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University Physics Volume 2

8.1 Capacitors and Capacitance

University Physics Volume 28.1 Capacitors and Capacitance
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  1. Preface
  2. Unit 1. Thermodynamics
    1. 1 Temperature and Heat
      1. Introduction
      2. 1.1 Temperature and Thermal Equilibrium
      3. 1.2 Thermometers and Temperature Scales
      4. 1.3 Thermal Expansion
      5. 1.4 Heat Transfer, Specific Heat, and Calorimetry
      6. 1.5 Phase Changes
      7. 1.6 Mechanisms of Heat Transfer
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    2. 2 The Kinetic Theory of Gases
      1. Introduction
      2. 2.1 Molecular Model of an Ideal Gas
      3. 2.2 Pressure, Temperature, and RMS Speed
      4. 2.3 Heat Capacity and Equipartition of Energy
      5. 2.4 Distribution of Molecular Speeds
      6. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    3. 3 The First Law of Thermodynamics
      1. Introduction
      2. 3.1 Thermodynamic Systems
      3. 3.2 Work, Heat, and Internal Energy
      4. 3.3 First Law of Thermodynamics
      5. 3.4 Thermodynamic Processes
      6. 3.5 Heat Capacities of an Ideal Gas
      7. 3.6 Adiabatic Processes for an Ideal Gas
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    4. 4 The Second Law of Thermodynamics
      1. Introduction
      2. 4.1 Reversible and Irreversible Processes
      3. 4.2 Heat Engines
      4. 4.3 Refrigerators and Heat Pumps
      5. 4.4 Statements of the Second Law of Thermodynamics
      6. 4.5 The Carnot Cycle
      7. 4.6 Entropy
      8. 4.7 Entropy on a Microscopic Scale
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
  3. Unit 2. Electricity and Magnetism
    1. 5 Electric Charges and Fields
      1. Introduction
      2. 5.1 Electric Charge
      3. 5.2 Conductors, Insulators, and Charging by Induction
      4. 5.3 Coulomb's Law
      5. 5.4 Electric Field
      6. 5.5 Calculating Electric Fields of Charge Distributions
      7. 5.6 Electric Field Lines
      8. 5.7 Electric Dipoles
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
    2. 6 Gauss's Law
      1. Introduction
      2. 6.1 Electric Flux
      3. 6.2 Explaining Gauss’s Law
      4. 6.3 Applying Gauss’s Law
      5. 6.4 Conductors in Electrostatic Equilibrium
      6. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    3. 7 Electric Potential
      1. Introduction
      2. 7.1 Electric Potential Energy
      3. 7.2 Electric Potential and Potential Difference
      4. 7.3 Calculations of Electric Potential
      5. 7.4 Determining Field from Potential
      6. 7.5 Equipotential Surfaces and Conductors
      7. 7.6 Applications of Electrostatics
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    4. 8 Capacitance
      1. Introduction
      2. 8.1 Capacitors and Capacitance
      3. 8.2 Capacitors in Series and in Parallel
      4. 8.3 Energy Stored in a Capacitor
      5. 8.4 Capacitor with a Dielectric
      6. 8.5 Molecular Model of a Dielectric
      7. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    5. 9 Current and Resistance
      1. Introduction
      2. 9.1 Electrical Current
      3. 9.2 Model of Conduction in Metals
      4. 9.3 Resistivity and Resistance
      5. 9.4 Ohm's Law
      6. 9.5 Electrical Energy and Power
      7. 9.6 Superconductors
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    6. 10 Direct-Current Circuits
      1. Introduction
      2. 10.1 Electromotive Force
      3. 10.2 Resistors in Series and Parallel
      4. 10.3 Kirchhoff's Rules
      5. 10.4 Electrical Measuring Instruments
      6. 10.5 RC Circuits
      7. 10.6 Household Wiring and Electrical Safety
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    7. 11 Magnetic Forces and Fields
      1. Introduction
      2. 11.1 Magnetism and Its Historical Discoveries
      3. 11.2 Magnetic Fields and Lines
      4. 11.3 Motion of a Charged Particle in a Magnetic Field
      5. 11.4 Magnetic Force on a Current-Carrying Conductor
      6. 11.5 Force and Torque on a Current Loop
      7. 11.6 The Hall Effect
      8. 11.7 Applications of Magnetic Forces and Fields
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    8. 12 Sources of Magnetic Fields
      1. Introduction
      2. 12.1 The Biot-Savart Law
      3. 12.2 Magnetic Field Due to a Thin Straight Wire
      4. 12.3 Magnetic Force between Two Parallel Currents
      5. 12.4 Magnetic Field of a Current Loop
      6. 12.5 Ampère’s Law
      7. 12.6 Solenoids and Toroids
      8. 12.7 Magnetism in Matter
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    9. 13 Electromagnetic Induction
      1. Introduction
      2. 13.1 Faraday’s Law
      3. 13.2 Lenz's Law
      4. 13.3 Motional Emf
      5. 13.4 Induced Electric Fields
      6. 13.5 Eddy Currents
      7. 13.6 Electric Generators and Back Emf
      8. 13.7 Applications of Electromagnetic Induction
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    10. 14 Inductance
      1. Introduction
      2. 14.1 Mutual Inductance
      3. 14.2 Self-Inductance and Inductors
      4. 14.3 Energy in a Magnetic Field
      5. 14.4 RL Circuits
      6. 14.5 Oscillations in an LC Circuit
      7. 14.6 RLC Series Circuits
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    11. 15 Alternating-Current Circuits
      1. Introduction
      2. 15.1 AC Sources
      3. 15.2 Simple AC Circuits
      4. 15.3 RLC Series Circuits with AC
      5. 15.4 Power in an AC Circuit
      6. 15.5 Resonance in an AC Circuit
      7. 15.6 Transformers
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    12. 16 Electromagnetic Waves
      1. Introduction
      2. 16.1 Maxwell’s Equations and Electromagnetic Waves
      3. 16.2 Plane Electromagnetic Waves
      4. 16.3 Energy Carried by Electromagnetic Waves
      5. 16.4 Momentum and Radiation Pressure
      6. 16.5 The Electromagnetic Spectrum
      7. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
  4. A | Units
  5. B | Conversion Factors
  6. C | Fundamental Constants
  7. D | Astronomical Data
  8. E | Mathematical Formulas
  9. F | Chemistry
  10. G | The Greek Alphabet
  11. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
    10. Chapter 10
    11. Chapter 11
    12. Chapter 12
    13. Chapter 13
    14. Chapter 14
    15. Chapter 15
    16. Chapter 16
  12. Index

