Capacitance

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Common symbol(s):C
SI unit:farad
 
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Common symbol(s):C
SI unit:farad

Capacitance is the ability of a body to store an electrical charge. Any object that can be electrically charged exhibits capacitance. A common form of energy storage device is a parallel-plate capacitor. In a parallel plate capacitor, capacitance is directly proportional to the surface area of the conductor plates and inversely proportional to the separation distance between the plates. If the charges on the plates are +q and −q, and V gives the voltage between the plates, then the capacitance C is given by

C={\frac  {q}{V}}.

which gives the voltage/current relationship

I(t)=C{\frac  {{\mathrm  {d}}V(t)}{{\mathrm  {d}}t}}.

The capacitance is a function only of the physical dimensions (geometry) of the conductors and the permittivity of the dielectric. It is independent of the potential difference between the conductors and the total charge on them.

The SI unit of capacitance is the farad (symbol: F), named after the English physicist Michael Faraday; a 1 farad capacitor when charged with 1 coulomb of electrical charge will have a potential difference of 1 volt between its plates.[1] Historically, a farad was regarded as an inconveniently large unit, both electrically and physically. Its subdivisions were invariably used, namely the microfarad, nanofarad and picofarad. More recently, technology has advanced such that capacitors of 1 farad and greater can be constructed in a structure little larger than a coin battery (so-called 'supercapacitors'). Such capacitors are principally used for energy storage replacing more traditional batteries.

The energy (measured in joules) stored in a capacitor is equal to the work done to charge it. Consider a capacitor of capacitance C, holding a charge +q on one plate and −q on the other. Moving a small element of charge dq from one plate to the other against the potential difference V = q/C requires the work dW:

{\mathrm  {d}}W={\frac  {q}{C}}\,{\mathrm  {d}}q

where W is the work measured in joules, q is the charge measured in coulombs and C is the capacitance, measured in farads.

The energy stored in a capacitor is found by integrating this equation. Starting with an uncharged capacitance (q = 0) and moving charge from one plate to the other until the plates have charge +Q and −Q requires the work W:

W_{{\text{charging}}}=\int _{0}^{Q}{\frac  {q}{C}}\,{\mathrm  {d}}q={\frac  {1}{2}}{\frac  {Q^{2}}{C}}={\frac  {1}{2}}QV={\frac  {1}{2}}CV^{2}=W_{{\text{stored}}}.

Capacitors[edit]

The capacitance of the majority of capacitors used in electronic circuits is generally several orders of magnitude smaller than the farad. The most common subunits of capacitance in use today are the microfarad (µF), nanofarad (nF), picofarad (pF), and, in microcircuits, femtofarad (fF). However, specially made supercapacitors can be much larger (as much as hundreds of farads), and parasitic capacitive elements can be less than a femtofarad.

Capacitance can be calculated if the geometry of the conductors and the dielectric properties of the insulator between the conductors are known. For example, the capacitance of a parallel-plate capacitor constructed of two parallel plates both of area A separated by a distance d is approximately equal to the following:

\ C=\varepsilon _{r}\varepsilon _{0}{\frac  {A}{d}}

where

C is the capacitance, in Farads;
A is the area of overlap of the two plates, in square meters;
εr is the relative static permittivity (sometimes called the dielectric constant) of the material between the plates (for a vacuum, εr = 1);
ε0 is the electric constant (ε0 ≈ 8.854×10−12 F m–1); and
d is the separation between the plates, in meters;

Capacitance is proportional to the area of overlap and inversely proportional to the separation between conducting sheets. The closer the sheets are to each other, the greater the capacitance. The equation is a good approximation if d is small compared to the other dimensions of the plates so the field in the capacitor over most of its area is uniform, and the so-called fringing field around the periphery provides a small contribution. In CGS units the equation has the form:[2]

C=\varepsilon _{r}{\frac  {A}{4\pi d}}

where C in this case has the units of length. Combining the SI equation for capacitance with the above equation for the energy stored in a capacitance, for a flat-plate capacitor the energy stored is:

W_{{\text{stored}}}={\frac  {1}{2}}CV^{2}={\frac  {1}{2}}\varepsilon _{{r}}\varepsilon _{{0}}{\frac  {A}{d}}V^{2}.

where W is the energy, in joules; C is the capacitance, in farads; and V is the voltage, in volts.

