week 5 notes
U of L
Popular in Fundamentals of Physics II
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This 4 page Class Notes was uploaded by Mary-elizabeth Notetaker on Friday September 23, 2016. The Class Notes belongs to Phys 222 at University of Louisville taught by Dr. Ming Yu in Fall 2016. Since its upload, it has received 3 views. For similar materials see Fundamentals of Physics II in Physics at University of Louisville.
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Date Created: 09/23/16
Week 5 Tuesday, September 20, 2016 11:13 AM Potentials and charged conductors? ○ Vsurface=? Wab=- PE All points on surface of charged conductor in electrostatic eq are at same potential Electric potential constant everywhere on surface and equal to inside value Eqiupotential surfaces- surface on which all points are at same potential V=Vb-Va=0 Wab= - PE=-q V=0 No work needed to move chrge at constant speed on equipot surface Electric field is perpendicular to equipot surface Equipot & EF lines Equipot- family of spheres centered on point charge Potential dec along direction of EF lines EF lines perpendicular to E pot at all points …dipole If has one pos and one neg,point closer to positive charge has higher potential and it decreases along EF line as you go towards the negative charge …parallel plates If one pos and one neg, points closer to pos have higher potential Electric field does 0 work along path perpendicular to plates Capacitance- quantity characterizing 2 parallel plates conductor and is indep of Q and V Capacitor- used in variety of electrical circuits Capacitance, C, of capacitor is defined as ratio of mag of the charge on either conductor(plate) to the Phys 222 Page 1 Capacitance, C, of capacitor is defined as ratio of mag of the charge on either conductor(plate) to the mag of the pot diff btwn the 2 plates Larger area can store more charge so enhances capacitance Small plate separation can dec pot diff and will enhance capacitance Ex) 2 parallel plates w area A separated by d.. Equal but opp charge(no matter plate size) Battery being connected pulls charge off one plate and transfers to another Transfer stops when ..capacitance of device depends on geometric arrangement of conductors If separated by air: Increasing voltage across 2 plates does what? C=(Q/ V) increases charge(Q=C V) Inc separation of plates? Dec capacity Capacitors in parallel When capacitors connected in circuit, electrons transferred from left(+..high potential) plates through battery to right plate(-..low pot), so L plate + and right plate - Capacitors reach max charge when charge flow ceases , why? Flow stops when V across capacitor = that of battery Total charge(Q)= sum of charges on capacitors: Q=Q1+Q2 Capacitors can be replaced w one capacitor w capacitance of Ceq (Ceq=C1+C2) Equivalent capacitor must have exactly the same external effects on circuits as the original capacitors Ceq must be larger than C1 and C2 Capacitors in serier As neg charge accumulates on plates of one group, equivalent amt neg charge is leaving plates of the other group(left w excess pos charge) ..mag of charge must be same on left plate(outer) of C1(group left w pos charge) and right plate(outer) of C2(group accumulating neg charge) For charges on inside plates…: Internal plates connected to each other but not battery so they are electrically neutral. Charges are induced by outside plates in series …Overall, all right plates gain -Q charge and all left plates gain +Q charge Equivalent capacitor can be found that performs same fx as series combo does Pot diffs add up to battery V ..equivalent capacitance of series combo is always less than any individual capacitor in series To combine capacitors: 2 or more capacitors connected in parallel, pot diff across them are same (V1=V2) Charge on each capacitor is proportional to its capacitance Capacitors add directly to give equivalent capacitance 2 or more unequal capacitors connected in series, carry same Q1=Q2 charge but pot diffs across them are not equal Capacitances add as reciprocals and the equivalent capacitance is always less than smallest indiv capacitor Complicated circuit can be reduced to one equivalent capacitor Replace capactiors in series or parallel w their equivalent, redraw Repeat until only one equivalent capacitor To find Q charge on or pot diff across one of the capacitors: Use Ceq(final eq capacitor) and work back thru circuit reuctions When working thru circuit diagrams, use these equations: Capacitance: Phys 222 Page 2 When working thru circuit diagrams, use these equations: Capacitance: E stored in capacitor: Battery- transfers charges from one plate to another when connected Stored E is same as work req'd to move charge onto plates done by battery Total work by battery & energy stored Max capacitance, 3 capacitors must be parallel Max pot diff, 2 batteries must be in a series Dielectric- insulating material that inc capacitance when placed btwn 2 plates of a capacitor Rubber, plastic, wax paper Capacitance mult by k(dielectric constant) when dielectric completely fills region btwn 2 plates Dielectric strength-max E field 2 capacitors in series: w/out dielectrics: w/ dielectrics: 2 capacitors in parallel: w/out dielectrics: Phys 222 Page 3 w/dielectrics: Polarization- occurs when separation btwn avg positions of neg and pos charge In capacitor, dielectric becomes polarized bc it is in E field that exists btwn plates Induced polarization in dielectric field from the 2 plates …dec net field in capacitor, dec pot diff across capacitor ..more charge needed to return capacitor to original pot diff Net effect of dielectric is to inc amt of charge stored on capacitor, then inc capacitance Current- rate at which charge flows perpendicularly thru a surface Q is amt of charge that flows thru area A in time interval t Instantaneous current- limit of avg current as time interval goes to 0 Steady current- avg and instantaneous currents are same Current direction is along dir pos charges would flow =conventional current dir In common conductor, current is due to net motion of neg charged e-'s Charge carrier- moving charge(+ or -) Phys 222 Page 4
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