SPH4U Electric Charges and Electric Fields Mr. LoRusso

Transcription

1 SPH4U lctric Chargs an lctric Fils Mr. LoRusso lctricity is th flow of lctric charg. Th Grks first obsrv lctrical forcs whn arly scintists rubb ambr with fur. Th notic thy coul attract small bits of straw with th ambr. Thy call this th ambr ffct. Th Grk wor for ambr is lktron an th ffct was call lktrics, hnc lctricity. Rnaissanc prio It was iscovr that two objcts rubb by an intical thir matrial woul caus th two to xprinc a rpulsiv forc. Convrsly, if th two objcts wr rubb by two iffrnt matrials, an attractiv forc coul b crat. This is whn th concpt of two charg typs, positiv an ngativ, cam about. Bn Franklin: was crit to naming chargs positiv an ngativ. H bliv lctricity was a flui. Thrfor, if a pic of ambr was rubb by fur, th fur woul absorb this lctric flui, causing th ambr to hav a ficit of lctric flui, an hnc a ngativ charg. His assumption turn out to b incorrct. In rality, lctrons ar posit on to th ambr from th fur. It is this rason by lctrons wr givn a ngativ charg, simply bcaus of an incorrct scintific assumption. An accint of history that has ha a prmannt impact on morn scinc an tchnology. Funamntal Law of Chargs Opposit chargs attract Similar chargs rpl Charg objcts attract som nutral objcts Basic structurs of mattr an thir chargs 19 lctrons ( ) ngativ charg C or 19 Protons ( p ) positiv charg C or 1 Nutrons ( n ) nutral charg 0C 1 Insulators: Do not allow th flow of lctric charg (Rubbr, air, most non-mtallic substancs) Conuctors: Do allow th flow of lctric charg (Coppr, aluminum, salt watr, most mtals) Coulomb s Law Charls Agustin Coulomb ( ) iscovr th rlationship btwn forc an lctric charg. H us a similar xprimnt to Nwton s for trmining th forc of gravity. k1 F 9 N m 8 N m Whr k is a constant with a valu of or an 1 an ar C masur in Coulombs (C )

2 SPH4U lctric Chargs an lctric Fils Mr. LoRusso lctric Fils lctric fils ar analogous to gravitational fils. Th forc of gravity varis pning on th mass of th falling objct yt th acclration u to gravity is constant for all objcts caught in th fil. M r m tst GMm Fg r Fg GM m r GM ag r k F F k tst k ε tst Masur of gravitational fil intnsity. Masur of lctric fil intnsity. Facts about lctric fils Th strngth of an lctric fil is inpnnt of th charg on th tst charg tst Positiv charg sourcs ( ) mit lctric fils as lins of forc, outwar from th charg. Ngativ charg sourcs (- ) mit lctric fils as lins of forc, inwar to th charg. lctric Fils ar vctor uantitis

3 SPH4U lctric Chargs an lctric Fils Mr. LoRusso lctric Fil Bhaviour an Spcial Cass Th bhaviour of lctric fil lins ar similar to that of magntic fil lins. Th abov fil lins rprsnt th lctric fil at any givn point. + r ε r r ε + ε nt - Spcial Cas Paralll Plats In th cas of th paralll plat, th fil nsity rmains constant btwn th two plats xcpt at th gs. Thrfor, F r is constant btwn th plats. Paralll plats ar us to acclrat charg particls. Th lctron gun of a tlvision an computrs monitors us paralll plat to acclrat a stram of lctrons that caus th scrn to illuminat. Q: Why ar th fil lins paralll btwn th plats? A: Symmtry. Consir th following vnly istribut ngativ charg Positiv tst charg F nt + ach plat consists of svral point chargs. Bcaus of symmtry, th nt forc will always b prpnicular to th plat vnly istribut positiv charg Th sum of F from n positiv chargs an n ngativ chargs rsult in a F that is to th plat nt

