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Home End HolisticTuition CashPlants Chapter 4: Electronics Form 5 1 Physics Next > The study of matter.

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Presentation on theme: "Home End HolisticTuition CashPlants Chapter 4: Electronics Form 5 1 Physics Next > The study of matter."— Presentation transcript:

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2 Home End HolisticTuition CashPlants Chapter 4: Electronics Form 5 1 Physics Next > The study of matter

3 Home End HolisticTuition CashPlants Objectives: (what you will learn) Objectives: (what you will learn) 1)uses of Cathode Ray Oscilloscope 2)understanding semiconductor diodes 3)understanding transistors 4)analysing logic gates Physics: Chapter 4 2 < Back Next >

4 Home End HolisticTuition CashPlants 3 < Back Next > Maltese-cross Tube Thermionic emission = emission of electrons from hot metal surface in vacuum Cathode rays = electrons moving at high speeds after acceleration through high potential difference A Maltese-cross tube is used to show the first two properties of cathode rays. Properties: 1.electrons moving at high speeds in straight lines 2.cause fluorescent material to emit light 3.deflected by magnetic field 4.deflected by electric field

5 Home End HolisticTuition CashPlants 4 < Back Next > The direction of deflection of cathode rays by magnetic field is found with Flemings left-hand rule. Maltese Cross Crookes Tube Invented in the 1880s by William Crookes during his investigations into the nature of cathode rays. It demonstrates that radiant matter is blocked by metal objects. Maltese-cross Tube

6 Home End HolisticTuition CashPlants 5 < Back Next > Cathode Ray Tube It is widely used throughout industry and in laboratories to test and adjust electronic equipment, and to follow rapid oscillations in electric voltages. The oscilloscope is capable of following changes that occur within billionths of a second. Special converters attached to oscilloscope can convert mechanical vibrations, sound waves, and other forms of oscillatory motion into electrical impulses that can be observed on the face of CRT. The Cathode Ray Tube (CRT)

7 Home End HolisticTuition CashPlants 6 < Back Next > The Cathode Ray Tube (CRT) Cathode Ray Tube

8 Home End HolisticTuition CashPlants 7 < Back Next > Cathode Ray Oscilloscope The Cathode Ray Oscilloscope (C.R.O.) is divided into 3 parts:Oscilloscope Electron gun Deflection system Fluorescent screen

9 Home End HolisticTuition CashPlants 8 < Back Next > Cathode Ray Oscilloscope Deflection system: Y-plates: electric field deflects electrons vertically X-plates: electric field deflects electrons horizontally Fluorescent screen: When fast electrons hit fluorescent screen, their kinetic energy is converted into light – a spot of light is seen on the screen The walls of C.R.O. after anode is coated with graphite and grounded to keep out external electric field Kinetic energy of electrons emerging from anode = eV ½ mv 2 = eV Electron gun: The cathode emits electrons when heated The grid controls the number of electrons reaching anodes – control with brightness knob The anode focus electrons into fine beam – control with focus knob The potential difference between anode and cathode accelerates electrons to high velocity 2 eV m Velocity, v = where e = charge of electron, m = mass of electron doctronics

10 Home End HolisticTuition CashPlants 9 < Back Next > Cathode Ray Oscilloscope Uses of C.R.O. 1. Measure potential difference Switch off time-base Connect voltage to be measured to Y-input d.c. voltage: if x = deflection of light spot, voltage = x n volts a.c. voltage: 2 x (peak voltage, V 0 ) = l n x l Given: Y-sensitivity = n V per division r.m.s. voltage, V rms = = volts V 0 1 l n 2 2 2

11 Home End HolisticTuition CashPlants 10 < Back Next > Cathode Ray Oscilloscope 2. Measure short time interval Switch on time-base; one horizontal division = time interval, T Pulse A represents sound detected by microphone Pulse B represents the echo Say, time interval between A and B is 3 divisions = 3 T If d = distance of wall from microphone 3. Display waveform Connect input voltage to Y-input Switch on time-base Adjust frequency to a steady trace formed on screen The trace or waveform is the graph of voltage V against time t Speed of sound, v = = Distance travelled2 d Time taken3 T AB 3 divisions wall d d microphone