Voltage-dependent capacitors[edit]

The dielectric constant for a number of very useful dielectrics changes as a function of the applied electrical field, for example ferroelectric materials, so the capacitance for these devices is more complex. For example, in charging such a capacitor the differential increase in voltage with charge is governed by:

\ dQ=C(V)\,dV

where the voltage dependence of capacitance, C(V), stems from the field, which in a large area parallel plate device is given by ε = V/d. This field polarizes the dielectric, which polarization, in the case of a ferroelectric, is a nonlinear S-shaped function of field, which, in the case of a large area parallel plate device, translates into a capacitance that is a nonlinear function of the voltage causing the field.[3][4]

Corresponding to the voltage-dependent capacitance, to charge the capacitor to voltage V an integral relation is found:

Q=\int _{0}^{V}C(V)\,dV\

which agrees with Q = CV only when C is voltage independent.

By the same token, the energy stored in the capacitor now is given by

dW=Q\,dV=\left[\int _{0}^{V}\ dV'\ C(V')\right]\ dV\ .

Integrating:

W=\int _{0}^{V}\ dV\ \int _{0}^{V}\ dV'\ C(V')=\int _{0}^{V}\ dV'\ \int _{{V'}}^{V}\ dV\ C(V')=\int _{0}^{V}\ dV'\left(V-V'\right)C(V')\ ,

where interchange of the order of integration is used.

The nonlinear capacitance of a microscope probe scanned along a ferroelectric surface is used to study the domain structure of ferroelectric materials.[5]

Another example of voltage dependent capacitance occurs in semiconductor devices such as semiconductor diodes, where the voltage dependence stems not from a change in dielectric constant but in a voltage dependence of the spacing between the charges on the two sides of the capacitor.[6] This effect is intentionally exploited in diode-like devices known as varicaps.

Frequency-dependent capacitors[edit]

If a capacitor is driven with a time-varying voltage that changes rapidly enough, then the polarization of the dielectric cannot follow the signal. As an example of the origin of this mechanism, the internal microscopic dipoles contributing to the dielectric constant cannot move instantly, and so as frequency of an applied alternating voltage increases, the dipole response is limited and the dielectric constant diminishes. A changing dielectric constant with frequency is referred to as dielectric dispersion, and is governed by dielectric relaxation processes, such as Debye relaxation. Under transient conditions, the displacement field can be expressed as (see electric susceptibility):

{\boldsymbol  {D(t)}}=\varepsilon _{0}\int _{{-\infty }}^{t}\ \varepsilon _{r}(t-t'){\boldsymbol  E}(t')\ dt',

indicating the lag in response by the time dependence of εr, calculated in principle from an underlying microscopic analysis, for example, of the dipole behavior in the dielectric. See, for example, linear response function.[7][8] The integral extends over the entire past history up to the present time. A Fourier transform in time then results in:

{\boldsymbol  D}(\omega )=\varepsilon _{0}\varepsilon _{r}(\omega ){\boldsymbol  E}(\omega )\ ,

where εr(ω) is now a complex function, with an imaginary part related to absorption of energy from the field by the medium. See permittivity. The capacitance, being proportional to the dielectric constant, also exhibits this frequency behavior. Fourier transforming Gauss's law with this form for displacement field:

I(\omega )=j\omega Q(\omega )=j\omega \oint _{{\Sigma }}{\boldsymbol  D}({\boldsymbol  r},\ \omega )\cdot d{\boldsymbol  {\Sigma }}\
=\left[G(\omega )+j\omega C(\omega )\right]V(\omega )={\frac  {V(\omega )}{Z(\omega )}}\ ,

where j is the imaginary unit, V(ω) is the voltage component at angular frequency ω, G(ω) is the real part of the current, called the conductance, and C(ω) determines the imaginary part of the current and is the capacitance. Z(ω) is the complex impedance.

When a parallel-plate capacitor is filled with a dielectric, the measurement of dielectric properties of the medium is based upon the relation:

\varepsilon _{r}(\omega )=\varepsilon '_{r}(\omega )-j\varepsilon ''_{r}(\omega )={\frac  {1}{j\omega Z(\omega )C_{0}}}={\frac  {C(\omega )}{C_{0}}}\ ,

where a single prime denotes the real part and a double prime the imaginary part, Z(ω) is the complex impedance with the dielectric present, C(ω) is the so-called complex capacitance with the dielectric present, and C0 is the capacitance without the dielectric.[9][10] (Measurement "without the dielectric" in principle means measurement in free space, an unattainable goal inasmuch as even the quantum vacuum is predicted to exhibit nonideal behavior, such as dichroism. For practical purposes, when measurement errors are taken into account, often a measurement in terrestrial vacuum, or simply a calculation of C0, is sufficiently accurate.[11])

Using this measurement method, the dielectric constant may exhibit a resonance at certain frequencies corresponding to characteristic response frequencies (excitation energies) of contributors to the dielectric constant. These resonances are the basis for a number of experimental techniques for detecting defects. The conductance method measures absorption as a function of frequency.[12] Alternatively, the time response of the capacitance can be used directly, as in deep-level transient spectroscopy.[13]

Another example of frequency dependent capacitance occurs with MOS capacitors, where the slow generation of minority carriers means that at high frequencies the capacitance measures only the majority carrier response, while at low frequencies both types of carrier respond.[14][15]

At optical frequencies, in semiconductors the dielectric constant exhibits structure related to the band structure of the solid. Sophisticated modulation spectroscopy measurement methods based upon modulating the crystal structure by pressure or by other stresses and observing the related changes in absorption or reflection of light have advanced our knowledge of these materials.[16]

Capacitance matrix[edit]

The discussion above is limited to the case of two conducting plates, although of arbitrary size and shape. The definition C=Q/V still holds for a single plate given a charge, in which case the field lines produced by that charge terminate as if the plate were at the center of an oppositely charged sphere at infinity.

C=Q/V does not apply when there are more than two charged plates, or when the net charge on the two plates is non-zero. To handle this case, Maxwell introduced his coefficients of potential. If three plates are given charges Q_{1},Q_{2},Q_{3}, then the voltage of plate 1 is given by

V_{1}=P_{{11}}Q_{1}+P_{{12}}Q_{2}+P_{{13}}Q_{3},

and similarly for the other voltages. Hermann von Helmholtz and Sir William Thomson showed that the coefficients of potential are symmetric, so that P_{{12}}=P_{{21}}, etc. Thus the system can be described by a collection of coefficients known as the elastance matrix or reciprocal capacitance matrix, which is defined as:

P_{{ij}}={\frac  {\partial V_{{i}}}{\partial Q_{{j}}}}

From this, the mutual capacitance C_{{m}} between two objects can be defined[17] by solving for the total charge Q and using C_{{m}}=Q/V.

C_{m}={\frac  {1}{(P_{{11}}+P_{{22}})-(P_{{12}}+P_{{21}})}}

Since no actual device holds perfectly equal and opposite charges on each of the two "plates", it is the mutual capacitance that is reported on capacitors.

The collection of coefficients C_{{ij}}={\frac  {\partial Q_{{i}}}{\partial V_{{j}}}} is known as the capacitance matrix,[18][19] and is the inverse of the elastance matrix.

Self-capacitance[edit]

In electrical circuits, the term capacitance is usually a shorthand for the mutual capacitance between two adjacent conductors, such as the two plates of a capacitor. However, for an isolated conductor there also exists a property called self-capacitance, which is the amount of electrical charge that must be added to an isolated conductor to raise its electrical potential by one unit (i.e. one volt, in most measurement systems).[20] The reference point for this potential is a theoretical hollow conducting sphere, of infinite radius, centered on the conductor. Using this method, the self-capacitance of a conducting sphere of radius R is given by:[21]

C=4\pi \varepsilon _{0}R\,

Example values of self-capacitance are:

The capacitative component of a coil, which reduces its impedance at high frequencies and can lead to resonance and self-oscillation, is also called self-capacitance[23] as well as stray or parasitic capacitance.

