Physics (CBSE 2007) Time: 3 hours Max. Marks: 70. General Instructions

Transcription

1 Physics (CSE 7) Tim: 3 hours Max Marks: 7 Gnral nstructions 1 ll qustions ar compulsory Thr is no ovrall choic Howvr, an intrnal choic has bn provi in on qustion of two marks, on qustion of thr marks an all thr qustions of fiv marks You hav to attmpt only on of th choic in such qustions 3 Qustion numbrs 1 to 5 ar vry short answr typ qustions, carrying on mark ach 4 Qustion numbrs 6 to 1 ar short answr typ qustions, carrying two marks ach 5 Qustion numbrs 13 to 4 ar also short answr typ qustions, carrying thr marks ach 6Qustions numbrs 5 to 7 ar long answr typ qustions, carrying fiv marks ach 7 Us of calculators is not prmitt Howvr, you may us log tabls, if ncssary 8 You may us th following valus of physical constants whrvr ncssary: c = ms -1 h = Js = C m = 1-7 T m -1 = C N -1 m = Nm C ε Mass of lctron, m = kg Mass of nutron, m n = kg oltzmann s constant k = JK -1 vogaro s numbr N = mol -1 aius of arth = 64 km 1 n lctron, an alpha-particl an a proton hav th sam kintic nrgy Which on of ths particls has th largst -rogli wavlngth? [1] Th kintic nrgy is P T = m Thrfor, th rogli wavlngth an kintic nrgy ar rlat by h λ = mt Thrfor, th wavlngth of th particl is invrsly propotional to th squar root of mass of th particl Thrfor, lctron has th largst wavlngth Why shoul th matrial us for making prmannt magnts hav high corcivity? [1] Th magntic fil n to b appli to magntiz th matrial is known as corcivity So matrials with high corcivity kp thir magntization vn in th prsnc of small magntic fils Thrfor, matrials with high corcivity ar us to mak prmannt magnts 3 Th raioactiv isotop D cays accoring to th squnc β - -particl 1 D D D f th mass numbr an atomic numbr of D ar 176 an 71 rspctivly, what is (a) th mass numbr (b) atomic numbr of D? [1] Emission of th b - incrass th atomic numbr by 1 an prsrvs th mass numbr Emission of th alpha particl crass th mass numbr by 4 an atomic numbr by 4 What will b th valus of input an for th oolan xprssion ( + ) i( i) = 1? [1] Th output is 1 if, ( i ) = 1 an ( + ) = 1 ( + ) = 1only whn = an = Physics Spcial markt ook _7 CSEin 1 1/1/11 1:: PM

2 Physics (CSE 7) f = an = ( + ) =1 5 Why is frquncy moulation prfrr ovr amplitu moulation for transmission of music? [1] Frquncy moulation has highr banwith an quality compar to amplitu moulation Sinc th information is transmitt in th form of frquncy variation th changs occurring to th amplitu of th signal o not affct th information transmitt, whras it bcoms a srious problm in amplitu moulation Thrfor, frquncy moulation is prfrr ovr ampli tu moulation to transmit music 6 Th output of an O gat is connct to both th inputs of a NND gat Draw th logic circuit of this combination of gats an writ its truth tabl [] C C D Draw a plot of potntial nrgy of a pair of nuclons as a function of thir sparation What is th significanc of ngativ potntial nrgy in th graph rawn? [] MV 1 r 1 3 r(fm) Whn potntial nrgy is ngativ, forc is attractiv 8 convx lns of rfractiv, inx 15 has a focal lngth of 18 cm in air Calculat th chang in its focal lngth whn it is immrs in watr of rfractiv inx 4/3 [] Th focal lngth of th lns in air or 1 n = ( - ) - f = 5 - f (1) 1 D f th lns is ipp in watr, th nw rfractiv inx is nglass 15 9 n = = = = n 4/ watr Th nw focal lngth = ( - ) - f () 1 Diviing th quation (1) by () w gt Or th nw focal lngth f 5 = = f f = 3 84 f = cm = 69 1 cm Thrfor th chang in focal lngth is f = f - f = 69 