...

Capacitors and Dielectrics

by taratuta

on
Category: Documents
227

views

Report

Comments

Transcript

Capacitors and Dielectrics
CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD
Figure 19.12 Charges and Fields (http://cnx.org/content/m42331/1.3/charges-and-fields_en.jar)
19.5 Capacitors and Dielectrics
A capacitor is a device used to store electric charge. Capacitors have applications ranging from filtering static out of radio reception to energy
storage in heart defibrillators. Typically, commercial capacitors have two conducting parts close to one another, but not touching, such as those in
Figure 19.13. (Most of the time an insulator is used between the two plates to provide separation—see the discussion on dielectrics below.) When
battery terminals are connected to an initially uncharged capacitor, equal amounts of positive and negative charge, +Q and – Q , are separated
into its two plates. The capacitor remains neutral overall, but we refer to it as storing a charge
Q in this circumstance.
Capacitor
A capacitor is a device used to store electric charge.
Figure 19.13 Both capacitors shown here were initially uncharged before being connected to a battery. They now have separated charges of
+Q
and
–Q
on their two
halves. (a) A parallel plate capacitor. (b) A rolled capacitor with an insulating material between its two conducting sheets.
The amount of charge
Q a capacitor can store depends on two major factors—the voltage applied and the capacitor’s physical characteristics, such
as its size.
The Amount of Charge
The amount of charge
Q a Capacitor Can Store
Q a capacitor can store depends on two major factors—the voltage applied and the capacitor’s physical characteristics,
such as its size.
A system composed of two identical, parallel conducting plates separated by a distance, as in Figure 19.14, is called a parallel plate capacitor. It is
easy to see the relationship between the voltage and the stored charge for a parallel plate capacitor, as shown in Figure 19.14. Each electric field line
starts on an individual positive charge and ends on a negative one, so that there will be more field lines if there is more charge. (Drawing a single field
line per charge is a convenience, only. We can draw many field lines for each charge, but the total number is proportional to the number of charges.)
The electric field strength is, thus, directly proportional to Q .
677
678
CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD
Figure 19.14 Electric field lines in this parallel plate capacitor, as always, start on positive charges and end on negative charges. Since the electric field strength is proportional
to the density of field lines, it is also proportional to the amount of charge on the capacitor.
The field is proportional to the charge:
E ∝ Q,
where the symbol
(19.45)
∝ means “proportional to.” From the discussion in Electric Potential in a Uniform Electric Field, we know that the voltage
V = Ed . Thus,
across parallel plates is
It follows, then, that
V ∝ E.
(19.46)
Q ∝ V.
(19.47)
V ∝ Q , and conversely,
This is true in general: The greater the voltage applied to any capacitor, the greater the charge stored in it.
Different capacitors will store different amounts of charge for the same applied voltage, depending on their physical characteristics. We define their
capacitance C to be such that the charge Q stored in a capacitor is proportional to C . The charge stored in a capacitor is given by
Q = CV.
(19.48)
This equation expresses the two major factors affecting the amount of charge stored. Those factors are the physical characteristics of the capacitor,
C , and the voltage, V . Rearranging the equation, we see that capacitance C is the amount of charge stored per volt, or
C=
Q
.
V
C=
Q
.
V
(19.49)
Capacitance
Capacitance
C is the amount of charge stored per volt, or
(19.50)
The unit of capacitance is the farad (F), named for Michael Faraday (1791–1867), an English scientist who contributed to the fields of
electromagnetism and electrochemistry. Since capacitance is charge per unit voltage, we see that a farad is a coulomb per volt, or
1 F = 1 C.
1V
(19.51)
A 1-farad capacitor would be able to store 1 coulomb (a very large amount of charge) with the application of only 1 volt. One farad is, thus, a very
⎛
⎛
⎞
–3 ⎞
large capacitance. Typical capacitors range from fractions of a picofarad ⎝1 pF = 10 –12 F⎠ to millifarads ⎝1 mF = 10 F⎠ .
This content is available for free at http://cnx.org/content/col11406/1.7
CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD
Figure 19.15 shows some common capacitors. Capacitors are primarily made of ceramic, glass, or plastic, depending upon purpose and size.
Insulating materials, called dielectrics, are commonly used in their construction, as discussed below.
Figure 19.15 Some typical capacitors. Size and value of capacitance are not necessarily related. (credit: Windell Oskay)
Parallel Plate Capacitor
A , separated by a distance d
V is applied to the capacitor, it stores a charge Q , as shown. We can see how its
The parallel plate capacitor shown in Figure 19.16 has two identical conducting plates, each having a surface area
(with no material between the plates). When a voltage
