physics spm 2015 chapter 9: electronics chapter 9: · pdf file · 2017-11-15physics...

╞╡§¥ Physics SPM 2015 Chapter 9: Electronics

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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. 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.

9.1.2 Maltese Cross Tube

Situation Results seen on

the fluorescent screen

Explanation

The low voltage is switched on; the extra high voltage is off

Shadow of the Maltese cross caused by the light emitted from the hot filament. No green shadow as there are no cathode rays.

Both low voltage and extra high voltage are switched on

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.

A magnetic bar is placed near the fluorescent screen

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 left-hand Fleming rule.



9.1.3 Perrin Tube (Deflection tube)

Situation Results seen on the fluorescent screen

The extra high voltage is switched off

The extra high voltage is switched on

(If P is positive)

(If Q is positive)

9.1.4 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

9.1.5 Cathode Ray Oscilloscope (CRO)

Uses: Measure potential difference Measure short time intervals Display wave forms



Functions of the components in a cathode ray oscilloscope

Part Component Function

Electron gun Filament Heat up the cathode

Cathode Release electrons (via thermionic emission)

Control grid Control number of electrons that flow (controls brightness of the bright spot on the screen)

Focusing anode Focus the cathode rays

Accelerating anode 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) Alternating current

Time-based switched off

Time-based switched on

9.1.6 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:

eV = ½ mv2

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



9.2 Semiconductors

9.2.1 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 p-type semiconductor

Type of foreign atoms added

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

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. 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

Typical semiconductor: Silicone

Symbol of a diode

Silicone diode graph which shows a

junction voltage of 0.6 V



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 Full-wave Rectification

Using a single diode: 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.



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

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

Capacitor discharging

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:

IE = IB + IC

Current magnification =

B

C

I

I

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



9.3.1 Transistors as amplifiers

Transistor as a current amplifier

Transistor as a sound amplifier

Component Function

Microphone Converts sound signals to electrical signals

Capacitor Prevents d.c. from flowing into the microphone and loudspeaker

Transistor Amplifies input signal

Loudspeaker Converts electrical signals to sound

9.3.2 Transistors as automatic switches

When resistance of R2 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

R1 and R2 act as potential dividers. To calculate base voltage:

total

total

base

base

R

V

R

V

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 ↓ IB × flows, IC × flows Light bulb does not light up

When dark: LDR resistance ↑ Base voltage ↑ IB flows, IC 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 ↑ IB flows, IC flows Alarm rings

When cold: Thermistor resistance ↑ Base voltage ↓ IB × flows, IC × flows Alarm does not ring

.



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

Input Output

0 1

1 0

OR

X = A + B

Input Output

0 0 0

0 1 1

1 0 1

1 1 1

AND

X = A • B

Input Output

0 0 0

0 1 0

1 0 0

1 1 1

NOR

X = B A

Input Output

0 0 1

0 1 0

1 0 0

1 1 0

NAND

X = B A

Input Output

0 0 1

0 1 1

1 0 1

1 1 0

END OF CHAPTER

physics spm 2015 chapter 9: electronics chapter 9: · pdf file · 2017-11-15physics...

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