1cm- 18cm = 51 1cm 9 Distinguish btwn th trms avrag valu an rms valu of an altrnating currnt Th instantanous currnt from an ac sourc is = 5 sin (314 t) ampr What ar th avrag an rms valus of th currnt? [] Th avrag valu is fin as avg T T = 1 T t = 1 T sin tt ω = (1) Sinc th currnt is positiv in on half cycl an ngativ in th othr, th avrag currnt bcoms zro Th rms valu is fin as th root of th avrag of squars of th currnt T = rms t t 1 T sin ω = () Sinc th valu of th currnt in ach half cycl is squar, th rms valu is finit Th rms valu is us to mntion th avrag powr transmitt W hav = 5 sin (314 t) ampr From (1), avg = n from (), For half cycl rms 5 = = = = avg π 1 = π 1 Writ th rlation for th forc acting on a charg carrir q moving with a vlocity through a magntic fil in vctor notation Using this rlation, uc th conitions unr which this forc will b (a) maximum (b) minimum [] Th forc acting on th charg particl which is moving in a magntic fil is Physics Spcial markt ook _7 CSEin 1/1/11 1::3 PM

3 Physics (CSE 7) 3 F = q( ν ) (1) F = qν sinθnˆ Thrfor, th currnt bcoms maximum whn angl btwn th vlocity an magntic fil is 9 an bcoms minimum whn th irction of vlocity is paralll to th irction of magntic fil 11 cylinrical mtallic wir is strtch to incras its lngth by 5% Calculat th prcntag chang in its rsistanc [] Whil strtching th wir, volum of th wir rmains constant nitial volum V = πr l 1 whr r is th raius of th wir an l is th lngth of th wir ftr strtching th wir th lngth of th wir bcoms l = l+ 5 l Th spac aroun a charg boy is fill with lins of forc, which ar known as lctric fil lins Elctric fil lins of th positiv charg point raially outwar from th charg + Elctric fil lins of th ngativ charg point towar th charg E for lon ( ) charg Th lctrical fil lins start from th positiv charg Thrfor, th fil lins for Q > ar Thrfor, th nw volum V = π r l( ) ut both volums ar qual r r 15 or r = = ( ) nitial rsistanc of th wir n th final rsistanc = πr lρ πr r 15 = πr l( 15) ρ= lρ = Thrfor, th rsistanc of th wir rmains constant 1 Th lctric fil E u to a point charg at any point nar it is fin as F E lim whr q is th tst charg an F = q q is th forc acting on it What is th physical significanc F of lim in this xprssion? Draw th lctric fil lins of a q q point charg Q whn (i) Q > an (ii) Q < O Dfin lctric flux Writ its S units sphrical rubbr balloon carris a charg that is uniformly istribut ovr its surfac s th balloon is blown up an incrass in siz, how os th total lctric flux coming out of th surfac chang? Giv rason [] Elctric fil intnsity at a point is fin as th lctric forc acting on a tiny tst charg q plac at that point ivi by th charg q F E = lim q q +Q Th lctrical fil lins n at th ngativ charg Thrfor, th lctrical fil lins for Q < ar O Q Th numbr of lctric fil lins passing through a givn ara is known as lctric flux t is also fin as th lctric fil, E, multipli by th componnt of ara prpnicular to th fil φ = Ei a S unit of lctric flux is volt mtr ccoring to Gauss s law, lctric fil is givn by QS φe = E i = S ε whr Q S is th nt charg nclos by th surfac an is th lctric constant Physics Spcial markt ook _7 CSEin 3 1/1/11 1::7 PM

4 4 Physics (CSE 7) Th total charg th surfac of th balloon ncloss is inpnnt of th siz of th balloon, Thrfor, th nt lctric flux coming out of th surfac of th balloon rmains constant 13 Duc an xprssion for th lctric potntial u to an lctric ipol at any point on its axis Mntion on contrasting fatur of lctric potntial of a ipol at a point as compar to that u to a singl charg [3] q +q x P Th istanc btwn th ngativ an positiv charg of th ipol is an th istanc of point P on th axis of th ipol from th positiv charg is x Thrfor, th lctric potntial kq kq kp V ( P)= - = kq = x x - + x - x For x >> V kp x Th lctric ipol potntial falls off at larg istancs as 1/r, not as 1/r, which is a charactristic of th potntial u to a singl charg x 1/r 1/r () Th lctric fil intnsity btwn th two infinitly paralll plats is E = 1 kε σ whr k is th ilctric constant of th mium btwn th two plats, an s is th surfac charg nsity Thrfor, lctric fil intnsity bfor insrting th ilctric slab is Q E = 1 = 1 i ε σ ε n aftr insrting th slab E = 1 Q f k = 1 ε σ kε Thrfor, aftr insrting th ilctric slab, th lctric fil intnsity btwn th slab crass by a factor of k (3) Th capacitanc of th paralll plat capacitor is givn by th quation C = kε whr k is th ilctric constant of th mium insrt btwn th plats, is th sparation btwn th plats an th is th ara of th plat Thrfor, th capacitanc bfor insrting th slab is C = ε i n aftr insrting th ilctric slab Potntial V singl charg V ipol Variation of potntial V with r in units of r -1 k C = ε f Thrfor, aftr insrting th ilctric slab, th capacitanc incrass by a factor of k 15 Stat Kirchhoff s ruls of currnt istribution in an lctrical ntwork Using ths ruls trmin th valu of th currnt in th lctric circuit givn blow [3] Ω 14 paralll plat capacitor, with ach plat of ara an sparation, is charg to a potntial iffrnc V Th battry us to charg it is thn isconnct ilctric slab of thicknss an ilctric constant k is now plac btwn th plats What chang, if any, will tak plac in th (a) charg on th plats (b) lctric fil intnsity btwn th plats (c) capacitanc of th capacitor Justify your answr in ach cas [3] (1) Elctric charg is a consrv quantity Thrfor, th charg on th plats rmains constant 1 4 Ω 3 8 V Ω 4 V (1) Kirchhoff s currnt law stats that th algbraic sum of th currnt ntring a junction an laving a junction is zro () Kirchoff s voltag law stats that th algbraic sum of th voltag in a currnt loop is zro Physics Spcial markt ook _7 CSEin 4 1/1/11 1::9 PM

5 Physics (CSE 7) 5 F 1 C 4 Ω Ω 3 8 V Ω y consiring th loop CD 4 V 4 Ω+ Ω= 4 V+ 8 V 3 or + = 1 (1) 3 y consiring th loop CDEF Ω+ Ω= 8 V 1 or 1+ = 4 () E D n th cas of smiconuctors, th numbr nsity (numbr of lctrons or hols) incrass with incras in tmpratur Thrfor, th rsistivity of smiconuctors crass with incras in tmpratur 17 Explain with th hlp of a labl iagram th unrlying principl an working of a stp-up transformr Why cannot such a vic b us to stp-up c voltag? O Draw a labl iagram of an ac gnrator Explain brifly its principl an working [3] Stp-up transformr: Transformr is a vic which works on th principl of mutual inuction transformr consists of two sts of coils, insulat from ach othr On of th coils, call th primary coil, has N p turns Th othr coil is call th sconary coil; it has N s turns y consiring th loop EF Ω+ 4 Ω= 4 V 1 3 y substituting () ing (4) to (1) y substituting 3 in (1) an y substituting in () + = (3) = (4) = 14 or 3 = 4 = 1-5= 7, = 4- = 4-7=-3 1 Th ngativ sign coms bcaus of th irctions of 1 an ar opposit to ach othr 16 Writ th mathmatical rlation for th rsistivity of a matrial in trms of rlaxation tim, numbr nsity an mass an charg of charg carrirs in it Explain, using this rlation, why th rsistivity of a mtal incrass an that of a smiconuctor crass with ris in tmpratur [3] Th rsistivity of th matrial ρ = m n τ whr n is th numbr of charg carrirs pr unit volum, is th charg of th lctron an t is th rlaxation tim Th rlaxation of tim of mtals crass with incras in tmpratur Thrfor, rsistivity of mtals incrass with tmpratur /C sourc Mor turns, highr voltag Currnt passing through th primary coil inucs a magntic fil aroun th sconary both th primary an th sconary ar woun on th sam iron cor Th flux u to th currnt in th primary passs through th sconary coil So if an altrnating currnt is passing through th primary, it proucs an altrnating magntic flux which links th sconary coil an inucs an mf in it Th inuc mf in th sconary coil is ε s =-N Φ s t Th sam magntic flux Φ inucs a back-mf in th primary also: ε p =-N p Φ t f th rsistanc of th transformr is vry low ε p = ν p n if only a littl currnt takn from th sconary or it is opn circuit, thn Thn ν s ε p = ν p =-N Φ s t an ν p =-N p or νs s = N νp N p Φ t Physics Spcial markt ook _7 CSEin 5 1/1/11 1::13 PM

6 6 Physics (CSE 7) stp-up transform is us to incras th voltag Thrfor, it has mor numbr of turns in th sconary an thrfor, th output voltag is, N νs = νp N Dirct currnt os not prouc a tim varying flux link with th sconary coil, Thrfor, a transformr cannot us to stp-up c voltags O s p Thrfor, ε = ε sin ωt Th inuc magntic fil bcoms maximum whn th coil is prpnicular to th magntic fil 18 Givn blow ar two lctric circuits an X L =3 X L =3 X C = (a) (b) Coil xl Calculat th ratio of powr factor of th circuit to th powr factor of circuit [3] N S For th circuit, th phas angl is φ = -1 XL -1 3 tan tan = = Slip rings Carbon brushs ltrnating mf Th vic us to gnrat ac voltag (or currnt) using th principl of lctromagntic inuction is call C gnrator n C gnrator consists of a mtallic coil call armatur rotating btwn th pols of th fil magnt, as shown in th figur Th axis of rotation of th coil is prpnicular to th irction of th magntic fil Th ns of th armatur coil ar wl to two mtallic rings call slip rings which rotat along with th armatur Two carbon brushs rub against th slip rings an th wirs which carry th powr to th xtrnal circuit ar connct to th carbon brushs s th armatur rotats in th magntic fil, th flux link to th coil changs an hnc inuc currnt is gnrat in th coil Whn th coil is rotat with constant angular vlocity w, th angl q btwn th magntic fil vctor an th ara vctor is θ= ωt( assumingθ = at t = ) Thrfor, th flux at tim t is Φ( t) = cosθ= cos ωt Thrfor, th mf inuc in th coil is ε =- N Φ = Nωsin ωt t Th maximum mf inuc is ε = N ω Thrfor th powr factor cos f = 3163 Th phas angl for th circuit is -1 XL - XC -1 φ = tan = tan = Thrfor th powr factor is cosφ = cos = 447 Thrfor ratio of th powr factor of th circuit to that of is cos φ 447 = = cos φ Dfin th trm rsolving powr of an astronomical tlscop How os it gt affct on (a) incrasing th aprtur of th objctiv lns? (b) incrasing th wavlngth of th light us? Justify your answr in ach cas [3] Th rsolving powr of a tlscop is th ability to istinguish btwn two objcts sparat by a small angular istanc a a λ D = ( ) 5 Thrfor, incrasing th aprtur siz of th tlscop incrass th rsolving powr of th tlscop lso, incrasing th wavlngth us crass th rsolving powr of th tlscop Writ any four charactristics of lctromagntic wavs Giv two uss ach of (a) aiowavs (b) Microwavs [3] Physics Spcial markt ook _7 CSEin 6 1/1/11 1::16 PM

7 Physics (CSE 7) 7 Elctromagntic wavs ar transvrs wavs consisting of oscillating lctric an magntic fils prpnicular to ach othr an prpnicular to th irction of propagation n lctromagntic wav has following charactristics: (a) mplitu Th amplitu of an lctromagntic wav is th amplitu of th lctric fil Th intnsity of th lctromagntic wav is proportional to th squar of th amplitu of th wav (b) Wavlngth Lik all othr wavs, th m wavs too hav a wll fin wavlngth Th wavlngths of lctromagntic wavs go from xtrmly long to xtrmly short an vrything in btwn Th wavlngths trmin how mattr rspons to th lctromagntic wav, an thos charactristics trmin th nam w giv that particular group of wavlngths (c) Vlocity Th m wav has a wll fin vlocity an th vlocity of light (vlocity of m wav) in vacuum is a univrsal constant not by th lttr c Vlocity of th m wav in any mium is givn by v = 1 εm whr is th prmittivity an m is th prmability of th mium n in vacuum () Frquncy c = = 3 1 ms 1 εm Th frquncy of any wavform quals th vlocity ivi by th wavlngth Th units of masurmnt ar in cycls pr scon or Hrtz Uss of raio wavs (1) aio wavs ar us to transmit raio an tlvision programs Cllular phons us raio wavs to transmit voic communication in th ultrahigh frquncy (UHF) ban () Uss of microwavs Microwavs ar us in raar systms Microwavs ar us in microwav ovns to cook foo 1 n a plot of photolctric currnt vrsus ano potntial, how os: (a) th saturation currnt vary with ano potntial for incint raiations of iffrnt frquncis but sam intnsity? (b) th stopping potntial vary for incint raiations of iffrnt intnsitis but sam frquncy? (c) photolctric currnt vary for iffrnt intnsitis but sam frquncy of incint raiations? Justify your answr in ach cas [3] (a) v 3 > v > v 1 v 3 Photolctric currnt v v 1 Saturation currnt V 3 V V 1 Collctor plat potntial taring potntial Variation of photolctric currnt with collctor plat potntial for iffrnt frquncis of incint raiation Th saturation currnt is a function of incint intnsity only Thrfor, th saturation currnt for th sam intnsity rmains th sam irrspctiv of th incint frquncy (b) Th stopping potntial is a function of incint frquncy So th stopping potntial rmains constant irrspctiv of th incint intnsity of th light Stopping potntial V taring potntial Photocurrnt 3 > > Collctor plat potntial Variation of photocurrnt with collctor plat potntial for iffrnt intnsity of incint raiation (c) Photolctric currnt is a function of th intnsity or th numbr of photons incint on th photolctric mtal Th photolctric currnt incrass linarly with incras in th intnsity of th light incint on th mtal Photolctric currnt ntnsity of light Variation of photolctric currnt with intnsity of light Physics Spcial markt ook _7 CSEin 7 1/1/11 1::17 PM

8 8 Physics (CSE 7) Calculat th amount of nrgy rlas uring th a-cay of 9 U 9 Th+ H Givn : 38 (a) atomic mass of 9 U= u 34 (b) atomic mass of 9 Th = u 4 (c) atomic mass of H = 4 6 u 1u= 931 5MV/c s this cay spontanous? Giv rason [3] Th nrgy mitt in th raction is E = mc m= M - M + M ( ) U Th H = ( ) = 456 Or th nrgy rlas is E = m MV/c E = 4 471Mv Th nrgy rlas in this procss is positiv, thrfor th procss is a spontanous procss 3 What is a igital signal? Explain th function of mom in ata communication Writ two avantags of igital communication [3] Digital signals ar thos which can tak only iscrt stpwis valus Morn igital signals consist of only two valus: zro an iscrt on Thy ar rprsnt by squar signals nformation sourc Puls ris Mssag signal Puls uration Puls fall Communication systm Transmitt signal Transmittr Channl civr Nois civ signal Puls amplitu Mssag signal lock iagram of a gnraliz communication systm Usr of information communication systm is th mans of transmitting information btwn two prsons or instrumnts(far or nar), on which is call transmittr which transmits ata an th othr is call rcivr which (or) rcivs ata Th physical mium us fors communication is call communication channl Dpning upon th typ of communication systm, a channl may b in th form of wirs or cabls conncting th transmittr an th rcivr or it may b wirlss Th purpos of th transmittr is to convrt th mssag signal prouc by th sourc of information into a form suitabl for transmission through th channl f th output of th information sourc is a non-lctrical signal lik a voic signal, a transucr convrts it to lctrical form bfor giving it as an input to th transmittr Whn a transmitt signal propagats along th channl it may gt istort u to channl imprfction Morovr, nois as to th transmitt signal an th rcivr rcivs a corrupt vrsion of th transmitt signal Th rcivr has th task of oprating on th rciv signal t rconstructs a rcognizabl form of th original mssag signal for livring it to th usr of information vantags of igital communication: (1) liabl communication; lss snsitivity to changs in nvironmntal conitions (tmpratur, tc) () Easy procssing lik ncryption an comprssion 4 Explain, with th hlp of a schmatic iagram, th principl an working of a Light Emitting Dio What critrion is kpt in min whil choosing th smiconuctor matrial for such a vic? Writ any two avantags of Light Emitting Dios ovr convntional incanscnt lamps [3] p njct lctrons n njct hols Th light mitting io is a havily op p-n junction io which unr forwar bias mits spontanous raiation Th io is ncapsulat with a transparnt covr so that mitt light can com out Whn th io is forwar bias th hols in th p-op rgion mov towar th n-rgion an lctrons in th n-op rgion mov towar th p-rgion t th pltion rgion thy rcombin an mit raiation Photons with nrgy slightly lss than or qual to th ban gap nrgy of th matrial ar mitt in th procss Thrfor, th color of th light mitt by th LED irctly pns upon th ban gap nrgy Thrfor, th smiconuctor us for th LED fabrication must hav a nrgy ban gap of at last 18 V (spctral rang of visibl light is from about 4 mm to 7 mm, i, from about 3 V to 18 V) LEDs hav th following avantags ovr convntional incanscnt lamps: (i) Low oprational voltag an lss powr (ii) Fast action an no warm-up tim rquir (iii) Th banwith of mitt light is 1 Å to 5 Å or in othr wors, it is narly (but not xactly) monochromatic Physics Spcial markt ook _7 CSEin 8 1/1/11 1::19 PM

9 Physics (CSE 7) 9 (iv) Long lif an ruggnss (v) Fast on-off switching capability 5 Draw a labl iagram of a moving coil galvanomtr Stat th principl on which it works Duc an xprssion for th torqu acting on a rctangular currntcarrying loop kpt in a uniform magntic fil Writ two factors on which th currnt snsitivity of a moving coil galvanomtr pn O Stat iot Savart law Us it to riv an xprssion for th magntic fil at th cntr of a circular loop of raius carrying a stay currnt Sktch th magntic fil lins for such a currnt-carrying loop [5] Torqu acting on a rctangular currnt carrying coil Pointr N Sp Coil Pivot Soft-iron cor Uniform raial magntic fil S Prmannt magnt N m q Lt th angl btwn th magntic fil an normal to th coil b q Thn th forcs on th arms C an D ar qual, opposit, an act along th axis of th coil, which conncts th cntrs of mass of C an D ing collinar along th axis thy cancl ach othr, rsulting in no nt forc or torqu Th forcs on arms an CD ar F 1 an F Thy too ar qual an opposit, with magnitus D F1= F= b ut thy ar not collinar an thus rsult in a coupl S f a currnt carrying loop is plac in a uniform magntic fil, a torqu acts on th loop which is proportional to th currnt in th loop This is th working principl of a moving coil galvanomtr Th galvanomtr consists of a coil, with many turns, fr to rotat about a fix axis, in a uniform raial magntic fil Thr is a cylinrical soft iron cor which not only maks th fil raial but also incrass th strngth of th magntic fil Whn a currnt flows through th coil, a torqu acts on it This torqu is givn by τ = N whr N is th numbr of turns of th coil, is th ara of th coil, is th currnt passing through th coil an is th magntic fil intnsity Th torqu tns to rotat th coil spring Sp provis a countr torqu kf that balancs th magntic torqu N; rsulting in a stay angular flction f F n quilibrium kf = N a/ sin q F 1 a/ a/ q m Th magnitu of th torqu on th loop is, τ = F a θ+ F a 1 sin sinθ τ = sinθ= sin θ k = N φ Th quantity in th brackt is call th galvanomtr constant Th snsitivity of th galvanomtr pns upon th numbr of turns of th coil, ara of th galvanomtr coil, pol strngth of th prmannt magnt an th spring constant O ccoring to iot Savart s law, th magnitu of th magntic fil is proportional to th currnt, th lmnt lngth l, an invrsly proportional to th squar of th istanc r ts irction is prpnicular to th plan containing l an r Physics Spcial markt ook _7 CSEin 9 1/1/11 1::1 PM

10 1 Physics (CSE 7) y substituting () in (1) l q y intgrating m = ( x + ) / 3 r m = ( x + ) i 3 / Th magntic fil lins ar m r = 3 r whr m Z l = m sinθ r is th constant of proportionality O Y l r x 1 q P x Th figur shows a circular loop carrying a stay currnt Th loop is plac in th y-z plan with its cntr at th origin O an has a raius Th x-axis is th axis of th loop W wish to calculat th magntic fil at th point P on this axis Lt x b th istanc of P from th cntr O of th loop Consir a conucting lmnt l of th loop Th magnitu of th magntic fil u to l is givn by th iot Savart law ut r = x + Thrfor, m l r = 3 r m = ( x + ) n th irction of th is as shown in th figur gain X (1) cos θ = () ( x + ) 1 / 6 What ar cohrnt sourcs? Why ar cohrnt sourcs rquir to prouc intrfrnc of light? Giv an xampl of intrfrnc of light in vryay lif n Young s oubl slit xprimnt, th two slits ar 3 cm apart an th scrn is plac at a istanc of 15 m away from th slits Th istanc btwn th cntral bright fring an fourth bright fring is 1 cm Calculat th wavlngth of light us O Stat th conition unr which th phnomnon of iffraction of light taks plac Driv an xprssion for th with of th cntral maximum u to iffraction of light at a singl slit slit of with a is illuminat by a monochromatic light of wavlngth 7 nm at normal incinc Calculat th valu for a position of (a) first minimum at an angl of iffraction of 3 (b) first maximum at an angl of iffraction of 3 [5] Two wavfronts which hav a constant phas rlationship constitut a cohrnt sourc of light Sinc th intrfrnc pattrns ar visibl only whn th phas iffrnc of th intrfring wavs ar constant, only cohrnt light sourcs can prouc a visibl intrfrnc pattrn Th colors apparing in th soup bubbl an oil slick in watr ar two xampls of intrfrnc of light in aily lif Th fring with of th givn pattrn is istanc btwn cntralan fourth fring β = = 4 lso Thrfor, β = λd 1 λ = β = D O = m 1 cm 4 Physics Spcial markt ook _7 CSEin 1 1/1/11 1::5 PM

11 Physics (CSE 7) 11 Diffraction occurs whn a obstacl is plac in th path of th bam an it is prominnt if th siz of th objct is comparabl to that of th wavlngth of th light incint on it Lt a sourc of monochromatic light b incint on a slit of finit with a, as shown in figur n iffraction of Fraunhofr typ, all rays passing through th slit ar approximatly paralll n aition, ach portion of th slit will act as a sourc of light wavs accoring to Huygns principl For simplicity w ivi th slit into two halvs t th first minimum, ach ray from th uppr half will b xactly 18 out of phas with a corrsponing ray from th lowr half a/ a/ q a sinq Th conition for th first minimum is or a sin θ = λ λ sin θ = a pplying th sam rasoning to th wavfronts from four qually spac points a istanc apart, th path iffrnc woul b δ = asin θ/4 an th conition for structiv intrfrnc is λ sinθ = a Th argumnt can b gnraliz to show that structiv intrfrnc will occur whn asinθ= mλ( forstructivintrfrnc) a q L sin 1 q O l/a l/a l/a l/a Th intnsity istribution of th iffraction pattrn is as givn in th figur W hav mλ a = sin θ Thrfor, at 3 an at a = sin 3-9 = 14 mm a = 3 5 = 137 mm sin 7 Stat th principl of working of p-n io as a rctifir Explain, with th hlp of a circuit iagram, th us of p-n io as a full wav rctifir Draw a sktch of th input an output wavforms O Draw th symbolic rprsntation of a (i) p-n-p, (ii) n-p-n transistor Why is th bas rgion of transistor thin an lightly op? With propr circuit iagram, show th biasing of a p-n-p transistor in common bas configuration Explain th movmnt of charg carrirs through iffrnt parts of th transistor in such a configuration an show that E = C + [5] Whn a p-n junction io is rvrs bias, th rsistanc offr by th io bcom vry larg Thrfor, most of th voltag rops across th io an no currnt passs through it Similarly whn a p-n junction io is forwar bias th rsistanc offr by th io bcoms minimum an so currnt passs through th circuit So if w apply an ac voltag to a p-n junction io uring th positiv half cycl, th io bcoms forwar bias an uring ngativ half cycl th io bcoms rvrs bias Thrfor, currnt passs through th circuit only uring th positiv half cycl C in + Full wav rctifir T + n C D 1 D DC out E F Out whr T transformr, D 1, D ios, -loa rsistanc During th positiv cycl of th ac th trminal of th transformr bcoms positiv an bcoms ngativ Thrfor, th io D 1 is forwar bias an D rvrs Physics Spcial markt ook _7 CSEin 11 1/1/11 1::8 PM

12 1 Physics (CSE 7) Hnc th currnt passs through th path EFC During th ngativ half cycl of th C, Dio D 1 bcoms positiv bias an D bcoms ngativ bias Thn th currnt flows along th path EFC During both th half cycls th currnt flow through th loa rsistanc is along th sam irction Th bas rgion of th transistor is ma thin to ruc th carrir rcombination bfor raching th collctor-bas junction, so that most of th mittr currnt passs through th collctor E C (a) Wavform at (i) t V EE iasing in p-n-p transistors V CC Wavform at Output wavform (across L ) Du to D 1 (ii) Du to D O (b) (c) Du to D 1 Du to D t t For biasing a p-n-p transistor th mittr junction is connct to +v voltag V EE an th collctor trminal to th ngativ of th cll with voltag V cc n this configuration th mittr-bas junction is forwar bias an collctor-bas junction is rvrs bias Sinc collctor potntial is lowr than th bas potntial most of th hols jct from th mittr to th bas pass to th collctor rgion an contribut to th collctor currnt y pplying th Kirchhoff s currnt law at th junction currnt ntring th junction = Currnt laving th junction E p n p C (i) Emittr Collctor E = c + (ii) Emittr as p-n-p transistor Collctor as n-p-n transistor Physics Spcial markt ook _7 CSEin 1 1/1/11 1::9 PM