TOPIK 9 (T5)

4.1 UNDERSTANDING CATHODE RAY OSCILLOSCOPE

 

1. Metal consists a large number of electrons

2. At room temperature, the electrons are free to move but remain inside

    the metal.

3. If the metal is heated at a high temperature, some of the free electrons

    may gain sufficient energy to escape from the metal.

4. Thermionic emission is the process of emission of electrons from

a heated metal’s surface.

5. Cathode rays tube.

 6. Properties of cathode rays

     a) Negatively charged particles called electrons.

     b)Travel in straight lines and cast sharp shadows.

     c)Travel at very high speed and have kinetic energy.

     d) Can cause fluorescence. (A process where the kinetic energy of the

          electrons is converted into light energy)

     e) Deflected by electric and magnetic fields.

7. Cathode rays can be used in

    a)  picture tube of a television

    b)  cathode ray oscilloscope (C.R.O)

    c)  visual display on a radar screen.

 8. Maltese cross cathode ray tube

 

 a) 6.0 V power supply is switched on,

       i)  the filament is heated.

      ii)  the Maltese cross shadow is formed on the  screen due to the light

           from the filament

b) EHT power supply is switched on,

      i)  a high voltage is applied between the cathode and anode

     ii)  causing electrons to accelerate at high speeds

    iii)  these electrons are the cathode rays

    iv)  cathode rays travel in straight lines

c) The Maltese Cross blocked the cathode rays causing a shadow to form

     on the screen.

d) The green screen formed around the shadow shows that the kinetic

     energy of the electron is converted into light energy when the

     electrons hit the fluorescent screen.

e) When a strong magnet is placed at the side of the Maltese Cross tube,

    the shadow formed is moved and distorted.  It shows that cathode rays

    are deflected by a magnetic  field.

UNDERSTANDING CATHODE RAY OSCILLOSCOPE (C.R.O)

1. Components in a (C.R.O)  

a)vacuum glass tube

b)an electron gun,

c)deflection system for deflecting the electron beam

d)fluorescent coated screen

2. Electron gun - produce a narrow beam of electrons

Component	Function
Filament	heat up the cathode.
Cathode	heated cathode emits electrons through the process of thermionic emissions.
Control grid	control the number of electrons in the electron beams.
Focusing anode	focus the electrons into a beam and to attract electrons from the area of the control grid.
Accelerating anode	accelerate the electron beam towards the screen.

3. Deflection System - allows the electron beam to be deflected from its straight-line path when it leaves the electron gun.

 

Component	Function
Y-plates	move the electron beam vertically up and down
X-plates	move the electron beam across the screen horizontally from left to right

4. Fluorescent Screen - coated on the inside surface with some fluorescent material such as phosphor or zinc sulfide.

Component	Function
Moving electrons	have kinetic energy and strikes the screen,
Fluorescent coating	converts the kinetic energy of the electrons into light energy.

  

5. Application of CRO

a) Measuring potential difference

b) Measuring short intervals

c) Displaying wave forms

4.2 UNDERSTANDING SEMICONDUCTOR  DIODES

Semiconductors

1. are metals whose conductivity lie between good conductors and insulators.

Conductor (Metals)	Good conductors of electricity	Very low resistance (generally)	Have free electrons that can move easily between atoms
Insulators	Poor conductors of electricity.	Very high resistance.	Have very few free electrons to  move about
Semiconductors	Conductivity between conductor and insulator.	Resistance  between conductors and insulator.	At 0 Kelvin behaves as an  insulator. Temperature increases conductivity increase,   resistance will be lowered.

2. Pure semiconductors

a)    Tetravalent elements

b)   Each atoms have 4 electrons in the outermost shell

c)    Examples: silicon, germanium and selenium

3. Doping

is a process of adding a certain amount of specific impurities (dopants) 

to a pure semiconductor to increase its electrical conductivity.

4. Type of semiconductors

a)    n-type

b)   p-type

Aspect	n-type Semiconductor	p-type Semiconductor
Pure semiconductor	silicon	silicon
Dopants material	phosphorus, arsenic, antimony	boron, aluminium, gallium
Function of the dopants material	produce an abundance of electrons	create an abundance of holes
Valens electrons of the dopant material	pentavalent atoms	trivalent atoms
Majority charge carriers	free electrons	the holes
Minority charge carriers	the holes	free electrons

SEMICONDUCTOR DIODES

1.Diode is

   a) made by joining a p-type and n-type semiconductors.

   b) a device that allows current to flow in one direction only

    (blocks it in the opposite directions).

   c) the simplest semiconductor device.

