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Capacitance and Dielectrics

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Capacitance and Dielectrics Any two conductors separated by an insulator forms a Capacitor. Definition: 1F =1 farad = 1 C/V = 1 coulomb /volt Chapter 24 – PowerPoint PPT presentation

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Title: Capacitance and Dielectrics


1
Capacitance and Dielectrics
Any two conductors separated by an insulator
forms a Capacitor. Definition 1F 1 farad 1
C/V 1 coulomb /volt
2
Capacitors and Capacitance (Chapter 24, Sec 1)
Coul/m2
Figure 24-2
Farads
1 ?F 1 x 10-6 Farads
1 pF 1 x 10-12 Farads
1 nF 1 x 10-9 Farads
3
Useful Definitions and Relationships
4
Practical Capacitors
Practical Values 100µF 1µF 0.01µF 100pF 1pF
5
Some examples of flat, cylindrical, and spherical
capacitors
  • See just how large a 1 F capacitor would be.
    Refer to Example 24.1.
  • Refer to Example 24.2 to calculate properties of
    a parallel-plate capacitor.
  • Follow Example 24.3 and Figure 24.5 to consider a
    spherical capacitor.
  • Follow Example 24.3 and Figure 24.5 to consider a
    cylindrical capacitor.

6
Capacitors in Series
All capacitors in series have the same charge Q
7
Capacitors in Parallel
All Capacitors in Parallel have the same voltage V
Figure 24-7
8
Capacitors in Series and Parallel - Example 24.6
9
Energy Storage in Capacitors
Let q equal the changing charge increasing from 0
to Q as the changing voltage v is increasing from
0 to Vab. We will determine the energy stored in
the capacitor when the charge reaches Q and the
voltage reaches Vab. (q and v are the
intermediate charge and charging voltage
joules
10
Example 24-7, Page 827 Text Transferring charge
and energy between capacitors
Calculate the initial charge Calculate the
initial stored energy Connect the
capacitors Calculate the resulting
voltage Calculate the charge distribution Calculat
e the energy change
11
Capacitor Dielectrics Solve Three Problems
  • Provides mechanical spacing between two large
    plates
  • Increases the maximum possible potential between
    plates.
  • For a given plate area the dielectric increases
    the capacitance.

12
Dielectrics change the potential difference
  • The potential between to parallel plates of a
    capacitor changes when the material between the
    plates changes. It does not matter if the plates
    are rolled into a tube as they are in Figure
    24.13 or if they are flat as shown in Figure
    24.14.

13
What Happens with a Dielectric
A
A
C
V ? V0 (Q unchanged)
C0
Therefore C ? C0
Figure 24-12
(Definition of Dielectric Constant)
(24-12)
(24-13)
14
Dielectric Constants
15
Field lines as dielectrics change
  • Moving from part (a) to part (b) of Figure 24.15
    shows the change induced by the dielectric.

16
Induced Charge and Polarization
Inserting the dielectric increases permittivity
by K, decreases E by 1/K and decreases energy
density by 1/K The E field does work on the
dielectric as it is inserted. Removing the
dielectric the energy is returned to the field.
17
Dielectrics (Chapter 24, Sec 4)
Induced Charge and Polarization
0
(24-15)
(24-18)
Therefore E ? E0
18
Dielectric Breakdown
V Ed
Vmax Emax d
where Emax is the dielectric strength of the
dielectric in volts/meter.
For dry air Emax 3 x 106 V/m For Mylar,
K3.1, Emax 9.3 x 106 V/m
d
19
Dielectric breakdown
  • A very strong electrical field can exceed the
    strength of the dielectric to contain it. Table
    24.2 at the bottom of the page lists some limits.

20
Electric Field Effect on Molecules
21
Polarization and electric field lines
22
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