CHAPTER 9: ELECTRONICS

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Roy Cannon
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CHAPTER 9: ELECTRONICS 9.1 Cathode Rays 9.1.1 Thermionic Emission Thermionic emission is the emission of electrons from a heated metal surface.

rate Surface area the larger the area, the higher the rate Type of metal different metals have different rates of emission Metal surface

high speed from the cathode to the anode. 9.1.

switched on A magnetic bar is placed near the fluorescent screen Results seen on the fluorescent screen Explanation Shadow of the Maltese

Green shadow of the Maltese cross caused by the electron beams overlap the shadow caused by the light emitted.

1 CHAPTER 9: ELECTRONICS 9.1 Cathode Rays 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 the beam of electrons which move at high speed from the cathode to the anode Maltese Cross Tube Situation The low voltage is switched on; the extra high voltage is off Both low voltage and extra high voltage are switched on A magnetic bar is placed near the fluorescent screen Results seen on the fluorescent screen Explanation Shadow of the Maltese cross caused by the light emitted from the hot filament. No green shadow as there are no cathode rays. 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. 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. Hoo Sze Yen Page 1 of 8

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

Has momentum and energy; produces fluorescent effect when connects with fluorescent items.

2 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) Characteristics of Cathode Rays Movement is in a straight line because it is light and has high velocity. Has momentum and energy; produces fluorescent effect when connects 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 Cathode Ray Oscilloscope (CRO) Uses: Measure potential difference Measure short time intervals Display wave forms Hoo Sze Yen Page 2 of 8

3 Functions of the components in a cathode ray oscilloscope Part Component Function Electron gun Filament Heat up the cathode Cathode Control grid Focusing anode Accelerating anode Release electrons (via thermionic emission) Control number of electrons that flow (controls brightness of the bright spot on the screen) Focus the cathode rays Accelerate the cathode rays Deflection system 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 Traps stray electrons CRO Reading No input Direct current (from dry cell) Time-based switched off Alternating current Time-based switched on Speed of Cathode Rays If potential energy provided by the potential difference = ev and kinetic energy is ½ mv 2, the relationship of a cathode ray is: Note: The time-base is connected to the X- plates and generates a time varying voltage as below: ev = ½ mv 2 Hoo Sze Yen Page 3 of 8

9.2 Semiconductors 9.2.1 Doping of Semiconductors Materials usually used in the electronics industry as semiconductors are silicone and germanium.

4 9.2 Semiconductors Doping of Semiconductors Materials usually used in the electronics industry as semiconductors are silicone and 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. n-type semiconductor Typical semiconductor: Silicone p-type semiconductor Type of foreign atoms Pentavalent atoms Trivalent atoms added Examples Antimony, arsenic, phosphorus Boron, gallium, indium, aluminium Major charge carrier Free electrons Positively-charged holes Minor charge carrier Positively-charged holes Free electrons Diodes A semiconductor diode is also known as a p-n junction. A diode allows current to flow in one direction only. Symbol of a diode A diode consists of a combination of an n-type and a p-type semiconductor. At the junction of these two semiconductors, the electrons from the n-type 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. Junction voltages for silicone and germanium are approximately 0.6 V and 0.1 V respectively. Forward Bias Reverse Bias Current can flow in forward bias connection because the depletion layer is thin Current cannot flow in reverse bias connection because the depletion layer is thick Silicone diode graph which shows a junction voltage of 0.6 V Hoo Sze Yen Page 4 of 8

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.

5 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: Full-wave Rectification Using four diodes (bridge rectifier): Half-wave Rectification with capacitor 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. Hoo Sze Yen Page 5 of 8

6 9.2.4 Capacitors Capacitors are used to smoothen the 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 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 Current magnification = I I C B where I E = emitter current [A] I B = base current [A] I C = collector current [A} Hoo Sze Yen Page 6 of 8

7 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 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 R base base V R total total Light sensitive switch Light-dependent resistor (LDR) changes resistance depending on presence of light. Low resistance when bright High resistance when dark When bright: LDR resistance Base voltage I B flows, I C flows Light bulb does not light up When dark: LDR resistance Base voltage I B flows, I C flows Light bulb lights up. Heat sensitive switch Thermistor is a heat-dependent resistor Low resistance when hot High resistance when cold When hot: Thermistor resistance Base voltage I B flows, I C flows Alarm rings. When cold: Thermistor resistance Base voltage I B flows, I C flows Alarm does not ring Hoo Sze Yen Page 7 of 8

8 9.4 Logic Gates Logic gates: electronic switches that have one or more input and only one output Truth table:a table which lists all possible situations for input and output through logic gates Gate Symbol Equivalent circuit Boolean equation Truth table NOT X = Ā OR X = A + B AND X = A B NOR X = A B NAND X = A B END OF CHAPTER Hoo Sze Yen Page 8 of 8

Maltase cross tube. D. Senthilkumar P a g e 1

Maltase cross tube. D. Senthilkumar P a g e 1 Thermionic Emission Maltase cross tube Definition: The emission of electrons when a metal is heated to a high temperature Explanation: In metals, there exist free electrons which are able to move around