin
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Reverse Bias
Current cannot flow in reverse bias connection because the depletion layer is thick
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Silicone diode graph which shows a unction unction volta volta e of 0.6 V
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╞╡§¥ Physics
Chapter 9: Electronics
SPM 2015
9.2.3 Diodes as Rectifiers Rectification is the process of converting alternating current to direct current. This is done with a diode as diodes allow current to flow only in one direction.
Potential difference from an alternating current source Half-wave Rectification Using a single diode:
Half-wave Rectification with capacitor
Full-wave Rectification Using four diodes (bridge rectifier):
Full-wave Rectification with capacitor
Note: The four-diode arrangement can be combined into a bridge rectifier. There are four terminals on a bridge rectifier: 2 to the a.c. source, and 2 to the resistor.
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╞╡§¥ Physics
Chapter 9: Electronics
SPM 2015
9.2.4 Capacitors Capacitors are used to smoothen the current. current. Using capacitors with full-wave rectification creates smoother current flows for optimal use with electrical appliances. Capacitor charging
Capacitor discharging
For the positive half-cycle, the diode is in forward bias Current flows through the capacitor and the resistor Capacitor is charged and energy is stored
For the negative half-cycle, the diode is in reverse bias Current is not allowed to flow through the diode Capacitor discharges and the energy stored is used to maintain the potential difference across the resistor
9.3
Transistors
Transistors are electronic devices that act as a transfer resistor to control the current and potential difference within an electronic circuit. Transistors are a combination of two types of semiconductors, i.e. type p and type n. Transistors have three electrodes: Base (B) Collector (C) Emitter (E)
Things you need to know about transistors: The collector current depends on the base current. When base current is zero, the collector current is zero. (The base current on the other hand does not depend depend on the collector current) A small change in the base current causes a big change in the collector current.
There are two types of transistors: n-p-n transistor
p-n-p transistor
For both n-p-n and p-n-p transistors: I E I B + I C E = C
Current magnification =
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where I C
I E = emitter current [A] E = I B = base current [A] I C = collector current [A} C =
I B
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╞╡§¥ Physics
Chapter 9: Electronics
SPM 2015
9.3.1 Transistors as amplifiers Transistor as a current amplifier
Transistor as a sound amplifier Component Microphone Capacitor Transistor Loudspeaker
Function Converts sound signals to electrical signals Prevents d.c. from flowing into the microphone and loudspeaker Amplifies input signal Converts electrical signals to sound
9.3.2 Transistors as automatic switches When
resistance of R 2 increases, the base voltage increases. This causes base current to flow into the transistor. If there is base current, there will be collector current; therefore the light bulb will light up R 1 and R 2 act as potential dividers. To calculate base voltage: V base Rbase
V total Rtotal
Light
sensitive switch Light-dependent resistor (LDR) changes resistance depending on presence of light. Low resistance when bright High resistance when dark When bright:
When dark:
LDR resistance ↓ Base voltage ↓
LDR resistance ↑ Base voltage ↑
I B × flows, I C flows C × Light bulb does not light up
I B flows, I C C flows Light bulb lights up
. Heat
sensitive switch Thermistor is a heat-dependent r esistor Low resistance when hot High resistance when cold When hot:
When cold:
Thermistor resistance ↓ Base voltage ↑
Thermistor resistance ↑ Base voltage ↓
I B flows, I C C flows Alarm rings
I B × flows, I C C × flows Alarm does not ring
.
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╞╡§¥ Physics 9.4
SPM 2015
Chapter 9: Electronics
Logic Gates
Logic gates: gates: electronic switches that have one or more input and only one output Truth table: table: a table which lists all possible situations for input and output through logic gates