4 SPH4U lctric Chargs an lctric Fils Mr. LoRusso lctric Potntial nrgy As with gravity, thr is a rlationship btwn forc an nrgy in lctrostatics. Using an analogy argumnt btwn gravity an lctrostatics Gravity GM 1M F g R F lctrostatics k 1 Using intgration, w can us th gravity uation to rnr th nrgy uation. g F R g R GM 1M g R Sinc gravity is an attractiv forc, gravitational potntial nrgy is ngativ, implying objcts ar stuck in a potntial wll. Using intgration, w can us th gravity uation to rnr th nrgy uation. F k1 Sinc lctrostatic forc can b an attractiv or a rpulsiv forc, lctrostatic potntial nrgy can b positiv or ngativ, implying that positiv lctrostatic potntials man that 1 an ar lik chargs (rpulsiv forcs) an ngativ lctrostatic potntials man that 1 an ar unlik chargs (attractiv forcs) xampl: How much work is on on a proton, if it mov from a istanc of.0m to infinity from a positiv charg sourc of C

5 SPH4U lctric Chargs an lctric Fils Mr. LoRusso Vs. Voltag In lctrostatics, th masur of lctric potntial is mor commonly us than lctric potntial nrgy. lctric potntial is fin as follows: Whr Volts, an V tst is th absolut potntial nrgy in Jouls, V is th absolut lctric potntial in tst is th charg on a positiv tst charg in Coulombs. Not: Th trm absolut implis a masurmnt rlativ to infinity. Charg sourc lctric Fil lins 1 volt of lctric potntial masur at a point in spac implis that it woul tak 1J of nrgy bring that charg from infinity to that point in spac. Howvr, masurmnts from infinity ar cumbrsom an usually of littl practical valu. In gnral it is th iffrncs in lctric potntial that ar of concrn. I.. Walking along a hallway at th 50 th floor is no mor angrous than walking along th 1 st floor, howvr falling own a flight of stairs from th 50 th to th 49 th floor is as angrous as falling from th 1 st floor to th basmnt. Consir th following A ε Moving a positiv tst charg from point A to B. Not: is inpnnt of th path at A at B B V A A V A V B B V B V b V Amount of nrgy ruir to mov a positiv tst charg ( ) from A to B A tst

6 SPH4U lctric Chargs an lctric Fils Mr. LoRusso lctrostatics an Work Work is fin as a forc through a istanc or as a chang of nrgy, or W F an W, Thrfor F. Furthrmor F F F F ε an ε ε F Spcial Cass- Th paralll plat First cas: Two charg paralll plats Th two paralll plats hav a fix amount of charg on ach. As iscuss arlir, th lctric fil btwn two paralll plats is constant no mattr whr a charg is locat btwn th plats (xcluing th gs of cours). This also implis that th fil strngth btwn th two plats is inpnnt of th plat sparation (rmmbr, th charg on ach plat is constant). Bas on th uation abov, ε, as th sparation incrass btwn th plats, th voltag must incras btwn th plats in orr to maintain a constant lctric fil. Scon cas: Two charg paralll plats, attach to a fix voltag supply In this cas th voltag is fix, thrfor accoring to th formulaε, as th istanc incrass, th lctric fil strngth must cras an as th istanc crass, th lctric fil strngth must incras. Not: th amount charg on ach plat is not fix in this cas. As th plats gt closr, th forc of lctrostatic attraction incrass, inucing mor lctrons from th powr supply to collct on th ngativ plat an mor lctrons to rc from th positiv plat to th positiv si of th powr supply.

7 SPH4U lctric Chargs an lctric Fils Mr. LoRusso Qustions: 5 1. Two point chargs,.0 10 C an C rspctivly, ar sparat by 10cm fin a) Th lctric potntial nrgy stor in th two chargs b) Th lctric forc btwn th two chargs. 5. Fin th voltag an th lctric fil crat by a C from a istanc of a).0m b) 4.0m 3. Two charg paralll plats ar sparat by a istanc of.0cm. Th voltag btwn th two plats is 500V. Fin a) Th lctric fil strngth btwn th two plats b) Assuming th plats ar no longr attach to a powr supply, fin th lctric fil strngth btwn th plats if th istanc is oubl. c) Assuming th plats ar no longr attach to a powr supply, fin th voltag btwn th plats if th istanc is oubl. 4. Two paralll plats ar attach to a 1000V powr supply. If th plats ar sparat by a istanc of 5.0cm, fin 31 a) th sp at which an lctron will arriv at th positiv plat ( m kg ) 7 m p b) th sp at which a proton woul arriv at th ngativ plat ( kg ) c) calculat a) an b) if th istanc btwn th two plats is oubl Using a paralll plat apparatus, fin th voltag ruir to balanc a kg ust particl if th ust particl has a charg of C an th plats ar sparat by 5.0cm.