physics final revision - electronics 2013.pdf

7/27/2019 Physics Final Revision - Electronics 2013.pdf

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Physics SPM 2013 Chapter 9: Electronics

Hoo Sze Yen www.physicsrox.com Page 1 of 9

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 the beam of electrons which move at high speed from the cathode to the anode.


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9.1.2 Maltese Cross Tube

Situation Results seen onthe fluorescentscreen

Explanation

The low voltage is

switched on; theextra high voltage isoff

Shadow of the Maltese cross caused by the light

emitted from the hot filament

Both low voltageand extra highvoltage areswitched on

Green shadow of the Maltese cross caused by theelectron beams

A magnetic bar isplaced near thefluorescent screen

The green shadow of the Maltese cross is deflected.Deflection is downwards if the north pole is placed nearthe screen.Direction of deflection can be determined by the left-

hand Fleming rule.

9.1.3 Perrin Tube (Deflection tube)

Situation Results seen on the fluorescent screen

The extra high voltage isswitched off

The extra high voltage isswitched on

(If P is positive) (If Q is positive)


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9.1.4 Electron Beam Characteristics

Movement is in a straight line because it is light and has high velocity. Has momentum and energy; fluorescent effect when connects with fluorescent items. Negatively charged deflected towards positive plates.

Can be deflected by magnetic fields. When collides with metal targets, kinetic energy 99% light and X-rays

9.1.5 Cathode Ray Oscilloscope (CRO)

CRO Reading No input Direct current (fromdry cell)

Alternating current

Time-basedswitched off

Time-basedswitched on

Cathode ray oscilloscopes are used to: Measure potential difference Measure short time intervals Display wave forms

Note:The time-base is connected to the X-plates and generates a time varying voltage as below:


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The input voltage is connected to the Y-plates.

9.1.6 Working Principle of the Cathode Ray Oscilloscope

9.1.7 Speed of Cathode Rays

If potential energy provided by the potential difference = eVand kinetic energy is mv2, the

relationship of a cathode ray is:

eV = mv2

9.2 Semiconductors

9.2.1 Doping of Semiconductors

Materials usually used in the electronics industry assemiconductors are silicone and germanium. Doping process is the addition of a small quantity of foreignobjects into a semiconductor to increase its conductivity. Theatom size of the foreign object has to be about the same size asthe atom size of the semiconductor.

n-type semiconductor p-type semiconductor

Type of foreign atomsadded

Pentavalent atoms Trivalent atoms

Examples Antimony, arsenic, phosphorus Boron, gallium, indium, aluminium

Major charge carrier Free electrons Positively-charged holes

Minor charge carrier Positively-charged holes Free electrons

Typical semiconductor: Silicone


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9.2.2 Diodes

A semiconductor diode is also known as a p-n junction.

A diode allows current to flow in one direction only.

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. The junction voltage of a diode must be overcome before current can flow. Junction voltages for silicone and germanium are approximately 0.6 V and 0.1

V respectively.

Forward Bias Reverse Bias

9.2.3 Diodes as Rectifiers

Rectification is the process of converting alternating current to direct current. This is done with adiode as diodes allow current to flow only in one direction.

Potential difference from an alternating current source

Half-wave Rectification Full-wave Rectification

Using a single diode: Using four diodes (bridge rectifier):

CRO

Silicone diode graph which shows ajunction voltage of 0.6 V


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9.2.4 Capacitors

Capacitors are used to smoothen out the current. Using capacitors with full-wave rectification createssmoother current flows for optimal use with electrical appliances.

Capacitor charging

For the positive half-cycle, the diode is inforward bias

Current flows through the capacitor and theresistor

Capacitor is charged and energy is stored

Capacitor discharging

For the negative half-cycle, the diode is inreverse bias

Current is not allowed to flow through thediode

Capacitor discharges and the energy storedis used to maintain the potential difference acrossthe resistor

Half-wave rectification with capacitor Full-wave rectification with capacitor

Circuitconnection

CRO trace

Note: The four-diode arrangement can becombined into a bridge rectifier. Thereare four terminals on a bridge rectifier: 2to the a.c. source, and 2 to the resistor.


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9.3 Transistors

Transistors are electronic devices that act as a transfer resistor to control the current and potentialdifference within an electronic circuit.

Transistors are a combination of two types of semiconductors, i.e. type p and type n. Transistors havethree electrodes:

Base (B)

Collector (C)

Emitter (E)

There are two types of transistors:

n-p-n transistor p-n-p transistor

Transistor basics:

For both n-p-n and p-n-p transistors:IE= IB + IC

Current magnification =

B

C

I

I

where IE= emitter current [A]IB = base current [A]IC= collector current [A}

Current magnification 50 150 for normal transistorsRB is a high resistance resistor to limit the flow of IB in the base current.

Note:

ICIB; i.e. IB = 0, IC= 0 IBIC; i.e. IC= 0, IB 0 A small change in IB causes a big change in IC.


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9.3.1 Transistors as amplifiers

Transistor as a current amplifier

Transistor as a sound amplifier

Component Function

Microphone Converts sound signals to electricalsignals

Capacitor Prevents d.c. from flowing into thetransistor and loudspeaker

Transistor Amplifies input signalLoudspeaker Converts electrical signals to sound

9.3.2 Transistors as automatic switches

R= 0, VR= 0 IB = 0, IC= 0R, VR IB, IC

When resistance of R2 increases, the potentialdifference across R2 increases. This causes current toflow through the base circuit

If there is base current, there will be collector current;therefore the light bulb will light up

Light controlled switch

Light-dependent resistor (LDR) changes resistancedepending on presence of light

Very high resistance in the dark

Low resistance in bright light

Heat controlled switch

Thermistor is a heat-dependent resistor

Resistance increases when it is cold

Resistance drops when it is hot


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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 the number of possible combinations Nforn input variables:

N = 2n

Gate Symbol Boolean equation Truth table

NOTX =

Input Output

0 1

1 0

OR X = A + B

Input Output

0 0 0

0 1 1

1 0 11 1 1

ANDX =A B

Input Output

0 0 0

0 1 0

1 0 0

1 1 1

NOR X = BA

Input Output

0 0 1

0 1 0

1 0 0

1 1 0

NAND X = BA

Input Output

0 0 1

0 1 0

1 0 01 1 0

END OF CHAPTER

physics final revision - electronics 2013.pdf

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