# chapter 25 .pdf

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Title: Fundamental of Physics

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Chapter 25

1. (a) The capacitance of the system is

C

q

70 pC

35

. pF.

V

20 V

(b) The capacitance is independent of q; it is still 3.5 pF.

(c) The potential difference becomes

V

q 200 pC

57 V.

C 35

. pF

2. Charge flows until the potential difference across the capacitor is the same as the

potential difference across the battery. The charge on the capacitor is then q = CV, and

this is the same as the total charge that has passed through the battery. Thus,

q = (25 10–6 F)(120 V) = 3.0 10–3 C.

3. THINK The capacitance of a parallel-plate capacitor is given by C = 0A/d, where A is

the area of each plate and d is the plate separation.

EXPRESS Since the plates are circular, the plate area is A = R2, where R is the radius

of a plate. The charge on the positive plate is given by q = CV, where V is the potential

difference across the plates.

ANALYZE (a) Substituting the values given, the capacitance is

C

0 R 2

d

8.8510

12

F m 8.2 102 m

2

3

1.310 m

1.44 1010 F 144 pF.

(b) Similarly, the charge on the plate when V = 120 V is

q = (1.44 10–10 F)(120 V) = 1.73 10–8 C = 17.3 nC.

LEARN Capacitance depends only on geometric factors, namely, the plate area and plate

separation.

4. (a) We use Eq. 25-17:

1115

1116

CHAPTER 25

C 4 0

40.0 mm 38.0 mm

ab

84.5 pF.

b a 8.99 109 Nm2 2 40.0 mm 38.0 mm

C

(b) Let the area required be A. Then C = 0A/(b – a), or

A

C b a

0

84.5pF 40.0 mm 38.0 mm 191cm2 .

8.85 10

12

C2 /N m2

5. Assuming conservation of volume, we find the radius of the combined spheres, then

use C = 40R to find the capacitance. When the drops combine, the volume is doubled. It

is then V = 2(4/3)R3. The new radius R' is given by

4p

4p

3

R 2 R3

3

3

R 21 3 R .

The new capacitance is

C 4p 0 R 4p 0 21 3 R 5.04p 0 R.

With R = 2.00 mm, we obtain C 5.04 8.851012 F m 2.00 103 m 2.80 1013 F .

6. (a) We use C = A0/d. The distance between the plates is

d

A 0 1.00 m

C

2

8.85 10

12

C2 /N m2

1.00 F

8.85 1012 m.

(b) Since d is much less than the size of an atom ( 10–10 m), this capacitor cannot be

constructed.

7. For a given potential difference V, the charge on the surface of the plate is

q Ne (nAd )e

where d is the depth from which the electrons come in the plate, and n is the density of

conduction electrons. The charge collected on the plate is related to the capacitance and

the potential difference by q CV (Eq. 25-1). Combining the two expressions leads to

C

d

ne .

A

V

With d / V ds / Vs 5.0 1014 m/V and n 8.49 1028 / m3 (see, for example, Sample

Problem 25.01 — “Charging the plates in a parallel-plate capacitor”), we obtain

1117

C

(8.49 1028 / m3 )(1.6 1019 C)(5.0 10 14 m/V) 6.79 104 F/m2 .

A

8. The equivalent capacitance is given by Ceq = q/V, where q is the total charge on all the

capacitors and V is the potential difference across any one of them. For N identical

capacitors in parallel, Ceq = NC, where C is the capacitance of one of them. Thus,

NC q / V and

q

1.00C

N

9.09 103 .

6

VC 110V 1.00 10 F

9. The charge that passes through meter A is

b

gb

g

q CeqV 3CV 3 25.0 F 4200 V 0.315 C.

10. The equivalent capacitance is

Ceq C3

b

gb

g

10.0 F 5.00 F

C1C2

4.00F

7.33 F.

C1 C2

10.0 F 5.00 F

11. The equivalent capacitance is

Ceq

C1 C2 C3 10.0 F 5.00 F 4.00 F 3.16 F.

C1 C2 C3

10.0 F 5.00 F 4.00 F

12. The two 6.0 F capacitors are in parallel and are consequently equivalent to

Ceq 12 F . Thus, the total charge stored (before the squeezing) is

qtotal CeqV 12 F (10.0V) 120 C.

