PHYS ,Fall 05, Term Exam #1, Oct., 12, 2005

Transcription

1 PHYS1444-,Fall 5, Trm Exam #1, Oct., 1, 5 Nam: Kys 1. circular ring of charg of raius an a total charg Q lis in th x-y plan with its cntr at th origin. small positiv tst charg q is plac at th origin. What is th irction of th forc on q, if Q is ngativ? a. is raially outwar towar th ring b. is raially inwar from th ring c. is in th k irction. has no irction sinc th forc is zro. Thr qual chargs ar locat in th x-y plan with coorinats (, m), (4m, m), an (4m, ). Which on of th following vctor has th irction th sam as that of th forc on th charg locat at (4m, m)? a. i + 4j b. 4i + j c. 9i + 16j. 16i + 9j. What is th irction of th lctric fil halfway btwn an lctron an a proton? a. towar th lctron b. towar th proton c. prpnicular to th lin from th lctron to th proton. unfin sinc th lctric fil is zro 4. Thr point chargs ach qual to 1 µc ar locat at x 1m, x m, an x m, rspctivly, on th x-axis. What is th magnitu of th lctric fil at th origin? a. c..5 1 N C b N C N C N C 5. What is th charg pr unit lngth, if a charg of 8.4 µc is vnly istribut aroun a ring of raius.1 m? a. 7 µc /m b. 1.6 µc /m c. 4. µc /m.. µc /m 6. n amount of charg Q is uniformly istribut ovr a sphrical surfac of raius. What is th magnitu of th lctric fil at th cntr of th sphr? a. zro b. Q c. Q 8πε. this is not possibl to trmin 7. If th arth's lctric fil is 1 N/C ownwar, what must b th charg on a 1 kg objct so that it woul b "wightlss" ( 5 points)? In orr for th objct to b wightlss all th forcs xrting on th objct must b balanc. Thus, th gravitational forc that pulls th objct ownwar shoul b balanc by th lctric forc btwn th Earth an th charg on th objct. F Mg 1kg 9.8 m s 9.8 N; F qe q1 N C g F F 9.8N 1 q; q C g Sinc th Earth s lctric fil is pointing ownwar, th Earth s lctric charg must b ngativ. In orr to gnrat a forc that balancs th gravitational forc, th lctric forc must b rpulsiv. Thus, th objct s charg must b th sam as th Earth s, ngativ. q C

2 PHYS1444-,Fall 5, Trm Exam #1, Oct., 1, 5 8. If th lctric flux through a rctangular ara is 5. Nm/C, an th lctric fil is thn oubl, what is th flux through th ara? a..5 Nm/C b. 5. Nm/C c. 1 Nm/C. Nm/C 9. If a charg is locat at th cntr of a sphrical volum an th lctric flux through th surfac of th sphr is Φ, what is th flux through th surfac if th raius of th sphr oubls? a..15 Φ b..5 Φ c. Φ. 8 Φ 1. n oxygn ion, O++, is nclos in a gaussian surfac. What is th flux laving that surfac? 8 a.. 1 Nm C b..6 1 Nm C 1 c Nm C. Nm C 11. Th lctric fil at all points on a clos surfac is 5 N/C outwars. If th ara of th surfac is 6. m, what is th nt charg nclos insi th surfac? a. 14 C b. 6 C c. 1.6 nc. 1 C 1. long straight lin of charg has a uniform positiv charg pr unit lngth λ. Th lin is partially nclos in a long rctangular box of lngth L an ns of ara, th lin running through th cntr of ach n. What is th lctric flux through th surfac of th box? (5 points) Q Φ E ; Q λ L ε Q λ L Φ E ε ε 1. Two qual chargs Q ar sparat by a istanc. On of th chargs is rlas an movs away from th othr u to th forc btwn thm. Whn th moving charg is a istanc from th othr charg, what is its kintic nrgy? (5 points) Q Q U ; U Q KE U U U

