  # Capacitance An RC circuit is in fact an open circuit. However, when the switch (S) is closed, electrons will rush toward one plate of the capacitor charging gradually the capacitor up to a maximum value (Q max).   When the capacitor is fully charged, the current through the circuit drops to zero.

 Q:    "Why is the total (equivalent) capacitance of a series connection always smaller than each of the capacitors connected in series?" A:  Look at a simple example when only two capacitors are connected in series. Then, the formula for the total (equivalent) capacitance is:   You can verify that Ceq is less than any of the capacitors connected in series by trying the case of three capacitors, or four capacitors, etc.   Do the algebra correctly, and you will always obtain Ceq expressed as the product of any of the individual capacitors and a factor that is less than 1 (a ratio with the denominator larger than the numerator). Q:     What does an electrical capacitor store? A:     An electrical capacitor stores a "charge Q."  An electrical capacitor is defined as two conducting components separated from each other and charged with different amounts of charge, Q1 and Q2.  The "charge stored in a capacitor", Q, is the charge imbalance between the two components: Q = Q =  Q2 - Q1. It is also correct to say that an electrical capacitor stores energy.  Since the two conducting components of the capacitor are charged with different amounts of charge, Q1 and Q2,   an electric field exists between them, and an electric field stores energy within the space in which the electric field exists (within the volume in space defined by the geometric arrangement, with respect to each other, of the two conducting components of the capacitor.) Q:     Many electronic devices carry a warning that removing the case, even with the power turned off, may cause electric shock.  Does this have anything to do with capacitors?  Why don't they discharge as soon as the power is cut? A:     When the power of the circuit is turned off, capacitors may remain charged because not all electrodes are grounded.  If one touches the charged electrode one receives an electric shock caused by the discharge of this electrode through the human body. Q:     Is it possible to break a capacitor by placing to much charge on it? A:     Yes.   To place too much "charge" on a capacitor means to establish a too high potential difference, V, between its conducting components.  If this potential difference is above what is called "breakage potential," an electric discharge results, destroying the capacitor.  If the capacitor contains an insulator between its conducting components, the insulator will be "burnt" at the atomic/molecular level.  A capacitor subjected to voltages exceeding the "breakage potential" cannot be recovered.  It should be discarded and replaced.  The value of  the "breakage potential" is given on a label attached to commercially available capacitors.

## References

### Equations

 Capacitance C = q/V Capacitance for a parallel-plate capacitor Capacitance for a cylindrical capacitor Capacitance for a spherical capacitor Capacitance of an isolated sphere C = 4  0R Equivalent capacitance of capacitors connected in parallel Equivalent capacitance of capacitors connected in series Electric potential energy of a charge capacitor Energy density (potential energy per unit volume) u =  0 E2 Gauss' Law with a dielectric # List of Topics

 Measurements Electric Potential Magnetism Electrical Circuits (AC) Optical Instruments: Mirrors and Lenses Electrostatics Capacitance Sources of Magnetic Fields Maxwell's Equations Interference Electric Fields Current and Resistance Magnetism in Matter Electromagnetic Waves Diffraction Electric Flux Electrical Circuits (DC) Electromagnetic Induction Interaction of Radiation with Matter: Reflection, Refraction, Polarization  ครั้งที่

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