application of Capacitors
• Tuning circuits of radios
• Electronic timing circuits
• Electronic flashguns
• Smoothing fluctuation in the output of power supplies in radio and TVs.
• Experiment in high energy particle accelerators, and especially those in fusion power research
EXAMPLE 26.1: A parallel-plate capacitor with a plate separation of 1 mm has a capacitance of 1 F. What is the area of each plate?
unit. lare very a is farad the
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m101.13
/1085.8
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12
3
0
mF
mFCdA
EXAMPLE 26.2: A parallel-plate capacitor has plates with dimensions 3 cm x 4 cm, separated by 2 mm. The plates are connected across a 60-V battery. Find: (a) the capacitance; (b) the magnitude of the charge on each plate.
C103.19
F)(60V)110(5.3
CV(b)Q
5.31pF 102
)102.1)(/1085.8(
)(
1012
10-
12-
3
23120
24
m
mmF
d
AC
a
mA
EXAMPLE 26.3: What is the capacitance of an isolated sphere of radius R?
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thatpredictsit , 6370km radius of sphere
conducting a isearth that theassume we
4C
)1/(4k sin
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/
0
0
If
R
ce
VQC
RkQV
EXAMPLE 26.4: A spherical capacitor consists of two con centric conducting spheres, as shown in Fig. 26.6. The inner sphere, of radius R1? has charge +Q. The charge on the outer shell of radius R2 is -Q. Find its capacitance.
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of ecapacitanc the torelated becan expression
)R-k(R
RR
/
)11
(
r
kQ--
/E
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12
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r
2
1
2
1
This
VQC
RRkQ
drEVV
rkQdrEsdE
sdEV
R
R
R
R
r
r
EXAMPLE 26.5: A cylindrical capacitor consists of a central conductor of radius a surrounded by a cylindrical shell of radius b, as shown in Fig. 26.7. A coaxial cable used for transmission of TV signals has this geometry. Usually, the outer sheath is grounded and shields the signal in the inner wire from electrical disturbances. A nylon, or Teflon, sleeve separates the inner wire from the sheath. Find the capacitance of a length L assuming that air is between the plates.
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dr ErdE
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r
ab
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r
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r
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ar
r
EXAMPLE 26.6: For the circuit in Fig. 26.10a, find: (a) the equivalent capacitance; (b) the charge and potential difference for each capacitor.
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eq
eq
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1111
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EXAMPLE 26.7-1: Two capacitors, C1= 5 μF and C2=3μ F, are initially in parallel with a 12-V battery, as in Fig. 26.11a. They are disconnected and then reconnected as shown in Fig26.11b. Note carefully the numbering on the plates. Find the charges, potential differences, and energies stored (a) in the initial state, and (b) in the final state.
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JVCVQU
JVCVQU
CQCQ
VC
Q
C
Q
a
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2
2
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1
EXAMPLE 26.7-2: Two capacitors, C1= 5 μF and C2=3μ F, are initially in parallel with a 12-V battery, as in Fig. 26.11a. They are disconnected and then reconnected as shown in Fig26.11b. Note carefully the numbering on the plates. Find the charges, potential differences, and energies stored (a) in the initial state, and (b) in the final state.
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charges The ously.instantane valuesfinalr reach theinot do
capacitors on the charges theresistance no is thereIf (2)
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note
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QC
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Q
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b
EXAMPLE 26.8: The "breakdown" field strength, at which dry air loses its insulating ability and allows a discharge to pass through it, is about 3 × 106 V/m. What is the energy density at this field strength?
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strength field breakdown (1)the
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)/103)(/1085.8(2
1
2
1
3
262212
2
0
mJ
mVmNC
EuE
EXAMPLE 26.9: Use Eq. 26.10 to derive the potential energy of a metal sphere of radius R with charge Q.
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2
)4()(2
1
2
1
r4d
R)(r
22
2
2
2
22
20
2
0
2
2
R
kQdrr
kQUE
drr
kQ
drrr
kQ
dVEdVudU
drVr
kQE
R
EE
EXAMPLE 26.10: A dielectric slab of thickness ; and dielec tric constant « is inserted into a parallel plate capacitor with plates of area A, separated by distance d, as shown in Fig. 26.19. Assume that the battery is disconnected before the slab is inserted. What is the capacitance?
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td
AVAVQC
ttd
ttdEVV
ttEtdEV
A
QE
totaltotal
total
D
/,1
)11
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)( )(V
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00
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00
0
Exercises of chapter 26
• Questions:
• Exercises:4,6,7,12,13,41,42,43
• Problems:1,2,3,4,5,6,11