3 PHYS1444-,Fall 5, Trm Exam #1, Oct., 1, If th potntial at a istanc from a ipol is Vo for istancs larg compar to th lngth of th ipol, thn what is th potntial at twic th istanc from th ipol? a. also Vo b. Vo/ c. Vo/4. Vo/8 15. If th potntial is givn by V xy - z-, what is th lctric fil has a y- componnt? a. x b. y c. x + y. x + y - 6z- 16. If a molcul of oxygn in air has a translational kintic nrgy of 1/4 V, what is its sp? Mass of an oxygn molcul is.u. (5 points) E V J KE mv v 88 m / s kg 17. What is th unit of capacitanc, th fara, quivalnt to? a. V/m b. C/V c. V/C. J/C 18. paralll plat capacitor has plats of ara.4 m an plat sparation of. mm. Th capacitor is connct to a 9. V battry. What is th capacitanc of th capacitor? a. 18 nf b. mf c. 18 µf. 18 µf 19. Paralll plat capacitor #1 has twic th plat ara an twic th plat sparation of capacitor #. What is th capacitanc of th scon capacitor? a. th sam as th first b. oubl th first c. quarupl th first. ight tims th first. If th polarity of th voltag appli to a 4. nf cylinrical capacitor is rvrs, what is th capacitanc? a..5 nf b. 4. nf c. -.5 nf. -4. nf 1. 1 pf capacitor is connct in paralll with a pf capacitor, th paralll combination thn bing connct in sris with a pf capacitor. What is th rsulting quivalnt capacitanc? a. 1.5 pf b. pf c. 4.5 pf. 6 pf

4 PHYS1444-,Fall 5, Trm Exam #1, Oct., 1, 5. If th voltag on a capacitor is oubl, what happns to th nrgy stor by th capacitor? a. stays th sam b. oubls c. quarupls. halvs. Thr 4 mf capacitors ar connct in sris across a 1 V battry. What is th total nrgy stor by this combination of capacitors? a. 4 mj b. 96 mj c. 88 mj. 178 mj 4. If a ilctric of ilctric constant 4 is insrt btwn th plats of a paralll plat capacitor filling th gap, what happns to th capacitanc? a. is unaffct b. incrass by a factor of 4 c. crass by a factor of 4. incrass by a factor of If a 1 currnt flows for two minuts, how many lctrons hav pass any point in th circuit (5 points)? Q 1 C s min 1 C s 1 sc 1. 1 C Q 1. 1 C 1 N C 6. Which way os th lctric currnt flow through a rsistor? a. from high potntial to low potntial b. from low potntial to high potntial c. can flow from any potntial to any iffrnt potntial. with no irction sinc it is not a vctor 7. Th tmpratur cofficint, α, of rsistivity for coppr is.68 o C-1. If a coppr wir has a rsistanc of 14 Ω at o C, what is its rsistanc at 8 o C? a. 4 Ω b. 71 Ω c. 146 Ω. 161 Ω 8. 1 watt toastr is connct to a 1-V C lin. What is th avrag powr issipat? a. W b. 1 W c. 17 W. 4 W 9. Coppr has on fr lctron pr atom, a nsity of kg m, an an atomic mass of 6.5 u. How many fr lctrons ar thr in a cubic cntimtr of coppr? a b c t vry low tmpraturs th rsistivity of som matrials bcoms zro as inicat by high prcision masurmnts. What is this phnomnon known as? a. suprrsistivity b. suprarsistivity c. suprconuctivity. BSCCO

5 PHYS1444-,Fall 5, Trm Exam #1, Oct., 1, 5 Usful formula an constants qq 1 Magnitu of th lctric forc btwn two chargs: F r Q Magnitu of th lctric fil gnrat by a charg Q: E r U Elctric potntial: V whr U is th lctric potntial nrgy. q Q Gauss law: Φ E ε b lationship btwn lctric potntial an th lctric fil: V E l or V V V Ex ; Ey ; Ez ; x y z Charg stor in a capacitor with capacitanc C u to potntial iffrnc V is: Q CV Capacitanc of a paralll plat capacitor is: C ε 1 Enrgy stor in a capacitor: U QV Ohm s Law: V I V Elctric Powr: P IV I vrag powr of a rsistanc in an C circuit: P IrmsVrms l sistanc: ρ, ρ th rsistivity, th cross sctional ara an L th lngth. Tmpratur pnnc of a conuctor rsistivity: ρt ρ 1 + α( T T) 1 Prmittivity of fr spac: ε C N m Charg of an lctron: q C -1 vogaro s numbr : N 6. 1 mol 7 tomic mass unit (1u): 1u kg 8 Sp of light: c 1 m s ba a