12 Home End HolisticTuition CashPlants 11 < Back Next > Semiconductor diodes Semiconductors have resistance between that of metals and insulators; e.g. carbons, germanium, silicon Pure semiconductor: negative charge carriers = positive charge carriers or free electrons = holes Doped semiconductor (with added impurity): n-type:free electrons > holes (impurity of valency 5; arsenic or phosphorus) p-type:holes > free electrons (impurity of valency 3, indium or gallium) pn + – p-n junction structure +– symbol +– actual diode band Semiconductor diode

13 Home End HolisticTuition CashPlants 12 < Back Next > Semiconductor diodes + + current Forward bias + + no current Reverse bias Ideal diode Allows current through when connected in forward bias Stops current when connected in reverse bias (infinite resistance)

14 Home End HolisticTuition CashPlants 13 < Back Next > Semiconductor diodes A diode is used as a rectifier to convert a.c. to d.c. VDVD VRVR R a.c. V Half-wave rectification Current only flows through the diode during the positive half cycle (as shown by +V). The voltage across the load, V R is direct voltage and the current is d.c.

15 Home End HolisticTuition CashPlants 14 < Back Next > Semiconductor diodes A capacitor, C is connected across load, R to smoothen voltage, V R. VDVD VRVR R a.c. V smoothing capacitor C

16 Home End HolisticTuition CashPlants 15 < Back Next > Semiconductor diodes 4 diodes are used in a bridge full-wave rectification. 2 diodes are used in a simple full-wave rectification.

17 Home End HolisticTuition CashPlants 16 < Back Next >Transistors n-p-n transistor B C E p-n-p transistor B C E B: base C: collector E: emitter Structure of an n-p-n transistor Some samples of the actual transistors Transistor is an electronic device containing at least 3 layers of semiconductor and electrical contacts, used in a circuit as amplifier, detector, or switch.

18 Home End HolisticTuition CashPlants 17 < Back Next >Transistors Transistor as a current amplifier The base current I b controls the collector current I c I c is many times larger than I b. When I b = 0, I c = 0 When I b changed, it is amplified by the transistor, producing larger change in I c. IbIb B C E mA µAµA IcIc

19 Home End HolisticTuition CashPlants 18 < Back Next >Transistors Transistor as a switch The transistor can be used as a switch to switch on a lamp, L. The light-dependent resistor (LDR) has resistance of 2 kΩ in bright light and 20 kΩ in the dark. During the day, resistance R 1 is much less than resistance R 2. So the potential difference across LDR is much smaller than across R 2. The base current I b is small, the collector current I c is small, and the relay is not activated. The lamp L is off. The reverse happens when in the dark. R 1 increases to maximum, potential difference across LDR increases, and I b increases. The transistor amplifies the increase resulting in large I c, thus activating relay and lamp L is switched on. Other devices may be used in place of LDR for other functions.

20 Home End HolisticTuition CashPlants 19 < Back Next > Logic Gates Logic gates Logic gates = switching circuits used in computers and electronic devices A logic gate has one or more inputs but only one output. Its action is summarized by an equation in Boolean algebra, or with a truth table. NOT logic gate It is also called the inverting buffer. AX InputOutput Boolean equation X = A Truth table

21 Home End HolisticTuition CashPlants 20 < Back Next > Logic Gates AND and NAND logic gates A B X = A B AND NAND A B X = A B 1 = 01 1 = 111 0 = 11 0 = 001 0 = 10 1 = 000 0 = 10 0 = 000 NANDANDBA OutputInput Making a NAND gate out of transistors and resistors

22 Home End HolisticTuition CashPlants 21 < Back Next > Logic Gates OR and NOR logic gates A B X = A + B NOR A B X = A + B OR 1 = 01 1 = 111 1 = 01 0 = 101 1 = 00 1 = 110 0 = 10 0 = 000 NORORBA OutputInput

23 Home End HolisticTuition CashPlants 22 Summary < Back What you have learned: 1.Uses of Cathode Ray Oscilloscope Thank You 2. 2.Semiconductor diodes 3. 3.Transistors 4. 4.Logic gates


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