Elastance[edit]

The reciprocal of capacitance is called elastance. The unit of elastance is the daraf, but is not recognised by SI.

Stray capacitance[edit]

Any two adjacent conductors can be considered a capacitor, although the capacitance will be small unless the conductors are close together for long distances or over a large area. This (often unwanted) effect is termed "stray capacitance". Stray capacitance can allow signals to leak between otherwise isolated circuits (an effect called crosstalk), and it can be a limiting factor for proper functioning of circuits at high frequency.

Stray capacitance is often encountered in amplifier circuits in the form of "feedthrough" capacitance that interconnects the input and output nodes (both defined relative to a common ground). It is often convenient for analytical purposes to replace this capacitance with a combination of one input-to-ground capacitance and one output-to-ground capacitance; the original configuration — including the input-to-output capacitance — is often referred to as a pi-configuration. Miller's theorem can be used to effect this replacement: it states that, if the gain ratio of two nodes is 1/K, then an impedance of Z connecting the two nodes can be replaced with a Z/(1 − k) impedance between the first node and ground and a KZ/(K − 1) impedance between the second node and ground. Since impedance varies inversely with capacitance, the internode capacitance, C, will be seen to have been replaced by a capacitance of KC from input to ground and a capacitance of (K − 1)C/K from output to ground. When the input-to-output gain is very large, the equivalent input-to-ground impedance is very small while the output-to-ground impedance is essentially equal to the original (input-to-output) impedance.

Capacitance of simple systems[edit]

Calculating the capacitance of a system amounts to solving the Laplace equation 2φ = 0 with a constant potential φ on the surface of the conductors. This is trivial in cases with high symmetry. There is no solution in terms of elementary functions in more complicated cases.

For quasi-two-dimensional situations analytic functions may be used to map different geometries to each other. See also Schwarz–Christoffel mapping.

Capacitance of simple systems
TypeCapacitanceComment
Parallel-plate capacitor\varepsilon A/dPlate CapacitorII.svg

ε: Permittivity

Coaxial cable{\frac  {2\pi \varepsilon l}{\ln \left(R_{{2}}/R_{{1}}\right)}}Cylindrical CapacitorII.svg

ε: Permittivity

Pair of parallel wires[24]{\frac  {\pi \varepsilon l}{\operatorname {arcosh}\left({\frac  {d}{2a}}\right)}}={\frac  {\pi \varepsilon l}{\ln \left({\frac  {d}{2a}}+{\sqrt  {{\frac  {d^{{2}}}{4a^{{2}}}}-1}}\right)}}Parallel Wire Capacitance.svg
Wire parallel to wall[24]{\frac  {2\pi \varepsilon l}{\operatorname {arcosh}\left({\frac  {d}{a}}\right)}}={\frac  {2\pi \varepsilon l}{\ln \left({\frac  {d}{a}}+{\sqrt  {{\frac  {d^{{2}}}{a^{{2}}}}-1}}\right)}}a: Wire radius
d: Distance, d > a
l: Wire length
Two parallel
coplanar strips[25]
\varepsilon l{\frac  {K\left({\sqrt  {1-k^{{2}}}}\right)}{K\left(k\right)}}d: Distance
w1, w2: Strip width
km: d/(2wm+d)

k2: k1k2
K: Elliptic integral
l: Length

Concentric spheres{\frac  {4\pi \varepsilon }{{\frac  {1}{R_{1}}}-{\frac  {1}{R_{2}}}}}Spherical Capacitor.svg

ε: Permittivity

Two spheres,
equal radius[26][27]
2\pi \varepsilon a\sum _{{n=1}}^{{\infty }}{\frac  {\sinh \left(\ln \left(D+{\sqrt  {D^{2}-1}}\right)\right)}{\sinh \left(n\ln \left(D+{\sqrt  {D^{2}-1}}\right)\right)}}
=2\pi \varepsilon a\left\{1+{\frac  {1}{2D}}+{\frac  {1}{4D^{2}}}+{\frac  {1}{8D^{{3}}}}+{\frac  {1}{8D^{{4}}}}+{\frac  {3}{32D^{{5}}}}+O\left({\frac  {1}{D^{{6}}}}\right)\right\}
=2\pi \varepsilon a\left\{\ln 2+\gamma -{\frac  {1}{2}}\ln \left({\frac  {d}{a}}-2\right)+O\left({\frac  {d}{a}}-2\right)\right\}
a: Radius
d: Distance, d > 2a
D = d/2a
γ: Euler's constant
Sphere in front of wall[26]4\pi \varepsilon a\sum _{{n=1}}^{{\infty }}{\frac  {\sinh \left(\ln \left(D+{\sqrt  {D^{{2}}-1}}\right)\right)}{\sinh \left(n\ln \left(D+{\sqrt  {D^{{2}}-1}}\right)\right)}}a: Radius
d: Distance, d > a
D = d/a
Sphere4\pi \varepsilon aa: Radius
Circular disc[28]8\varepsilon aa: Radius
Thin straight wire,
finite length[29][30][31]
{\frac  {2\pi \varepsilon l}{\Lambda }}\left\{1+{\frac  {1}{\Lambda }}\left(1-\ln 2\right)+{\frac  {1}{\Lambda ^{{2}}}}\left[1+\left(1-\ln 2\right)^{{2}}-{\frac  {\pi ^{{2}}}{12}}\right]+O\left({\frac  {1}{\Lambda ^{{3}}}}\right)\right\}a: Wire radius
l: Length
Λ: ln(l/a)

See also[edit]

References[edit]

  1. ^ http://www.collinsdictionary.com/dictionary/english/farad
  2. ^ The Physics Problem Solver, 1986, Google books link
  3. ^ Carlos Paz de Araujo, Ramamoorthy Ramesh, George W Taylor (Editors) (2001). Science and Technology of Integrated Ferroelectrics: Selected Papers from Eleven Years of the Proceedings of the International Symposium on Integrated Ferroelectrics. CRC Press. Figure 2, p. 504. ISBN 90-5699-704-1. 
  4. ^ Solomon Musikant (1991). What Every Engineer Should Know about Ceramics. CRC Press. Figure 3.9, p. 43. ISBN 0-8247-8498-7. 
  5. ^ Yasuo Cho (2005). Scanning Nonlinear Dielectric Microscope (in Polar Oxides; R Waser, U Böttger & S Tiedke, editors ed.). Wiley-VCH. Chapter 16. ISBN 3-527-40532-1. 
  6. ^ Simon M. Sze, Kwok K. Ng (2006). Physics of Semiconductor Devices (3rd Edition ed.). Wiley. Figure 25, p. 121. ISBN 0-470-06830-2. 
  7. ^ Gabriele Giuliani, Giovanni Vignale (2005). Quantum Theory of the Electron Liquid. Cambridge University Press. p. 111. ISBN 0-521-82112-6. 
  8. ^ Jørgen Rammer (2007). Quantum Field Theory of Non-equilibrium States. Cambridge University Press. p. 158. ISBN 0-521-87499-8. 
  9. ^ Horst Czichos, Tetsuya Saito, Leslie Smith (2006). Springer Handbook of Materials Measurement Methods. Springer. p. 475. ISBN 3-540-20785-6. 
  10. ^ William Coffey, Yu. P. Kalmykov (2006). Fractals, diffusion and relaxation in disordered complex systems..Part A. Wiley. p. 17. ISBN 0-470-04607-4. 
  11. ^ J. Obrzut, A. Anopchenko and R. Nozaki, "Broadband Permittivity Measurements of High Dielectric Constant Films", Proceedings of the IEEE: Instrumentation and Measurement Technology Conference, 2005, pp. 1350–1353, 16–19 May 2005, Ottawa ISBN 0-7803-8879-8 doi:10.1109/IMTC.2005.1604368
  12. ^ Dieter K Schroder (2006). Semiconductor Material and Device Characterization (3rd Edition ed.). Wiley. p. 347 ff. ISBN 0-471-73906-5. 
  13. ^ Dieter K Schroder (2006). Semiconductor Material and Device Characterization (3rd Edition ed.). Wiley. p. 270 ff. ISBN 0-471-73906-5. 
  14. ^ Simon M. Sze, Kwok K. Ng (2006). Physics of Semiconductor Devices (3rd Edition ed.). Wiley. p. 217. ISBN 0-470-06830-2. 
  15. ^ Safa O. Kasap, Peter Capper (2006). Springer Handbook of Electronic and Photonic Materials. Springer. Figure 20.22, p. 425. 
  16. ^ PY Yu and Manuel Cardona (2001). Fundamentals of Semiconductors (3rd Edition ed.). Springer. p. §6.6 Modulation Spectroscopy. ISBN 3-540-25470-6. 
  17. ^ Jackson, John David (1999), Classical Electrodynamic (3rd. ed.), USA: John Wiley & Sons, Inc., p. 43, ISBN 978-0-471-30932-1 
  18. ^ Maxwell, James (1873), "3", A treatise on electricity and magnetism, Volume 1, Clarendon Press, pp. 88ff 
  19. ^ "Capacitance". Retrieved 20 September 2010. 
  20. ^ William D. Greason (1992). Electrostatic discharge in electronics. Research Studies Press. p. 48. ISBN 978-0-86380-136-5. Retrieved 4 December 2011. 
  21. ^ Lecture notes; University of New South Wales
  22. ^ Tipler, Paul; Mosca, Gene (2004), Physics for scientists and engineers (5th ed.), Macmillan, p. 752, ISBN 978-0-7167-0810-0 
  23. ^ Massarini, A.; Kazimierczuk, M.K. (1997). "Self-capacitance of inductors". IEEE Transactions on Power Electronics 12 (4): 671–676. doi:10.1109/63.602562. : example of use of term self-capacitance
  24. ^ a b Jackson, J. D. (1975). Classical Electrodynamics. Wiley. p. 80. 
  25. ^ Binns; Lawrenson (1973). Analysis and computation of electric and magnetic field problems. Pergamon Press. ISBN 978-0-08-016638-4. 
  26. ^ a b Maxwell, J. C. (1873). A Treatise on Electricity and Magnetism. Dover. pp. 266 ff. ISBN 0-486-60637-6. 
  27. ^ Rawlins, A. D. (1985). "Note on the Capacitance of Two Closely Separated Spheres". IMA Journal of Applied Mathematics 34 (1): 119–120. doi:10.1093/imamat/34.1.119. 
  28. ^ Jackson, J. D. (1975). Classical Electrodynamics. Wiley. p. 128, problem 3.3. 
  29. ^ Maxwell, J. C. (1878). "On the electrical capacity of a long narrow cylinder and of a disk of sensible thickness". Proc. London Math. Soc. IX: 94–101. doi:10.1112/plms/s1-9.1.94. 
  30. ^ Vainshtein, L. A. (1962). "Static boundary problems for a hollow cylinder of finite length. III Approximate formulas". Zh. Tekh. Fiz. 32: 1165–1173. 
  31. ^ Jackson, J. D. (2000). "Charge density on thin straight wire, revisited". Am. J. Phys 68 (9): 789–799. Bibcode:2000AmJPh..68..789J. doi:10.1119/1.1302908. 

Further reading[edit]

  • Tipler, Paul (1998). Physics for Scientists and Engineers: Vol. 2: Electricity and Magnetism, Light (4th ed.). W. H. Freeman. ISBN 1-57259-492-6
  • Serway, Raymond; Jewett, John (2003). Physics for Scientists and Engineers (6 ed.). Brooks Cole. ISBN 0-534-40842-7
  • Saslow, Wayne M.(2002). Electricity, Magnetism, and Light. Thomson Learning. ISBN 0-12-619455-6. See Chapter 8, and especially pp. 255–259 for coefficients of potential.