ELECTRICITY: Part 2 Thermionic emission
-metals: outer electrons which move randomly = free or conduction electrons as they can move freely through the metal
-metal: heated > some free/ conduction electrons gain energy > escape from surface of metal > thermionic emission
-can be produced by electrically heating a fine tungsten filament

DEFLECTION BY ELECTRIC FIELD
-moves towards + > parabolic path
-greater electric field strength > greater deflection
DEFLECTION BY MAGNETIC FIELD
-use Flemming's Left-hand rule (current = opp. direction of electron flow), circular path



Cathode Ray Oscilloscope, CRO (screens, monitors)-consists of electron gun, deflecting plates, fluorescent screen

ELECTRON GUN
-consists of glass tube at v. low pressure (near vacuum > e^-s not scattered by gas molecules)
-e^-s accelerated at high speeds shoot through hole of positive potential anode in a fine beam of electrons
-electron beams = cathode ray, produced at cathode
-in a CRO contains a grid, grid connected to -ve potential > controls # of e-s reaching screen > brightness control

DEFLECTING PLATES
-2 pairs, voltage applied across each pair to deflect beam
₁Y-plates > create vertical difference > vertical deflection
₂X-plates > create horizontal difference > horizontal deflection

FLUORESCENT SCREEN
-screen coated with fluorescent salt (zinc sulphide), electrons hit screen > salt produces flash of light > spot on screen

Time base = measure of time for oscilloscope

-on: saw-tooth voltage across X-plates > constant speed of electron beam sweeping across screen
-each cycle; beam: left > right of screen > jump back very quickly for next sweep
-speed of spot varied by setting time base control
-knowing period of each cycle, T > know speed of beam sweeping across screen

Measuring Voltage
-voltage to be measured applied to Y-plates via Y-input terminals, time base off
-deflection of beam by electric field ∝ voltage applied, gain of Y-input determines sensitivity of oscilloscope

Waveforms can be displayed on the oscilloscope when time base is on and there is a voltage at the Y-input

CRO as timer

-w/ time base on, CRO can measure short intervals of time
-time and distance for sound to travel a specific distance can be measured
ssspeed can by determined
-a pulse of sound produced by microphone (1st pulse on CRO) > pulse bounces off
wall and received by microphone (2nd pulse)
-distance, x, between 2 pulses : a measure of time taken for sound to travel to and
fro the distance between microphone and wall

Resistors

Colour Code:

Black = 0	Brown = 1	Red = 2	Orange = 3	Yellow = 4
Green = 5	Blue = 6	Violet = 7	Gray = 8	White = 9

power ratings = max rate of dissipation of electrical energy as heat before it is damaged
power rating exceeded > resistor heat up > R increase > resistor burnt out if heats up too much

Potentiometer/ Potential divider; controls pd across a device, allows pd to be varied, even to 0

transducer: device which can convert 1 form of energy to another, energy converted to elec energy is used to create more free e^-s

Thermistor; input transducer made of semi-conductor; heat > elec energy (fire alarms, temperature measurement)
R decreases as temp ^ (temperature increase due to direct heating or passing current through it)
Light Dependant Resistor, l.d.r.; input transducer made of semi-conductor; light energy > elec energy
R decreases as light ^, light > more energy absorbed > more free electrons released for conduction of electricity
Uses:-1light operator, give info on light intensity, camera, 2light-operated switch


Capacitor; a device to store electric charge
-consists of a pair of metal plates separated by air or insulator (dielectric)

Charging capacitor

S: closed > e^-s on B go to +ve b > B, +ve > e^-s from a attracted to A by B > A, -ve (plates A & B: charge w/ equal # of charges)
e^- flow stops; pd of plates = pd of emf source

S; closed: t = 0 , V_C = 0, V_R = ,
when current flows >capacitor charged,
V_C ^, V_R , (I = V_R /R), e^- flow stops;
V_C = ,
V_R = 0, I = 0
capacitor fully charged

Discharging capacitor

S closed: electrons A > B
stored electrostatic energy in charge capacitor drives e-s round circuit to B, when fully discharged, stored energy transformed to elec. energy dissipated as heat due to resistance in resistor and wires

t = 0 , V_C = V_R, I = V_R/R,
discharging; e^- flow: A to B,
V_C , (charges ) > I (V_R ),
when V_C =0, I = 0, fully discharged



Diode; semiconductor device: allows current to flow easily in 1 direction only
forward bias; R very small, negligible
reverse bias; R very large, no current flow

Use of diode
-for rectification; converting ac to dc

Transistor; electronic switches or amplifiers
as a switch; a certain amount of voltage is supplied to base to activate transistor
< a specific amt; like an open switch > reverse bias
> a specific amt; like a closed switch, forward bias


Digital Logic Circuits; ATMs
-digital logic: input and output only have 2 levels of V: high(+5V)/low (0V)
Logic gate- electronic system which gives an output only under certain input conditions

Gate type	Symbol	Truth table	Statement
NOT		A C 0 1 1 0	Output NOT same as input
AND		A B C 0 0 0 0 1 0 1 0 0 1 1 1	Output high if input A AND B are high
OR		A B C 0 0 0 0 1 1 1 0 1 1 1 1	Output high if input A OR B OR both are high
NAND		A B C 0 0 1 0 1 1 1 0 1 1 1 0	Output NOT high if input A AND B are high
NOR		A B C 0 0 1 0 1 0 1 0 0 1 1 0	Output NOT high if input A OR B or more inputs are high

Thermal Physics
Waves & Sound
Radioactivity
Measurements in Physics
Forces
Energy
Pressure
Light
Magnetism
Electrostatic Charging

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