2.pn junction is formed when a n-type and p-type semiconductors are

joined together. The boundary between the p-type and n- type regions 

is called the junction.

3.Depletion layer is the neutral region around the junction which has no
charge carriers (left with neither holes nor free electrons). The conductivity 
of electric is poor

4. Forward bias

a) p-type is connected to the positive terminal of a battery.

b) n-type is connected to the negative terminal of a battery.

c)The diode conducts current because the holes from the p-type material 

and the electrons from the n-type material are able to cross over the junction. 

Hence the light bulb will light up.

 

5.Reversed bias

a)n-type is connected to the positive terminal of the battery.

b) p-type is connected to the negative terminal of the battery.

c) Reversed polarity causes a very small current flows as both electrons 

and holes are pulled away from the junction.

d) The depletion region widen  and the current will cease. Hence the bulb 

does not light up.

6.Diodes as rectifiers

a) A rectifier is an electrical device that converts alternating current to direct current.

b) Rectification is a process to convert an alternating  current into a direct current  

     by using a diode.

c) Two type of rectification:

i)  Half-wave rectification

ii) Full-wave rectification

7. Rectification

a) A process where only half of every cycle of an alternating current is made to

    flow in  one direction only.

b) No rectification

8. Half-wave rectification by using one diode

a)    A diode is connected in series with the resistor

b)   Diode can only allow current to flow in one direction. Current will only 

     flow in the first half-cycle when the diode in forward bias.

c)    Current is blocked in the second half-cycle when the diode is in reverse bias.

9. Full-wave rectification by using four diodes

a)    In the first half, the current flows from     A – P – T – U – R – B  

b)   the second half, the current flows from   B – Q – T – U – S – A 

 

10. The use of a capacitor in a rectifier circuit 

a)   Capacitor is used to smooth out the output current and output voltage

b)   When the current pass through the capacitor, the capacitor is charged and 
     stores energy.

c)    When there is no current pass through the capacitor, the capacitor is discharged.
d) The energy from it is used to produce voltage across the resistor. As a result it produces a smooth dc output.

4.3  TRANSISTOR

1.    A transistor has three leads connected to the emitter, E, base, B and 

     collector, C

2.    The emitter, E emits or sends charge carriers through the thin base, B layer 

      to be collected by the collector, C

3.    Two-type of transistor: 

      i) n-p-n transistor 

    ii ) p-n-p transistor

 

4.    I_B is too small compared to I_C.    [ I_B <<<<<< I_C]

     i)  I_B unit is μA

    ii)  I_C unit is mA

5.    Transistor as a current amplifier

     a) A small change in the base current, results in a big change in the 

         collector current, 

     b) Base current is amplified by the transistor. Hence M₂ is brighter than M₁

 6. Transistor as an automatic switch

a.    Suitable resistor R₁ and a variable resistor R₂ is used .

b.    Voltage at base terminal can be adjusted to switch the transistor on or off.

c.    If R₂  increases, base voltage also increases

d.    When the base voltage reaches certain minimum value, the base current 

     flows and switches the transistor on. 

e.    Examples of application of automatic switch are

i)             Light control switch

ii)            Fire alarm / Heat detector / Smoke detector

        

 Light controlled switch

Examples:

a.    Name terminal P, Q and R.

b.    What is the function of Y

c.    In order to light up the bulb, the potential difference across EF must be

     at least 4V.

    i) What is the resistance of X when the bulb lights up?

   ii) What happen to the light bulb if X has resistance of I KΩ?

d.    What must be done to the circuit so that the bulb is switched off in the bright light and on in the dark.

Answer:  

a.   P = base

      Q = collector

      R = emitter

b.  To limit the amount of current flowing into the base terminalc.

 

d. Replace X with a light dependent resistor (LDR)

4.4  ANALYSING LOGIC GATES

1. Logic gate is a circuit that has

a)    one or more input signals, but

b)   only one output signal.

c)    act as switching circuits in electronic systems

d)   work using tiny transistors as switches,

e)    manufactured as integrated circuit (IC), with each chip holding several gate

            f)  The input or inputs are on the left of the symbol.

           g)  The output is on the right

            h) Each input and output can be either

2. Five logic gates:

1. AND
2. OR
3. NOT
4. NAND
5. NOR

3. Symbol and Truth Table of the logic gates: