Physics Ideas to Implementation

8/12/2019 Physics Ideas to Implementation

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CATHODE RAY AND DEVELOPMENT OF TELEVISION

The Discovery of Cathode Rays

It was thought gases were electric insulators. Heinrich Geissler refned

a vacuum pump so it could be made to evacuate a glass tube to within

0.01 o normal air pressure. Julius Plucker his riend! took tubes and

sealed a metal plate called an electrode" to each end o the tube. #he

electrodes made electrical connections through the galss and were

sealed to maintain the partial vacuum in the tbe. #hese were then

connected to high voltage sourge.

#he evacuated tubes actuall$ conducted an electric current. #he glass

at the positive end or anode! o tube glowed with a pale green light.

%hat t$pe o invisible ra$ caused this &uorescence. &ourescence is

emission o light rom a material when it is e'posed to streams o

particles or e'ternal radiation.

%hatever this ra$ was" it was thought to have originated at thenegative electrode or cathode. (ugene Goldstein" stud$ing these same


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e)ects" names the ra$s that caused the glow *cathode ra$s* and thetubes were known as cathode ra$ tubes o discharge tubes.

(arl$ e'perimentors used these tubes to investigate all o theproperties o cathode ra$s and +ra$s.

,athode ra$s are now used in tv" computer

#here are now known to be streams o electrons emitted within anevacuated tube rom a cathode negative lectrode !to an anode positive electrode!.

- ,# is a sealed glass tube rom which most o the air is removed b$ avacuum pump. - beam o electrons travles rom the cathode to anodeand can be de&ected b$ electrical and or magnetic felds.

/ischarge #ubes

/ischarge tubes evacuated to di)erent ait pressures were ound toproduce di)erent e)ects.

-t low pressures" electrons can accelerate to aster speeds beorecolliding with gas particles. Initiall$" a current will &ow even thoughnothing can be seen. #he frst e)ect seen is a stead$ luminousdischarge knows as *glow discharge.

-s pressure is lowered urther" a number o colourul e)ects seen

-s frst" most o tube is occupied b$ birght region called positivecolumn which apperas to start rom the anode and is broken into serieso bands or striations. triations are separated b$ dark spaces.

,olour o discharge depends on gas used. In low pressure air positivecolum is a brilliant pink and negative glow is deep blue.

2or ever$da$ use the$ are in neon light signs.3 #he$ are long tubeswith most air removed. - small amound o gas is introduces" which"when e'cited b$ a high potential glows. (g when added gas is neon"the kinetic energ$ o electrons is enough to inoise the gas around thecathode causing the emission o a reddish light.


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2lourescent tubes in the home have mercur$ vapour at low temps. #Helight produced is in the 45 region o the electromagnetic spectrum. #6produce visible light" a thin coating o apowder is spread on the insidesurace o the tube. #He 45 radiation causing this coating to &uorescewith the amiliar white light.

Eect of e!ectric "e!ds o# cathode rays

7 felds we know8 electric" magnetic" gravitational. (lectric feld e'istsin region where electric charge e'pereiences orce. /irection o electricfeld is direction in which positive charge e'perienes orce when placedin electric feld.

9ines radiate rom centre. adiate in all directions or positive. adiall$centred ore negative.

2irst8 postive" second8negative" third" opposite" ourth like negative" fth like positive.

4niorm electric felds

- uniorm electric feld can be made b$ placing charges on two parallelplates which are separated b$ a small distance compared with their

lengths.

(lectric feld (! bw : parallel plates.


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(lectric feld is uniorm e'cept at edges where it bulges slightl$


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Protection against lightning8 Pointed conductors

9ighting strikes have massive electrical discharge over short period otime. -s cloud moves over the ground" negative charge in the cloudrepels electrons in the ground producing potential di)erence betweencloud and ground.

%hen P.d 9arge enough to overcome the resistance or air" there isdischarge seen as lightning.

;en


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#he Photocopier >achine 8 ,harged particles

Photocopiers which scan image and produce dr$ image i.e one htat

doesn?t use speacial photosensitive paper!" were developed b$ ,hester,arlson in a process called 'erograph$.

emiconductors@ selenium" arsenic" and tellurium @ act asphotoconductors. #hat is the$ act as insulators in the dark and electricconductors in the light.

- thin la$er o semiconductor material deposited on drum. -t start ocop$ c$cle" drum given a unirom electrostatic charge. Page to becopied is illuminated with strong light and an image o page is ormedb$ lens on the charged photoconductor. %hite areas light upphotoconductor and it becomes a conductos to its chrage leaks awa$

to matal backing. ;lack areas remain. #his latent image is thendeveloped.

- fne powder o small glass beads" covered with a black toner" isgentl$ brushed over drum. #oner sticks onl$ where there is remainingcharge" that is the black areas o original image.

Ae't a blank sheetr o paper is rolled over drum" and toner is


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transerred to the paper givign an e'act image o the original. #heimage" is * f'ed* b$ heating page. /rum is cleared or ne't cop$ c$cle.

J.J #hompson

%ork on cathode ra$s

incorporated charged plates inside # to veri$ h$pothesis b$ ,rookesthat cathode ra$s would be de&ected b$ electric felds.

IA thompson?s e'p" cathode ra$s passed between parallel platesconnected to batter$.

6bserved direction o ra$s moved towards positivel$ charged plate"showing that ra$s behaved as negative charges.

>illikan?s oil drop e'periment

obert -. >illikan" used uniorm electric feld bw : parallel plates toinvestigate properties o charge.

pra$ed oil drops into appartatus.


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ome drops drited into region ; and came under in&uence o electricfeld (. -s enetered" the$ were momentaril$ e'posed to + ra$s"resulting in some drops become chraged.

(arth?s gravitation feld e'erts orce down weight! which can becounteracted b$ an electric feld produced bw parallel plates b$ asource o variable voltage. paces bw plates can be viewed throughmicroscope. ;$ careul ad


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velocit$ o dop when the electric feld was turned o)" and it ell underin&uence o gravit$ alone. ;$ using e=uations rom &uid mechanics" hecould calculate radius o drop. ;$ using an oil with a known densit$" hedetermined it?s mass.

His fnding showed charge on drop wasn?t arbitar$. ;ut that charge

alwa$s occured in packets or multiples o some smallest value. #hisvalue was calculated to be

B elementar$ charge" charge ound on electron

I =uestion ask what is the charge e'isting o oil drop8 I suspended8mgB(= thereor =BmgC(

/irection o electric feld is alwa$s positive to negative

Eects of Ma$#etic Fie!ds o# Cathode Rays

>agnetic felds e'ert orces o electric currents" that is" on movingcharged particles.

#he direction o the orce is given b$ the HP I positive particle"direction o conventional current is that o velocit$ i negative"opposite to velocit$! -D- HP or positive 9HP or negative!

In HP8 thumb B 5elocit$or positive@use 9HP or negative" akavelocit$ is opposite! fngers B magnetic feld palmBorce. ;4# #%I#8

#humb can change angle i the velocit$ is at an angle theta tomagnetic feld


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Aow the particle with charge = and velocit$ v ma$ or ma$ not moveperpendicularl$ to magntic feld o strength ;. #hereore ormula is newin picture!.

#hereore i velocit$ and >agnetic feld perpendicular sinE0!B1thereore 2B=5;. I parallel" sin0!B0 thereore 2B0

I something is towards $ou" then downwards" awa$ rom $ou"upwards" right" let" normal.

Path o electron in a magnetic fled + + + + is circular" as magneticorce is acting

+ + + +perpendicular to the velocit$ o the

+ + + + electron

Deter%i#i#$ the char$e&to&%ass ratio of cathode rays

2ollowing rom his e'periment cathode ra$s de&ected b$ electricfelds! he measured charge@to@mass ratio o cathode ra$ particlescalled electrons.

He built a ,# with charged parallel plates called capacitor plates! toprovide a uniorm electric feld and source o uniorm magnetic feld.

%Ith apparatus he investigated e)ect o cathode ra$s passing throughboth felds oriented at right angles to each other and this had e)ect oproducing orces on cathode ra$s that directl$ opposed each other.

('p has two stages8

1. 5ar$ing the magnetic feld and electric felds until their opposing orcescancelled" leaving the cathode ra$s unde&ected. #his e)ect is

;$ e=uating magnetic and electric orce e=uations" #hompsondetermined velocit$ o cathode ra$ particles.


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2. -ppl$ing the same strength magnetic feld alone! and determining theradius o the circle path travelled b$ the charged particles in themagnetic feld.

#hompson combined results and obtained the magnitude o the charge@to@

mass ratio or the charged particles that constituted cathode ra$s.

Ph$sics 2act

%hen charged particles eneter electric feld the$ ollow a tra


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Cathode Rays & 'aves or Partic!es(

,rooke?s ound cathode ra$s did not penetrate metals and travelled instraight lines.

Initiall$ thought the$ were electromagnetic waves as similar bahviourto light but discounted" as crookes ound ra$s were de&ected b$magnetic felds 8 an e)ect not seen in light.

#his debate continued with scientists b$ observing e'periments andh$pothesising i its like light with short wavelengths" or matter withnegativel$ charged and moving with great velocit$ until JJ #hompsonshowed ra$s were streams o negativel$ charged particles 8 we call

electrons now.

(arlier inconsistent behaviour o ra$s was problem.

6bserations that ftted the wave model8

3. traight line travel

4. I opa=ue ob


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10. lower than light

>ain restirction or particle theor$ was abscence o de&ection inelectric felds" but JJ showed this was due to ra$s themselves. It?s dueto conductivit$ conerred on the rarefed gas b$ cathode ra$s.>easuring conductivit$" it decreased rapidl$ with e'haustion o gas. -t

high e'hausions there might be a chance o detecting de&ection ocathode ra$s b$ and electtrostatic orce.

%ithin tube" cathode ra$s ionised gas. Ions were attracted to plate withopposite charge and the line@up o ions neutralised the charge onplate" allowing ra$s to pass una)ected.

-ter evacuating chamber" #hompson observed de&ection towardspositive plates" confrming them as negativel$ charged particles.

-bilit$ o cathode ra$s to penetrate thin metal oilds still not e'plained.-nswer not in properties o cathode ra$" but model o atom. I atom notsolid" possible or small particles to pass through oil. (rnest utherordchanged the model o atom.

+@a$s8 /iscover$ and -pplication

%ilhelm ontgen e'perimented with cathode ra$s. 2ound in dark room"a screen covered with sensitive &ourescent material barium platino@c$anide! glowed when placed near the end o cathode ra$ tube.


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ince cathode ra$s couldn?t pass through glass at end o tube" hededuced that this &uorescence must be due to new orm o radiation.,alled radiation +ra$s.

9ater research shows + ra$s wre produced when high speed electronsinteracted with matter" like glass in ,#.

+@ra$s later ound to be electromagnetic waves" similar to light" smallerwavelenght.

Properties8 Penetrate man$ substances" e'pose photographic flm"cause certian substances to &uoresce" be re&ected and reracted.

,ommon use8 /iagnosing illness or in


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#elevision

4ses ,# as output device.

,olour television camera records images through G; flters.Inormation transmitted to reciever which directs signal to 7 electronguns G;!.

Pic reconstituted on screen b$ an additive process involving 7 colouredphospphors. (ach electron gun stimulated its phosphor.

creen made o F: horiontal lines o dots. ,urrent in each coil

energises the de&ection coils and is varied to scan screen twice oreach image

(lectrons sweep across the screen" building up picture. (ach picormed rom : passes o the electron beam. the odd@numbered linesare drawn frst" then the beam ?&ies?back to start adn ?draws in? evennumbered lines. (ach scan takes1C0 seconds" less than retina retainsimage so screen doesn?t &icker.

/ots o phosphorescent pain on screen convert energ$ o electronbeam into light. -ter beign e'cited" phosphorescent continues to emitlight or a longer time" which helps to minimise screen &icker. In colour

tv" phosphoresecnt ots come in the G;" and colour ormed b$combining the 7.

: electron beams scan screen the$ come rom slighting di)rerentdirections throught holes in a shadow mask to control brightness o 7sets o phosphores.

,athode a$ 6scilloscopes


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,6 makes voltage *visible.*

4ses ,# to displa$ a variet$ o electrical signals. Horiontal de&ectionis usuall$ provided b$ a time bases or sweep generator! which allowsthe voltage o vertical a'is! to be pltted as a unction o time onhoriontal a'ists!

#imebase control allows technician to select a variet$ o sweep rates.#his sets the time per division or the fguere drawn on screen. ;ottonotar$ switches control the amplitude o displa$ed waveorm. (achcentimetre o grid can be used to measure voltage o input waveorm.

Input waveorm enters rom let on diagram. 6ne part o signal isamplifed" and the voltage produced goes to vertical de&ecting plates.6#her part passes to the trigger and time base and is then passed tohoriontal de&ection plates.

,ombination o both signals produce waveorm.

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THE PHOTOELECTRIC EFFECT AND *LAC+ *ODY RADIATION

Ma,-e!!.s Theory of E!ectro%a$#etic 'aves

;ased on observations" that changing magnetic feld induces anelectric feld in the region around a magner" and that a magnetic feldis induced in the region around a current carr$ing conduction" James,lerk >a'well concluded that mutual induction o time and space


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changing electric and magnetic felds should allow the ollowingunending se=uence o events8

11. - time@var$ing feld in one region produces a time@ and space@var$ingmagnetic feld at all points around it.

12.#his var$ing magnetic feld then similiarl$ produces a var$ing electricfeld

13. In an electromagnetic distubance is started at one loaction eg.vibrating charges in radio antenna! the disturbance can travel out todistant point through the mutual generation o electric and magneticfelds.

14.#he eletric and magnetic felds propogate through space in the orm oan electromagnetic wave

4pshot was electromagnetic waves are sel@propogating.

6nce started it can continue orever.

>a'well?s theor$ gave defnite connection bw light and electricit$.

His e=uations predicted light and all electromagnetic waves must betransverse waves and travel at light speed estimated 7.1110KmCs.Implied that ull range o re=uencies o electromagnetic waves shoulde'ist -D- electromagnetic spectrum.

Hei#rich Hert/ a#d E,)eri%e#ts -ith Radio 'aves

>a'wells two important predictions were8

15. (lectromagnetic waves could e'ist with man$ di)erent re=uencies

16. -ll such waves should propogate through space at light speed

Hert verifed these

He thought he could produce some o electromagnetic waves withre=uencies other than that o visible light b$ creating rapidl$oscillating electric feld with an induction coil that caused a rapidsparking across a gap in a conducting cicuit.

4sed an induction coil to produce sparks between the sphericalelectrodes o the transmitter. He observed that when a small length owire was bent into a loop so that there was a small gap and held nearthe sparking induction coil" a spark would


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generator b$ transmitter. 2irst evidence o electromagnetic waves.

Hert showed that waves originating orm electrodes connected toinduction coil behaved as it the$ awere polarised b$ rotating thereceiever loop. %hen detector loop was perpendicular to transmittergap" radio waves rom the gap produced no spark. park in recieverwas caused b$ the electric current set up in the conducting wire. %henthe detector loop was parallel to hte sphereical electrodes attached tothe induction coil" the spark in the receiver was at ma'imum. -tintermediate angles it was prooportionall$ elss. It showed newl$ madewaves were polarised.


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I het was correct" and (> waves travel through space rom the coil tothe loop. he reasoned" there must be a small dela$ bw appearance o1st and :nd spark. #he spark in detector cant occur at e'actl$ thesame time as the spark in induction coil as even travelling at lightspeed takes some time.

#o measure wavelenght he ocnnected both transmitter and deterctorloop with a length o wire. He?d alread$ shown b$ roating second loop"that waves produced b$ sparking behaved as i the$ were polarised.

#he spaek in reciever was caused b$ electric current set up inconducting wire. -t intermediate angles" intererence o currentsprovided a measure o the wavelength o radio waves through air.

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Physics Ideas to Implementation