Learning Objectives

By the end of this section, you will be able to:
  • Explain the concepts of a capacitor and its capacitance
  • Describe how to evaluate the capacitance of a system of conductors

A capacitor is a device used to store electrical charge and electrical energy. It consists of at least two electrical conductors separated by a distance. (Note that such electrical conductors are sometimes referred to as “electrodes,” but more correctly, they are “capacitor plates.”) The space between capacitors may simply be a vacuum, and, in that case, a capacitor is then known as a “vacuum capacitor.” However, the space is usually filled with an insulating material known as a dielectric. (You will learn more about dielectrics in the sections on dielectrics later in this chapter.) The amount of storage in a capacitor is determined by a property called capacitance, which you will learn more about a bit later in this section.

Capacitors have applications ranging from filtering static from 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 8.2. Most of the time, a dielectric is used between the two plates. When battery terminals are connected to an initially uncharged capacitor, the battery potential moves a small amount of charge of magnitude Q from the positive plate to the negative plate. The capacitor remains neutral overall, but with charges +Q+Q and QQ residing on opposite plates.

Figure a shows two plates placed parallel to each other, a distance of d apart. Each plate is connected to one terminal of a battery. Figure b shows sheets of conductors and dielectric stacked alternately and rolled together. Each sheet of conductor is connected to one terminal of a battery. In both figures, the charge is plus Q and minus Q for the plates connected to the positive and negative terminals respectively.
Figure 8.2 Both capacitors shown here were initially uncharged before being connected to a battery. They now have charges of +Q+Q and QQ (respectively) on their plates. (a) A parallel-plate capacitor consists of two plates of opposite charge with area A separated by distance d. (b) A rolled capacitor has a dielectric material between its two conducting sheets (plates).

A system composed of two identical parallel-conducting plates separated by a distance is called a parallel-plate capacitor (Figure 8.3). The magnitude of the electrical field in the space between the parallel plates is E=σ/ε0E=σ/ε0, where σσ denotes the surface charge density on one plate (recall that σσ is the charge Q per the surface area A). Thus, the magnitude of the field is directly proportional to Q.