(a) and (b) As a result of the squeezing, one of the capacitors is now 12 F (due to the

inverse proportionality between C and d in Eq. 25-9), which represents an increase of

6.0 F and thus a charge increase of

qtotal CeqV 6.0 F (10.0V) 60 C .

13. THINK Charge remains conserved when a fully charged capacitor is connected to an

uncharged capacitor.

EXPRESS The charge initially on the charged capacitor is given by q = C1V0, where C1

= 100 pF is the capacitance and V0 = 50 V is the initial potential difference. After the

battery is disconnected and the second capacitor wired in parallel to the first, the charge

1118

CHAPTER 25

on the first capacitor is q1 = C1V, where V = 35 V is the new potential difference. Since

charge is conserved in the process, the charge on the second capacitor is q2 = q – q1,

where C2 is the capacitance of the second capacitor.

ANALYZE Substituting C1V0 for q and C1V for q1, we obtain q2 = C1(V0 – V). The

potential difference across the second capacitor is also V, so the capacitance of the second

capacitor is

q V V

50 V 35V

C2 2 0

C1

100 pF 42.86 pF 43pF.

V

V

35V

LEARN Capacitors in parallel have the same potential difference. To verify charge

conservation explicitly, we note that the initial charge on the first capacitor is

q C1V0 (100 pF)(50 V) 5000 pC. After the connection, the charges on each capacitor

are

q1 C1V (100 pF)(35 V) 3500 pC

q2 C2V (42.86 pF)(35 V) 1500 pC.

Indeed, q = q1 + q2.

14. (a) The potential difference across C1 is V1 = 10.0 V. Thus,

q1 = C1V1 = (10.0 F)(10.0 V) = 1.00 10–4 C.

(b) Let C = 10.0 F. We first consider the three-capacitor combination consisting of C2

and its two closest neighbors, each of capacitance C. The equivalent capacitance of this

combination is

CC

Ceq C 2 1.50 C.

C C2

Also, the voltage drop across this combination is

V

CV1

CV1

0.40V1 .

C Ceq C 1.50 C

Since this voltage difference is divided equally between C2 and the one connected in

series with it, the voltage difference across C2 satisfies V2 = V/2 = V1/5. Thus

10.0V

5

q2 C2V2 10.0 F

2.00 10 C.

5

15. (a) First, the equivalent capacitance of the two 4.00 F capacitors connected in series

is given by 4.00 F/2 = 2.00 F. This combination is then connected in parallel with two

other 2.00-F capacitors (one on each side), resulting in an equivalent capacitance C =

3(2.00 F) = 6.00 F. This is now seen to be in series with another combination, which

1119

consists of the two 3.0-F capacitors connected in parallel (which are themselves

equivalent to C' = 2(3.00 F) = 6.00 F). Thus, the equivalent capacitance of the circuit

is

CC 6.00 F 6.00 F

Ceq

3.00 F.

C C

6.00 F 6.00 F

(b) Let V = 20.0 V be the potential difference supplied by the battery. Then

q = CeqV = (3.00 F)(20.0 V) = 6.00 10–5 C.

(c) The potential difference across C1 is given by

V1

6.00 F 20.0V 10.0V .

CV

C C 6.00 F 6.00 F

(d) The charge carried by C1 is q1 = C1V1= (3.00 F)(10.0 V) = 3.00 10–5 C.

(e) The potential difference across C2 is given by V2 = V – V1 = 20.0 V – 10.0 V = 10.0 V.

(f) The charge carried by C2 is q2 = C2V2 = (2.00 F)(10.0 V) = 2.00 10–5 C.

(g) Since this voltage difference V2 is divided equally between C3 and the other 4.00-F

capacitors connected in series with it, the voltage difference across C3 is given by V3 =

V2/2 = 10.0 V/2 = 5.00 V.

(h) Thus, q3 = C3V3 = (4.00 F)(5.00 V) = 2.00 10–5 C.

16. We determine each capacitance from the slope of the appropriate line in the graph.

Thus, C1 = (12 C)/(2.0 V) = 6.0 F. Similarly, C2 = 4.0 F and C3 = 2.0 F. The total

equivalent capacitance is given by

C C2 C3

1

1

1

,

1

C123 C1 C2 C3 C1 (C2 C3 )

or

C123

C1 (C2 C3 ) (6.0 F)(4.0 F 2.0 F) 36

F 3.0 F .

