Presentation is loading. Please wait.

Presentation is loading. Please wait.

Form 5 Physics Next > The study of matter Chapter 4: Electronics 1.

Similar presentations

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

1 Form 5 Physics Next > The study of matter Chapter 4: Electronics 1

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

3 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 < Back Next > 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 3

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

5 The Cathode Ray Tube (CRT)
The oscilloscope is capable of following changes that occur within billionths of a second. It is widely used throughout industry and in laboratories to test and adjust electronic equipment, and to follow rapid oscillations in electric voltages. < Back Next > 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. 5 The Cathode Ray Tube (CRT)

6 The Cathode Ray Tube (CRT)
< Back Next > 6 The Cathode Ray Tube (CRT)

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

8 Cathode Ray Oscilloscope
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 < Back Next > 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 doctronics Kinetic energy of electrons emerging from anode = eV ½ mv2 = eV 8 2eV m Velocity, v = where e = charge of electron, m = mass of electron

9 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 = xn volts a.c. voltage: 2 x (peak voltage, V0) = ln r.m.s. voltage, Vrms = = volts V ln √ √2 2 < Back Next > x l Given: Y-sensitivity = n V per division 9

10 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 = 3T If d = distance of wall from microphone Speed of sound, v = = Distance travelled 2d Time taken 3T < Back Next > wall d microphone A B 3 divisions 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 10

11 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) < Back Next > Semiconductor diode p n + p-n junction structure + actual diode band + symbol 11

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

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

14 Semiconductor diodes A capacitor, C is connected across load, R to smoothen voltage, VR. < Back Next > VD VR R a.c.V smoothing capacitor C 14

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

16 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. n-p-n transistor B C E p-n-p transistor B C E < Back Next > Some samples of the actual transistors B: base C: collector E: emitter Structure of an n-p-n transistor 16

17 Transistors 17 Transistor as a current amplifier
The base current Ib controls the collector current Ic Ic is many times larger than Ib. When Ib = 0, Ic = 0 When Ib changed, it is amplified by the transistor, producing larger change in Ic. < Back Next > Ib B C E mA µA Ic 17

18 Transistors 18 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 R1 is much less than resistance R2. So the potential difference across LDR is much smaller than across R2. The base current Ib is small, the collector current Ic is small, and the relay is not activated. The lamp L is off. The reverse happens when in the dark. R1 increases to maximum, potential difference across LDR increases, and Ib increases. < Back Next > The transistor amplifies the increase resulting in large Ic, thus activating relay and lamp L is switched on. Other devices may be used in place of LDR for other functions. 18

19 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. < Back Next > NOT logic gate It is also called the inverting buffer. A X Input Output Boolean equation X = A Truth table 19

20 Making a NAND gate out of transistors and resistors
Logic Gates AND and NAND logic gates A B X = A • B AND A B X = A • B NAND Making a NAND gate out of transistors and resistors < Back Next > 1 = 0 1 • 1 = 1 1 0 = 1 1 • 0 = 0 0 • 1 = 0 0 • 0 = 0 NAND AND B A Output Input 20

21 Logic Gates 21 1 = 0 1 • 1 = 1 1 1 • 0 = 1 0 • 1 = 1 0 = 1 0 • 0 = 0
OR and NOR logic gates A OR X = A + B B A NOR X = A + B < Back Next > B 1 = 0 1 • 1 = 1 1 1 • 0 = 1 0 • 1 = 1 0 = 1 0 • 0 = 0 NOR OR B A Output Input 21

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

Download ppt "Form 5 Physics Next > The study of matter Chapter 4: Electronics 1."

Similar presentations

Ads by Google