Physics 2102 Lecture: 08 THU 18 FEB Jonathan Dowling Physics 2102 Lecture: 08 THU 18 FEB Capacitance II 25.4–7

Capacitors in Parallel: V=Constant An ISOLATED wire is an equipotential surface: V=Constant Capacitors in parallel have SAME potential difference but NOT ALWAYS same charge! VAB = VCD = V Qtotal = Q1 + Q2 CeqV = C1V + C2V Ceq = C1 + C2 Equivalent parallel capacitance = sum of capacitances V = VAB = VA –VB V = VCD = VC –VD VA VB VC VD A B C D C1 C2 Q1 Q2 V=V Ceq Qtotal PAR-V (Parallel: V the Same)

Capacitors in Series: Q=Constant Isolated Wire: Q=Q1=Q2=Constant Q1 = Q2 = Q = Constant VAC = VAB + VBC A B C C1 C2 Q1 Q2 SERI-Q: Series Q the Same Q = Q1 = Q2 SERIES: Q is same for all capacitors Total potential difference = sum of V Ceq

Capacitors in Parallel and in Series Cpar = C1 + C2 Vpar = V1 = V2 Qpar = Q1 + Q2 C1 C2 Q1 Q2 Ceq Qeq In series : 1/Cser = 1/C1 + 1/C2 Vser = V1  + V2 Qser= Q1 = Q2 C1 C2 Q1 Q2

Example: Parallel or Series? Parallel: Circuit Splits Cleanly in Two (Constant V) What is the charge on each capacitor? C1=10 F C3=30 F C2=20 F 120V Qi = CiV V = 120V on ALL Capacitors (PAR-V) Q1 = (10 F)(120V) = 1200 C Q2 = (20 F)(120V) = 2400 C Q3 = (30 F)(120V) = 3600 C Note that: Total charge (7200 C) is shared between the 3 capacitors in the ratio C1:C2:C3 — i.e. 1:2:3

Example: Parallel or Series Series: Isolated Islands (Constant Q) What is the potential difference across each capacitor? C1=10mF C2=20mF C3=30mF Q = CserV Q is same for all capacitors (SERI-Q) Combined Cser is given by: 120V Ceq = 5.46 F (solve above equation) Q = CeqV = (5.46 F)(120V) = 655 C V1= Q/C1 = (655 C)/(10 F) = 65.5 V V2= Q/C2 = (655 C)/(20 F) = 32.75 V V3= Q/C3 = (655 C)/(30 F) = 21.8 V Note: 120V is shared in the ratio of INVERSE capacitances i.e. (1):(1/2):(1/3) (largest C gets smallest V)

Example: Series or Parallel? Neither: Circuit Compilation Needed! 10F 5F 10V In the circuit shown, what is the charge on the 10F capacitor? 10 F 10F 10V The two 5F capacitors are in parallel Replace by 10F Then, we have two 10F capacitors in series So, there is 5V across the 10 F capacitor of interest by symmetry Hence, Q = (10F )(5V) = 50C

Energy U Stored in a Capacitor Start out with uncharged capacitor Transfer small amount of charge dq from one plate to the other until charge on each plate has magnitude Q How much work was needed? dq

Energy Stored in Electric Field of Capacitor Energy stored in capacitor: U = Q2/(2C) = CV2/2 View the energy as stored in ELECTRIC FIELD For example, parallel plate capacitor: Energy DENSITY = energy/volume = u = volume = Ad General expression for any region with vacuum (or air)

Dielectric Constant C =  A/d DIELECTRIC +Q –Q If the space between capacitor plates is filled by a dielectric, the capacitance INCREASES by a factor  This is a useful, working definition for dielectric constant. Typical values of are 10–200 +Q –Q DIELECTRIC C =  A/d The  and the constant o are both called dielectric constants. The  has no units (dimensionless).

Atomic View Emol Molecules set up counter E field Emol that somewhat cancels out capacitor field Ecap. This avoids sparking (dielectric breakdown) by keeping field inside dielectric small. Hence the bigger the dielectric constant the more charge you can store on the capacitor. Ecap

Example dielectric slab Capacitor has charge Q, voltage V Battery remains connected while dielectric slab is inserted. Do the following increase, decrease or stay the same: Potential difference? Capacitance? Charge? Electric field? dielectric slab

Example dielectric slab Initial values: capacitance = C; charge = Q; potential difference = V; electric field = E; Battery remains connected V is FIXED; Vnew = V (same) Cnew = kC (increases) Qnew = (kC)V = kQ (increases). Since Vnew = V, Enew = V/d=E (same) dielectric slab Energy stored? u=e0E2/2 => u=ke0E2/2 = eE2/2

Summary Any two charged conductors form a capacitor. Capacitance : C= Q/V Simple Capacitors: Parallel plates: C = e0 A/d Spherical : C = 4 e0 ab/(b-a) Cylindrical: C = 2 e0 L/ln(b/a) Capacitors in series: same charge, not necessarily equal potential; equivalent capacitance 1/Ceq=1/C1+1/C2+… Capacitors in parallel: same potential; not necessarily same charge; equivalent capacitance Ceq=C1+C2+… Energy in a capacitor: U=Q2/2C=CV2/2; energy density u=0E2/2 Capacitor with a dielectric: capacitance increases C’=kC