C1 C2 C3 6.0 F 4.0 F 2.0 F 12

This implies that the charge on capacitor 1 is q1 (3.0 F)(6.0 V) = 18 C. The voltage

across capacitor 1 is therefore V1 = (18 C)/(6.0 F) = 3.0 V. From the discussion in

section 25-4, we conclude that the voltage across capacitor 2 must be 6.0 V – 3.0 V = 3.0

V. Consequently, the charge on capacitor 2 is (4.0 F)(3.0 V) = 12 C.

17. (a) and (b) The original potential difference V1 across C1 is

1120

CHAPTER 25

V1

CeqV

C1 C2

3.16 F100.0 V 21.1V.

10.0 F 5.00 F

Thus V1 = 100.0 V – 21.1 V = 78.9 V and

q1 = C1V1 = (10.0 F)(78.9 V) = 7.89 10–4 C.

18. We note that the voltage across C3 is V3 = (12 V – 2 V – 5 V) = 5 V. Thus, its charge

is q3 = C3 V3 = 4 C.

(a) Therefore, since C1, C2 and C3 are in series (so they have the same charge), then

4 C

C1 = 2 V = 2.0 F .

(b) Similarly, C2 = 4/5 = 0.80 F.

19. (a) and (b) We note that the charge on C3 is q3 = 12 C – 8.0 C = 4.0 C. Since the

charge on C4 is q4 = 8.0 C, then the voltage across it is q4/C4 = 2.0 V. Consequently, the

voltage V3 across C3 is 2.0 V C3 = q3/V3 = 2.0 F.

Now C3 and C4 are in parallel and are thus equivalent to 6 F capacitor which would then

be in series with C2 ; thus, Eq 25-20 leads to an equivalence of 2.0 F which is to be

thought of as being in series with the unknown C1 . We know that the total effective

capacitance of the circuit (in the sense of what the battery “sees” when it is hooked up) is

(12 C)/Vbattery = 4 F/3. Using Eq 25-20 again, we find

1

1

3

+ C =

2F

4F

1

C1 = 4.0 F .

20. For maximum capacitance the two groups of plates must face each other with

maximum area. In this case the whole capacitor consists of (n – 1) identical single

capacitors connected in parallel. Each capacitor has surface area A and plate separation d

so its capacitance is given by C0 = 0A/d. Thus, the total capacitance of the combination is

C n 1 C0

n 1 0 A (8 1)(8.851012 C2 /N m2 )(1.25104 m2 ) 2.28 1012 F.

d

3.40 103 m

21. THINK After the switches are closed, the potential differences across the capacitors

are the same and they are connected in parallel.

EXPRESS The potential difference from a to b is given by Vab = Q/Ceq, where Q is the

net charge on the combination and Ceq is the equivalent capacitance.

1121

ANALYZE (a) The equivalent capacitance is Ceq = C1 + C2 = 4.0 10–6 F. The total

charge on the combination is the net charge on either pair of connected plates. The initial

charge on capacitor 1 is

q1 C1V 1.0 106 F 100 V 1.0 104 C

and the initial charge on capacitor 2 is

q2 C2V 3.0 106 F 100 V 3.0 104 C.

With opposite polarities, the net charge on the combination is

Q = 3.0 10–4 C – 1.0 10–4 C = 2.0 10–4 C.

The potential difference is

Q 2.0 104 C

Vab

50 V.

Ceq 4.0 106 F

(b) The charge on capacitor 1 is now q1 C1Vab = (1.0 10–6 F)(50 V) = 5.0 10–5 C.

(c) The charge on capacitor 2 is now q2 C2Vab = (3.0 10–6 F)(50 V) = 1.5 10–4 C.

LEARN The potential difference Vab 50 V is half of the original V ( = 100 V), so the

final charges on the capacitors are also halved.

22. We do not employ energy conservation since, in reaching equilibrium, some energy is

dissipated either as heat or radio waves. Charge is conserved; therefore, if Q = C1Vbat =

100 C, and q1, q2 and q3 are the charges on C1, C2 and C3 after the switch is thrown to

the right and equilibrium is reached, then

Q = q1 + q2 + q3.