capacitance depends on A and d by considering the characteristics of the Coulomb force. We know that like charges repel, unlike charges attract,
and the force between charges decreases with distance. So it seems quite reasonable that the bigger the plates are, the more charge they can
store—because the charges can spread out more. Thus C should be greater for larger A . Similarly, the closer the plates are together, the greater
the attraction of the opposite charges on them. So
C should be greater for smaller d .
Figure 19.16 Parallel plate capacitor with plates separated by a distance
d . Each plate has an area A .
It can be shown that for a parallel plate capacitor there are only two factors (
plate capacitor in equation form is given by
C = ε0 A.
d
Capacitance of a Parallel Plate Capacitor
C = ε0 A
d
A and d ) that affect its capacitance C . The capacitance of a parallel
(19.52)
(19.53)
A is the area of one plate in square meters, and d is the distance between the plates in meters. The constant ε 0 is the permittivity of free space;
its numerical value in SI units is
ε 0 = 8.85×10 – 12 F/m . The units of F/m are equivalent to C 2 /N · m 2 . The small numerical value of ε 0 is
related to the large size of the farad. A parallel plate capacitor must have a large area to have a capacitance approaching a farad. (Note that the
above equation is valid when the parallel plates are separated by air or free space. When another material is placed between the plates, the equation
is modified, as discussed below.)
679
680
CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD
Example 19.8 Capacitance and Charge Stored in a Parallel Plate Capacitor
(a) What is the capacitance of a parallel plate capacitor with metal plates, each of area
stored in this capacitor if a voltage of 3.00×10 3 V is applied to it?
1.00 m 2 , separated by 1.00 mm? (b) What charge is
Strategy
Finding the capacitance
using the equation
C is a straightforward application of the equation C = ε 0 A / d . Once C is found, the charge stored can be found
Q = CV .
Solution for (a)
Entering the given values into the equation for the capacitance of a parallel plate capacitor yields
F ⎞ 1.00 m 2
C = ε 0 A = ⎛⎝8.85×10 –12 m
⎠
d
1.00×10 –3 m
= 8.85×10 –9 F = 8.85 nF.
(19.54)
Discussion for (a)
This small value for the capacitance indicates how difficult it is to make a device with a large capacitance. Special techniques help, such as using
very large area thin foils placed close together.
Solution for (b)
The charge stored in any capacitor is given by the equation
Q = CV . Entering the known values into this equation gives
Q = CV = ⎛⎝8.85×10 –9 F⎞⎠⎛⎝3.00×10 3 V⎞⎠
(19.55)
= 26.6 µC.
Discussion for (b)
This charge is only slightly greater than those found in typical static electricity. Since air breaks down at about
cannot be stored on this capacitor by increasing the voltage.
3.00×10 6 V/m , more charge
Another interesting biological example dealing with electric potential is found in the cell’s plasma membrane. The membrane sets a cell off from its
surroundings and also allows ions to selectively pass in and out of the cell. There is a potential difference across the membrane of about –70 mV .
This is due to the mainly negatively charged ions in the cell and the predominance of positively charged sodium ( Na + ) ions outside. Things change
when a nerve cell is stimulated. Na + ions are allowed to pass through the membrane into the cell, producing a positive membrane potential—the
nerve signal. The cell membrane is about 7 to 10 nm thick. An approximate value of the electric field across it is given by
–3
E = V = –70×10–9 V = –9×10 6 V/m.
d
8×10 m
(19.56)
This electric field is enough to cause a breakdown in air.
Dielectric
The previous example highlights the difficulty of storing a large amount of charge in capacitors. If
then the maximum voltage must be reduced proportionally to avoid breakdown (since
d is made smaller to produce a larger capacitance,
E = V / d ). An important solution to this difficulty is to put an
d make
insulating material, called a dielectric, between the plates of a capacitor and allow d to be as small as possible. Not only does the smaller
the capacitance greater, but many insulators can withstand greater electric fields than air before breaking down.
There is another benefit to using a dielectric in a capacitor. Depending on the material used, the capacitance is greater than that given by the
equation
C = ε 0 A by a factor κ , called the dielectric constant. A parallel plate capacitor with a dielectric between its plates has a capacitance
d
given by
C = κε 0 A (parallel plate capacitor with dielectric).
d
(19.57)
Values of the dielectric constant κ for various materials are given in Table 19.1. Note that κ for vacuum is exactly 1, and so the above equation is
valid in that case, too. If a dielectric is used, perhaps by placing Teflon between the plates of the capacitor in Example 19.8, then the capacitance is
greater by the factor κ , which for Teflon is 2.1.
Take-Home Experiment: Building a Capacitor
How large a capacitor can you make using a chewing gum wrapper? The plates will be the aluminum foil, and the separation (dielectric) in
between will be the paper.
This content is available for free at http://cnx.org/content/col11406/1.7
CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD
Table 19.1 Dielectric Constants and Dielectric Strengths for Various Materials
at 20ºC
Material
Dielectric constant
κ
Dielectric strength (V/m)
Vacuum
1.00000
—
Air
1.00059
3×10 6
Bakelite
4.9
24×10 6
Fused quartz
3.78
8×10 6
Neoprene rubber 6.7
12×10 6
Nylon
3.4
14×10 6
Paper
3.7
16×10 6
Polystyrene
2.56
24×10 6
Pyrex glass
5.6
14×10 6
Silicon oil
2.5
15×10 6
Strontium titanate 233
8×10 6
Teflon
2.1
60×10 6
Water
80
—
Note also that the dielectric constant for air is very close to 1, so that air-filled capacitors act much like those with vacuum between their plates except
that the air can become conductive if the electric field strength becomes too great. (Recall that E = V / d for a parallel plate capacitor.) Also shown
in Table 19.1 are maximum electric field strengths in V/m, called dielectric strengths, for several materials. These are the fields above which the
material begins to break down and conduct. The dielectric strength imposes a limit on the voltage that can be applied for a given plate separation. For
instance, in Example 19.8, the separation is 1.00 mm, and so the voltage limit for air is
V = E⋅d
= (3×10 6 V/m)(1.00×10 −3 m)
= 3000 V.
(19.58)
6
However, the limit for a 1.00 mm separation filled with Teflon is 60,000 V, since the dielectric strength of Teflon is 60×10 V/m. So the same
capacitor filled with Teflon has a greater capacitance and can be subjected to a much greater voltage. Using the capacitance we calculated in the
above example for the air-filled parallel plate capacitor, we find that the Teflon-filled capacitor can store a maximum charge of
Q = CV
= κC air V
(19.59)
= (2.1)(8.85 nF)(6.0×10 4 V)
= 1.1 mC.
This is 42 times the charge of the same air-filled capacitor.
Dielectric Strength
The maximum electric field strength above which an insulating material begins to break down and conduct is called its dielectric strength.
Microscopically, how does a dielectric increase capacitance? Polarization of the insulator is responsible. The more easily it is polarized, the greater its
dielectric constant κ . Water, for example, is a polar molecule because one end of the molecule has a slight positive charge and the other end has a
slight negative charge. The polarity of water causes it to have a relatively large dielectric constant of 80. The effect of polarization can be best
explained in terms of the characteristics of the Coulomb force. Figure 19.17 shows the separation of charge schematically in the molecules of a
dielectric material placed between the charged plates of a capacitor. The Coulomb force between the closest ends of the molecules and the charge
on the plates is attractive and very strong, since they are very close together. This attracts more charge onto the plates than if the space were empty
and the opposite charges were a distance d away.
681
682
CHAPTER 19 | ELECTRIC POTENTIAL AND ELECTRIC FIELD
Figure 19.17 (a) The molecules in the insulating material between the plates of a capacitor are polarized by the charged plates. This produces a layer of opposite charge on
the surface of the dielectric that attracts more charge onto the plate, increasing its capacitance. (b) The dielectric reduces the electric field strength inside the capacitor,
resulting in a smaller voltage between the plates for the same charge. The capacitor stores the same charge for a smaller voltage, implying that it has a larger capacitance
because of the dielectric.
Another way to understand how a dielectric increases capacitance is to consider its effect on the electric field inside the capacitor. Figure 19.17(b)
shows the electric field lines with a dielectric in place. Since the field lines end on charges in the dielectric, there are fewer of them going from one
side of the capacitor to the other. So the electric field strength is less than if there were a vacuum between the plates, even though the same charge
is on the plates. The voltage between the plates is V = Ed , so it too is reduced by the dielectric. Thus there is a smaller voltage V for the same
charge
Q ; since C = Q / V , the capacitance C is greater.
The dielectric constant is generally defined to be
κ = E 0 / E , or the ratio of the electric field in a vacuum to that in the dielectric material, and is
intimately related to the polarizability of the material.
Things Great and Small
The Submicroscopic Origin of Polarization
Polarization is a separation of charge within an atom or molecule. As has been noted, the planetary model of the atom pictures it as having a
positive nucleus orbited by negative electrons, analogous to the planets orbiting the Sun. Although this model is not completely accurate, it is
very helpful in explaining a vast range of phenomena and will be refined elsewhere, such as in Atomic Physics. The submicroscopic origin of
polarization can be modeled as shown in Figure 19.18.
Figure 19.18 Artist’s conception of a polarized atom. The orbits of electrons around the nucleus are shifted slightly by the external charges (shown exaggerated). The resulting
separation of charge within the atom means that it is polarized. Note that the unlike charge is now closer to the external charges, causing the polarization.
This content is available for free at http://cnx.org/content/col11406/1.7
Fly UP