Two parallel plates are connected to a battery. The plate connected to the positive terminal has positive charges on it marked by the plus sign. Similarly, the other plate has minus signs on it. Arrows are shown between the plates, from the positive plate to the negative one. The space between the plates has the formula E proportional to Q.
Figure 8.3 The charge separation in a capacitor shows that the charges remain on the surfaces of the capacitor plates. Electrical field lines in a parallel-plate capacitor begin with positive charges and end with negative charges. The magnitude of the electrical field in the space between the plates is in direct proportion to the amount of charge on the capacitor.

Capacitors with different physical characteristics (such as shape and size of their plates) store different amounts of charge for the same applied voltage V across their plates. The capacitance C of a capacitor is defined as the ratio of the maximum charge Q that can be stored in a capacitor to the applied voltage V across its plates. In other words, capacitance is the largest amount of charge per volt that can be stored on the device:

C=QV.C=QV.
(8.1)

The SI unit of capacitance is the farad (F), named after Michael Faraday (1791–1867). Since capacitance is the charge per unit voltage, one farad is one coulomb per one volt, or

1F=1C1V.1F=1C1V.

By definition, a 1.0-F capacitor is able to store 1.0 C of charge (a very large amount of charge) when the potential difference between its plates is only 1.0 V. One farad is therefore a very large capacitance. Typical capacitance values range from picofarads (1pF=1012F)(1pF=1012F) to millifarads (1mF=103F)(1mF=103F), which also includes microfarads (1μF=10−6F1μF=10−6F). Capacitors can be produced in various shapes and sizes (Figure 8.4).

A photograph of different types of capacitors.
Figure 8.4 These are some typical capacitors used in electronic devices. A capacitor’s size is not necessarily related to its capacitance value. (credit: Windell Oskay)

Calculation of Capacitance

We can calculate the capacitance of a pair of conductors with the standard approach that follows.

Problem-Solving Strategy: Calculating Capacitance

  1. Assume that the capacitor has a charge Q.
  2. Determine the electrical field EE between the conductors. If symmetry is present in the arrangement of conductors, you may be able to use Gauss’s law for this calculation.
  3. Find the potential difference between the conductors from
    VBVA=ABE·dl,VBVA=ABE·dl,
    (8.2)

    where the path of integration leads from one conductor to the other. The magnitude of the potential difference is then V=|VBVA|V=|VBVA|.
  4. With V known, obtain the capacitance directly from Equation 8.1.

To show how this procedure works, we now calculate the capacitances of parallel-plate, spherical, and cylindrical capacitors. In all cases, we assume vacuum capacitors (empty capacitors) with no dielectric substance in the space between conductors.

Parallel-Plate Capacitor

The parallel-plate capacitor (Figure 8.5) has two identical conducting plates, each having a surface area A, separated by a distance d. When a voltage V is applied to the capacitor, it stores a charge Q, as shown. We can see how its capacitance may depend on A and d by considering characteristics of the Coulomb force. We know that force between the charges increases with charge values and decreases with the distance between them. We should expect that the bigger the plates are, the more charge they can store. Thus, C should be greater for a larger value of A. Similarly, the closer the plates are together, the greater the attraction of the opposite charges on them. Therefore, C should be greater for a smaller d.

Figure shows two parallel plates separated by a distance of d, with each one connected to one terminal of a battery. Electric field lines are shown as arrows from the positive plate to the negative one. The plate area is labeled A.
Figure 8.5 In a parallel-plate capacitor with plates separated by a distance d, each plate has the same surface area A.

We define the surface charge density σσ on the plates as

σ=QA.σ=QA.

We know from previous chapters that when d is small, the electrical field between the plates is fairly uniform (ignoring edge effects) and that its magnitude is given by

E=σε0,E=σε0,

where the constant ε0ε0 is the permittivity of free space, ε0=8.85×10−12F/m.ε0=8.85×10−12F/m. The SI unit of F/m is equivalent to C2/N·m2.C2/N·m2. Since the electrical field EE between the plates is uniform, the potential difference between the plates is

V=Ed=σdε0=Qdε0A.V=Ed=σdε0=Qdε0A.

Therefore Equation 8.1 gives the capacitance of a parallel-plate capacitor as

C=QV=QQd/ε0A=ε0Ad.C=QV=QQd/ε0A=ε0Ad.
(8.3)

Notice from this equation that capacitance is a function only of the geometry and what material fills the space between the plates (in this case, vacuum) of this capacitor. In fact, this is true not only for a parallel-plate capacitor, but for all capacitors: The capacitance is independent of Q or V. If the charge changes, the potential changes correspondingly so that Q/V remains constant.

