Unit 7 Charg-to-ass ratio of th lctron Kywords: J. J. Thoson, Lorntz Forc, Magntic Filds Objctiv: Obsrv th rsults of lctron ba influncd by th agntic fild and calculat th charg-to-ass ratio of th lctron. Apparatus: apparatus (th xtrnal for of th Modl J433- Lorntz Forc Donstrator), powr supply(0~300v;0~0v;6v), connctor. Principl: Whn th cathod is hatd, it shoots rays at th scrn in th back of anod. If a U-shapd agnt ovs across th tub, th light-spot will dflct up and down. J.J. Thoson had don th xprint in 1897. His ost faous achivnt was th discovry of th 'lctron' at that ti. H not only did th rsarch about th charactrs of rays fro th cathod but also discussd its quantity. At th sa ti, J.J. Thoson also obtaind th charg-to-ass ratio of th lctron. J.J. Thoson's xprint and subsqunt thoris won hi th Nobl Priz in physics in 1906. Bsids, Millikan asurd th lntary charg in 1909. Bcaus of thir distributions, th ass of lctron is asily calculatd and th charactrs of lctron ar uch or obvious. As w known, whn a charg "q" ovs with a vlocity "v" vrtically to th agntic v fild, it is influncd by a forc "F", which is v v F = qv B (1) According to th "right-hand rul", th cntriptal forc "F" is prpndicular to "v" as a constant. Th charg is oving in th way of unifor otion. As shown in figur 1, th dirction of agntic fild is out prpndicular to th papr. 7-1
Fig. 1 Suppos that th ass of lctron is "", and by unifor otion w can gt: () Whr "a" is acclration "r" is th radius of circl Cobin (1) and (), w can gt th quation: v r = Bv (3) (Considr scalar only, not vctor) Th nrgy of lctron fro an lctronic gun is: V = 1 v (4) Whr "V" is th potntial fro nd to nd of th lctronic gun Substitut (4) into (3), w can gt th charg-to-ass ratio of th lctron: V = B r (5) Th coil of agntic fild w us in this xprint is Hlhotzs Coil. Whn ths two coils ar st sid by sid and th distanc btwn th is "R" (As shown in figur ), thy gnrat a narly unifor agntic fild to th cntr point C. Th agntic fild parallls th axis of two circular cntrs. According to th following quation, th agnitud of agntic fild is: Ni B = 8 µ 0 (6) 15R 7 µ 0 = 4π 10 Wbr/ap. N: th nubrs of coils ar 145 (on sid) i: currnt R: th radius of agntizing coil is 14 c Fro (5) (6): 15RV = 3µ N i r 0 (7) 7-
Thrfor, w can obtain th ratio by substituting th nubrs of Hlhotzs Coils, radius of coil, th radius of charg in unifor otion, th potntial fro nd to nd of th lctronic gun, and th currnt of coils into quation (7). Instructions: ( I ) Obsrv th lctron ba incidnt vrtically to th agntic fild 1. Th connction is shown in figur 4.. Turn th powr on. Bfor opration, b sur that ach switch is adjustd to iniu. St th dflctor voltag adjust to OFF. St th currnt dirction in th agntizing coil to CLOCKWISE. Turn th acclrator voltag adjust knob clockwis to incras th voltag and you ay s a rctilinar orbit of th oving lctrons. 3. Kp th acclrator lctrod voltag at 150V. Vary th aount of currnt for totally 10 tis and rpat th procdur to find ach radius of circular trac. (Th rang is fro 0.9A to 1.4A) Masur ach radius of circl and do th plot vrsus currnt. 4. Kp th agntizing currnt as constant. Vary th aount of th acclrator lctrod voltag for totally 10 tis and rpat th procdur to find ach radius of circular trac. (Th rang of th acclrator lctrod voltag is fro 150V to 300V) Masur ach radius of circl and do th plot vrsus voltag. 5. Calculat th charg-to-ass ratio of th lctron ( ) and th avrag of th. ( II ) Obsrv th lctron ba incidnt vrtically to th agntic fild 1. Discuss th rlation btwn th pointr point out with an angl and th radius of th trac: Vary th aount of pointr point out angl for totally 7 tis and asur ach radius of circl.. Do th plot of radius to th pointr point out angl θ and th plot vrsus sinθ. Qustions: Q1: What is th thory aount of th ratio? Q: How will th lctron ba b whn you adjust th acclrator lctrod voltag? Q3:How will th lctron ba b whn you adjust th agnitud and th 7-3
dirction of th agntizing currnt? Q4: Why can you s th lctron ba? Is th glass containr vacuous? Q5: Is th trac of oving lctrons influncd by trrstrial agntis? Rark: 1. Whn you plug in th connctor, b carful of th pol.. Th tub of Lorntz Forc is a short lifti lctron dvic. In ordr to xtnd th lifti, b sur to st th acclration voltag adjust to iniu bfor opration. Aftr pr-waring for 5 inuts, turning th acclrator voltag to 100V~00V and th orbit of th oving lctrons can b sn. 3. Whn xprint is finishd, turn th acclrator voltag to zro, thn turn th powr switch to "OFF". 4. Ti of th xprint ay not xcd on hour in ordr to xtnd th lifti. Fig. th connction of apparatus S 1 :5 V S :0~300 V S 3 :0~0 V th powr switch of lap (in ordr to hat th filant) th powr switch of acclrator voltag (in ordr to acclrat th lctrons) th powr switch of agntic filds 7-4
Fig. 3 apparatus of Currnt of th agntic coil Acclrator voltag Fig 4. Th connction of apparatus 7-5