Transcript Physics 2102 Spring 2002 Lecture 8

Physics 2102
Jonathan Dowling
Physics 2102
Lecture 8
Capacitors II
Capacitors in parallel and in series
• In parallel :
– Ceq = C1 + C2
– Veq=V1=V2
– Qeq=Q1+Q2
• In series :
– 1/Ceq = 1/C1 + 1/C2
– Veq=V1 +V2
– Qeq=Q1=Q2
Q1
C1
Q2
C2
Qeq
Q1
Q2
C1
C2
Ceq
Example 1
What is the charge on each capacitor?
• Q = CV; V = 120 V
• Q1 = (10 mF)(120V) = 1200 mC
• Q2 = (20 mF)(120V) = 2400 mC
• Q3 = (30 mF)(120V) = 3600 mC
Note that:
• Total charge (7200 mC) is shared
between the 3 capacitors in the ratio
C1:C2:C3 — i.e. 1:2:3
10 mF
20 mF
30 mF
120V
Example 2
What is the potential difference across each capacitor?
• Q = CV; Q is same for all capacitors 10 mF
• Combined C is given by:
1
1
1
1



Ceq (10mF ) (20mF ) (30mF )
20 mF
120V
30 mF
• Ceq = 5.46 mF
• Q = CV = (5.46 mF)(120V) = 655 mC
• V1= Q/C1 = (655 mC)/(10 mF) = 65.5 V Note: 120V is shared in the
• V2= Q/C2 = (655 mC)/(20 mF) = 32.75 V ratio of INVERSE
capacitances
• V3= Q/C3 = (655 mC)/(30 mF) = 21.8 V
i.e.1:(1/2):(1/3)
(largest C gets smallest V)
Example 3
10 mF
In the circuit shown, what
is the charge on the 10mF
capacitor?
5 mF
• The two 5mF capacitors are in
parallel
• Replace by 10mF
• Then, we have two 10mF
capacitors in series
• So, there is 5V across the 10mF
capacitor of interest
• Hence, Q = (10mF )(5V) = 50mC
5 mF
10V
10 mF
10 mF
10V
Energy 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?
Q
Q
dq
2
2
q
Q
CV
U   Vdq   dq 

C
2C
2
0
0
Energy Stored in Electric Field
• 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 =
2
Q
Q
0  Q  0E 2
Q



U



2


2CAd 2  0 A  Ad 2 0 A
2

A
2
0




2
2
2
 d 
volume = Ad
General
expression for
any region with
vacuum (or air)
Example
• 10mF capacitor is initially charged to 120V.
20mF capacitor is initially uncharged.
• Switch is closed, equilibrium is reached.
• How much energy is dissipated in the process?
10mF (C1)
Initial charge on 10mF = (10mF)(120V)= 1200mC
20mF (C2)
After switch is closed, let charges = Q1 and Q2.
Charge is conserved: Q1 + Q2 = 1200mC
• Q1 = 400mC
Q2
Also, Vfinal is same: Q1  Q2
• Q2 = 800mC
Q1 
C1 C 2
2
• Vfinal= Q1/C1 = 40 V
Initial energy stored = (1/2)C1Vinitial2 = (0.5)(10mF)(120)2 = 72mJ
Final energy stored = (1/2)(C1 + C2)Vfinal2 = (0.5)(30mF)(40)2 = 24mJ
Energy lost (dissipated) = 48mJ
Dielectric Constant
DIELECTRIC
+Q -–Q
C =  A/d
• 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
Example
• 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
• Initial values:
capacitance = C; charge = Q;
potential difference = V;
electric field = E;
• Battery remains connected
• V is FIXED; Vnew = V (same)
• Cnew = C (increases)
• Qnew = (C)V = Q (increases).
• Since Vnew = V, Enew = E (same)
dielectric
slab
Energy stored? u=0E2/2 => u=0E2/2 = E2/2
Summary
• Any two charged conductors form a capacitor.
• Capacitance : C= Q/V
• Simple Capacitors:
Parallel plates: C = 0 A/d
Spherical : C = 4p 0 ab/(b-a)
Cylindrical: C = 2p 0 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’=C