Example 8.1

Capacitance and Charge Stored in a Parallel-Plate Capacitor (a) What is the capacitance of an empty parallel-plate capacitor with metal plates that each have an area of 1.00m21.00m2, separated by 1.00 mm? (b) How much charge is stored in this capacitor if a voltage of 3.00×103V3.00×103V is applied to it?

Strategy Finding the capacitance C is a straightforward application of Equation 8.3. Once we find C, we can find the charge stored by using Equation 8.1.

Solution

  1. Entering the given values into Equation 8.3 yields
    C=ε0Ad=(8.85×10−12Fm)1.00m21.00×10−3m=8.85×10−9F=8.85nF.C=ε0Ad=(8.85×10−12Fm)1.00m21.00×10−3m=8.85×10−9F=8.85nF.

    This small capacitance value indicates how difficult it is to make a device with a large capacitance.
  2. Inverting Equation 8.1 and entering the known values into this equation gives
    Q=CV=(8.85×10−9F)(3.00×103V)=26.6μC.Q=CV=(8.85×10−9F)(3.00×103V)=26.6μC.

Significance This charge is only slightly greater than those found in typical static electricity applications. Since air breaks down (becomes conductive) at an electrical field strength of about 3.0 MV/m, no more charge can be stored on this capacitor by increasing the voltage.

Example 8.2

A 1-F Parallel-Plate Capacitor Suppose you wish to construct a parallel-plate capacitor with a capacitance of 1.0 F. What area must you use for each plate if the plates are separated by 1.0 mm?

Solution Rearranging Equation 8.3, we obtain

A=Cdε0=(1.0F)(1.0×10−3m)8.85×10−12F/m=1.1×108m2.A=Cdε0=(1.0F)(1.0×10−3m)8.85×10−12F/m=1.1×108m2.

Each square plate would have to be 10 km across. It used to be a common prank to ask a student to go to the laboratory stockroom and request a 1-F parallel-plate capacitor, until stockroom attendants got tired of the joke.

Check Your Understanding 8.1

The capacitance of a parallel-plate capacitor is 2.0 pF. If the area of each plate is 2.4cm22.4cm2, what is the plate separation?

Check Your Understanding 8.2

Verify that σ/Vσ/V and ε0/dε0/d have the same physical units.

Spherical Capacitor

A spherical capacitor is another set of conductors whose capacitance can be easily determined (Figure 8.6). It consists of two concentric conducting spherical shells of radii R1R1 (inner shell) and R2R2 (outer shell). The shells are given equal and opposite charges +Q+Q and QQ, respectively. From symmetry, the electrical field between the shells is directed radially outward. We can obtain the magnitude of the field by applying Gauss’s law over a spherical Gaussian surface of radius r concentric with the shells. The enclosed charge is +Q+Q; therefore we have

SE·n^dA=E(4πr2)=Qε0.SE·n^dA=E(4πr2)=Qε0.

Thus, the electrical field between the conductors is

E=14πε0Qr2r^.E=14πε0Qr2r^.

We substitute this EE into Equation 8.2 and integrate along a radial path between the shells:

V=R1R2E·dl=R1R2(14πε0Qr2r^)·(r^dr)=Q4πε0R1R2drr2=Q4πε0(1R11R2).V=R1R2E·dl=R1R2(14πε0Qr2r^)·(r^dr)=Q4πε0R1R2drr2=Q4πε0(1R11R2).

In this equation, the potential difference between the plates is V=(V2V1)=V1V2V=(V2V1)=V1V2. We substitute this result into Equation 8.1 to find the capacitance of a spherical capacitor:

C=QV=4πε0R1R2R2R1.C=QV=4πε0R1R2R2R1.
(8.4)
The cross section of a spherical capacitor is shown in the form of two concentric circles. The radius of the smaller one is R subscript 1 and that of the larger one is R subscript 2. The smaller one has positive signs on it and the larger one has negative signs on it. Arrows radiate outwards from the inner circle to the outer circle. In between the two, is a third circle, with radius r, shown as a dotted line. This is labeled Gaussian surface.
Figure 8.6 A spherical capacitor consists of two concentric conducting spheres. Note that the charges on a conductor reside on its surface.

Example 8.3

Capacitance of an Isolated Sphere Calculate the capacitance of a single isolated conducting sphere of radius R1R1 and compare it with Equation 8.4 in the limit as R2R2.

Strategy We assume that the charge on the sphere is Q, and so we follow the four steps outlined earlier. We also assume the other conductor to be a concentric hollow sphere of infinite radius.