Since the parallel pair C2 and C3 are identical, it is clear that q2 = q3. They are in parallel

with C1 so that V1=V3, or

q1 q3

C1 C3

which leads to q1 = q3/2. Therefore,

Q (q3 / 2) q3 q3 5q3 / 2

which yields q3 = 2Q / 5 2(100 C) / 5 40 C and consequently q1 = q3/2 = 20 C.

1122

CHAPTER 25

23. We note that the total equivalent capacitance is C123 = [(C3)1 + (C1 + C2)1]1 = 6 F.

(a) Thus, the charge that passed point a is C123 Vbatt = (6 F)(12 V) = 72 C. Dividing this

by the value e = 1.60 1019 C gives the number of electrons: 4.5 1014, which travel to

the left, toward the positive terminal of the battery.

(b) The equivalent capacitance of the parallel pair is C12 = C1 + C2 = 12 F. Thus, the

voltage across the pair (which is the same as the voltage across C1 and C2 individually) is

72 C

= 6 V.

12 F

Thus, the charge on C1 is

q1 (4 F)(6 V) = 24 C,

and dividing this by e gives N1 q1 / e 1.5 1014 , the number of electrons that have

passed (upward) through point b.

(c) Similarly, the charge on C2 is q2 (8 F)(6 V) = 48 C, and dividing this by e gives

N2 q2 / e 3.0 1014 , the number of electrons which have passed (upward) through

point c.

(d) Finally, since C3 is in series with the battery, its charge is the same charge that passed

through the battery (the same as passed through the switch). Thus, 4.5 1014 electrons

passed rightward though point d. By leaving the rightmost plate of C3, that plate is then

the positive plate of the fully charged capacitor, making its leftmost plate (the one closest

to the negative terminal of the battery) the negative plate, as it should be.

(e) As stated in (b), the electrons travel up through point b.

(f) As stated in (c), the electrons travel up through point c.

24. Using Equation 25-14, the capacitances are

C1

2 0 L1

2 (8.85 1012 C2 /N m 2 )(0.050 m)

2.53 pF

ln(b1 / a1 )

ln(15 mm/5.0 mm)

2 0 L2

2 (8.85 1012 C2 /N m 2 )(0.090 m)

C2

3.61 pF .

ln(b2 / a2 )

ln(10 mm/2.5 mm)

Initially, the total equivalent capacitance is

CC

1

1

1 C1 C2

(2.53 pF)(3.61 pF)

C12 1 2

1.49 pF ,

C12 C1 C2

C1C2

C1 C2

2.53 pF 3.61 pF

1123

and the charge on the positive plate of each one is (1.49 pF)(10 V) = 14.9 pC. Next,

capacitor 2 is modified as described in the problem, with the effect that

C2

2 0 L2

2 (8.85 1012 C2 /N m2 )(0.090 m)

2.17 pF .

ln(b2 / a2 )

ln(25 mm/2.5 mm)

The new total equivalent capacitance is

C12

C1C2

(2.53 pF)(2.17 pF)

1.17 pF

C1 C2

2.53 pF 2.17 pF

and the new charge on the positive plate of each one is (1.17 pF)(10 V) = 11.7 pC. Thus

we see that the charge transferred from the battery (considered in absolute value) as a

result of the modification is 14.9 pC – 11.7 pC = 3.2 pC.

(a) This charge, divided by e gives the number of electrons that pass point P. Thus,

N

3.2 1012 C

2.0 107 .

1.6 1019 C

(b) These electrons move rightward in the figure (that is, away from the battery) since the

positive plates (the ones closest to point P) of the capacitors have suffered a decrease in

their positive charges. The usual reason for a metal plate to be positive is that it has more

protons than electrons. Thus, in this problem some electrons have “returned” to the

positive plates (making them less positive).

25. Equation 23-14 applies to each of these capacitors. Bearing in mind that = q/A, we

find the total charge to be

qtotal = q1 + q2 = 1 A1 + 2 A2 = o E1 A1 + o E2 A2 = 3.6 pC

where we have been careful to convert cm2 to m2 by dividing by 104.

26. Initially the capacitors C1, C2, and C3 form a combination equivalent to a single

capacitor which we denote C123. This obeys the equation

C C2 C3

1

1

1

.

1

C123 C1 C2 C3 C1 (C2 C3 )

Hence, using q = C123V and the fact that q = q1 = C1 V1 , we arrive at

V1

C2 C3

q1 q C123

V

V .

C1 C1 C1

C1 C2 C3

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