Solution On the outside of an isolated conducting sphere, the electrical field is given by Equation 8.2. The magnitude of the potential difference between the surface of an isolated sphere and infinity is

V=R1+E·dl=Q4πε0R1+1r2r^·(r^dr)=Q4πε0R1+drr2=14πε0QR1.V=R1+E·dl=Q4πε0R1+1r2r^·(r^dr)=Q4πε0R1+drr2=14πε0QR1.

The capacitance of an isolated sphere is therefore

C=QV=Q4πε0R1Q=4πε0R1.C=QV=Q4πε0R1Q=4πε0R1.

Significance The same result can be obtained by taking the limit of Equation 8.4 as R2R2. A single isolated sphere is therefore equivalent to a spherical capacitor whose outer shell has an infinitely large radius.

Check Your Understanding 8.3

The radius of the outer sphere of a spherical capacitor is five times the radius of its inner shell. What are the dimensions of this capacitor if its capacitance is 5.00 pF?

Cylindrical Capacitor

A cylindrical capacitor consists of two concentric, conducting cylinders (Figure 8.7). The inner cylinder, of radius R1R1, may either be a shell or be completely solid. The outer cylinder is a shell of inner radius R2R2. We assume that the length of each cylinder is l and that the excess charges +Q+Q and QQ reside on the inner and outer cylinders, respectively.

Figure shows two concentric cylinders. The inner one, with radius R1 has positive signs on it. The outer one, with radius R2 has negative signs on it. Arrows marked E are shown radiating from the inner one to the outer one. A third cylinder, with radius r, is shown as a dotted line between the two. This is labeled Gaussian surface.
Figure 8.7 A cylindrical capacitor consists of two concentric, conducting cylinders. Here, the charge on the outer surface of the inner cylinder is positive (indicated by ++) and the charge on the inner surface of the outer cylinder is negative (indicated by ).

With edge effects ignored, the electrical field between the conductors is directed radially outward from the common axis of the cylinders. Using the Gaussian surface shown in Figure 8.7, we have

SE·n^dA=E(2πrl)=Qε0.SE·n^dA=E(2πrl)=Qε0.

Therefore, the electrical field between the cylinders is

E=12πε0Qrlr^.E=12πε0Qrlr^.
(8.5)

Here r^r^ is the unit radial vector along the radius of the cylinder. We can substitute into Equation 8.2 and find the potential difference between the cylinders:

V=R1R2E·dlp=Q2πε0lR1R21rr^·(r^dr)=Q2πε0lR1R2drr=Q2πε0llnr|R1R2=Q2πε0llnR2R1.V=R1R2E·dlp=Q2πε0lR1R21rr^·(r^dr)=Q2πε0lR1R2drr=Q2πε0llnr|R1R2=Q2πε0llnR2R1.

Thus, the capacitance of a cylindrical capacitor is

C=QV=2πε0lln(R2/R1).C=QV=2πε0lln(R2/R1).
(8.6)

As in other cases, this capacitance depends only on the geometry of the conductor arrangement. An important application of Equation 8.6 is the determination of the capacitance per unit length of a coaxial cable, which is commonly used to transmit time-varying electrical signals. A coaxial cable consists of two concentric, cylindrical conductors separated by an insulating material. (Here, we assume a vacuum between the conductors, but the physics is qualitatively almost the same when the space between the conductors is filled by a dielectric.) This configuration shields the electrical signal propagating down the inner conductor from stray electrical fields external to the cable. Current flows in opposite directions in the inner and the outer conductors, with the outer conductor usually grounded. Now, from Equation 8.6, the capacitance per unit length of the coaxial cable is given by

Cl=2πε0ln(R2/R1).Cl=2πε0ln(R2/R1).

In practical applications, it is important to select specific values of C/l. This can be accomplished with appropriate choices of radii of the conductors and of the insulating material between them.

Check Your Understanding 8.4

When a cylindrical capacitor is given a charge of 0.500 nC, a potential difference of 20.0 V is measured between the cylinders. (a) What is the capacitance of this system? (b) If the cylinders are 1.0 m long, what is the ratio of their radii?

Several types of practical capacitors are shown in Figure 8.4. Common capacitors are often made of two small pieces of metal foil separated by two small pieces of insulation (see Figure 8.2(b)). The metal foil and insulation are encased in a protective coating, and two metal leads are used for connecting the foils to an external circuit. Some common insulating materials are mica, ceramic, paper, and Teflon™ non-stick coating.

Another popular type of capacitor is an electrolytic capacitor. It consists of an oxidized metal in a conducting paste. The main advantage of an electrolytic capacitor is its high capacitance relative to other common types of capacitors. For example, capacitance of one type of aluminum electrolytic capacitor can be as high as 1.0 F. However, you must be careful when using an electrolytic capacitor in a circuit, because it only functions correctly when the metal foil is at a higher potential than the conducting paste. When reverse polarization occurs, electrolytic action destroys the oxide film. This type of capacitor cannot be connected across an alternating current source, because half of the time, ac voltage would have the wrong polarity, as an alternating current reverses its polarity (see Alternating-Current Circuts on alternating-current circuits).

A variable air capacitor (Figure 8.8) has two sets of parallel plates. One set of plates is fixed (indicated as “stator”), and the other set of plates is attached to a shaft that can be rotated (indicated as “rotor”). By turning the shaft, the cross-sectional area in the overlap of the plates can be changed; therefore, the capacitance of this system can be tuned to a desired value. Capacitor tuning has applications in any type of radio transmission and in receiving radio signals from electronic devices. Any time you tune your car radio to your favorite station, think of capacitance.

A photograph of a device with discrete components is shown. One component is the variable air capacitor. It has two parts, a stator and a rotor. The stator has parallel plates of metal and is fixed to the device. The rotor has parallel plates of metal attached to a shaft. The stator and rotor are arranged in a way that their plates are alternately stacked.
Figure 8.8 In a variable air capacitor, capacitance can be tuned by changing the effective area of the plates. (credit: modification of work by Robbie Sproule)

The symbols shown in Figure 8.9 are circuit representations of various types of capacitors. We generally use the symbol shown in Figure 8.9(a). The symbol in Figure 8.9(c) represents a variable-capacitance capacitor. Notice the similarity of these symbols to the symmetry of a parallel-plate capacitor. An electrolytic capacitor is represented by the symbol in part Figure 8.9(b), where the curved plate indicates the negative terminal.

Figure a shows two vertical lines. Figure b shows a vertical line to the left and another, slightly curved vertical line to the right. Figure c shows two vertical lines and an arrow cutting across them diagonally. In all figures, each line is connected to a horizontal line on the outside.
Figure 8.9 This shows three different circuit representations of capacitors. The symbol in (a) is the most commonly used one. The symbol in (b) represents an electrolytic capacitor. The symbol in (c) represents a variable-capacitance capacitor.

An interesting applied example of a capacitor model comes from cell biology and deals with the electrical potential in the plasma membrane of a living cell (Figure 8.10). Cell membranes separate cells from their surroundings but allow some selected ions to pass in or out of the cell. The potential difference across a membrane is about 70 mV. The cell membrane may be 7 to 10 nm thick. Treating the cell membrane as a nano-sized capacitor, the estimate of the smallest electrical field strength across its ‘plates’ yields the value E=Vd=70×10−3V10×10−9m=7×106V/m>3MV/mE=Vd=70×10−3V10×10−9m=7×106V/m>3MV/m.

This magnitude of electrical field is great enough to create an electrical spark in the air.

The figure shows a cell membrane with negative signs on the inner boundary and positive signs on the outer boundary. Chloride ions are outside the cell. Diffusion moves them toward the cell while Coulomb force is shown pointing outwards. Some chloride ions are shown passing through the membrane to the inside. Potassium ions are shown inside the cell. Diffusion moves them out towards the membrane while Coulomb force is shown pointing inwards. Some potassium ions are shown passing the membrane to the outside. Sodium ions are outside the cell. Both Coulomb force and diffusion are shown pointing towards the cell. Some sodium ions are shown within the cell.
Figure 8.10 The semipermeable membrane of a biological cell has different concentrations of ions on its interior surface than on its exterior. Diffusion moves the K+K+ (potassium) and ClCl (chloride) ions in the directions shown, until the Coulomb force halts further transfer. In this way, the exterior of the membrane acquires a positive charge and its interior surface acquires a negative charge, creating a potential difference across the membrane. The membrane is normally impermeable to Na+ (sodium ions).

Interactive

Visit the PhET Explorations: Capacitor Lab to explore how a capacitor works. Change the size of the plates and add a dielectric to see the effect on capacitance. Change the voltage and see charges built up on the plates. Observe the electrical field in the capacitor. Measure the voltage and the electrical field.

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