# Capacitance

views updated May 18 2018

# Capacitance

The farad, the unit of capacitance

Energy storage in capacitors

Capacitance and alternating current

Capacitance and direct current

Dielectrics

Dielectric strength

Working voltage

Capacitors as a cause of electronics equipment failures

The significance of capacitance

Resources

Capacitance is the capacity of a system of conducting surfaces separated by an insulator to store electrical charge. A device designed to have capacitance is called a capacitor. The voltage between the two conducting parts of a capacitor, and the amount of energy stored in the capacitor, stays constant until the quantity of charge stored is changed. In this sense, a capacitor is akin to a storage battery.

## The farad, the unit of capacitance

The unit of capacitance is the farad (F), in honor of Michael Faradays work with electrostatics. When a 1-farad capacitance store 1 Coulomb the result will be 1 volt. The coulomb (C) is the basic unit of electrical charge, equal to 6.2422× 1018 charges the size carried by an electron or by a proton.

Practical capacitors may have as small a value as a few trillionths of a F or as large as several F.

## Energy storage in capacitors

Work is performed to accumulate charge in a capacitor. Each additional electron stored must overcome the repelling force caused by the charge previously stored. Energy storage increases as the square of the voltage across a capacitor. This often considerable energy can be used later.

Capacitors used as energy reservoirs can deliver very tiny or very powerful pulses of energy, depending on their size. A capacitor can also discharge or recharge rapidly or slowly, depending on the application. Being a passive device, a capacitor can only be recharged by some power source. The power source needs only to be large enough to supply the average energy needed by the charge-discharge cycle of the capacitor in question. Inexpensive audio amplifiers may use large capacitors to provide high power peaks required by occasional loud sounds. Quiet intervals allow the capacitor to recharge before the next power burst.

## Capacitance and alternating current

A capacitor effectively conducts alternating current even though electrons do not cross from one plate to other plate. Alternating current that appears to pass through a capacitor is actually the charge and discharge current resulting from the constantly-changing voltage across the capacitor. AA capacitors opposition to alternating current is called reactance. Higher capacitance introduces less reactance and higher frequencies result in lower reactance. An ideal capacitance appears as a purely reactive (non-resistive) load in a alternating-current circuit. All real-world capacitors, however, have inductance and resistance as well as capacitance.

## Capacitance and direct current

In a direct-current circuit, a series capacitor will permit only a single pulse of charging current when the circuit voltage is changed. The charging current quickly falls to almost zero as a capacitor charges from a constant-voltage source. Capacitors are sometimes used in circuits to oppose direct current. They may block direct current while simultaneously passing a superimposed alternating currents. A blocking capacitor is commonly used to separate alternating and direct current components.

## Dielectrics

Dielectrics are the insulating materials used between the conducting plates of capacitors. Dielectrics increase capacitance or provide better insulation between the plates. Dielectrics materials exhibit very little ability to conduct electric charge. Mylar, paper, mica, and ceramics are commonly-used dielectrics. When extremely high capacitance is required, a thin film of aluminum oxide on etched aluminum plates is used as a dielectric.

Dielectrics have a property called polarizability. A dielectric placed within an electric field appears to have electric charge on its surfaces even though the insulator remains electrically neutral. Each of the dielectrics molecules is stretched when the electric field causes its negative charges to be pulled toward the positive-charged capacitor plate and the molecules positive charges are pulled toward the negative plate. This polarization strain causes each dielectric molecule to act as a voltage source. These voltages add in series aiding as do the voltage from several cells making up the battery in a flashlight. A phantom charge appears on each surface of the dielectric, canceling much of the electric field produced by the real charges. The greater the polarization developed by a dielectric, the larger the quantity of real charge the capacitor must store to develop a given voltage. The capacitance appears to increase as a result of dielectric polarization.

The capacitance multiplier for any dielectric is called its dielectric constant. The dielectric constant of a perfect vacuum is defined as exactly 1. Common dielectrics have dielectrics constants in the range of

### KEY TERMS

Alternating current Electric current that flows first in one direction, then in the other; abbreviated AC.

Direct current (DC) Electrical current that always flows in the same direction.

Electric field The concept used to describe how one electric charge exerts force on another, distant electric charge.

Electron A negatively charged particle, ordinarily occurring as part of an atom. The atoms electrons form a sort of cloud about the nucleus.

Farad The unit of capacitance, equal to 1 Volt per Coulomb.

Neutral No net charge, when positive and negative charges cancel.

Open circuit A physical break in a circuit path that stops the current.

Polarizability Possible asymmetrical charge distribution in a molecule.

Power supply A source of electrical energy used to supply a circuit.

Proton The positively-charged particle in atoms.

Short circuit Unwanted bypass of the expected current path in a circuit.

Voltage Ratio of electrical potential energy to the quantity of charge.

2 to 4. Using a higher quality dielectric increases the capacitance by a factor equal to the dielectric constant.

## Dielectric strength

Dielectric strength is the measure of a dielectrics ability to resist electric stress without losing its insulating capabilities. A high dielectric constant does not always correspond to high dielectric strength. Distilled water has a fairly high dielectric constant, but it has poor dielectric strength. Water, therefore, is not a useful dielectric for capacitors because it breaks down too easily. Some ceramics have dielectric constants as high as 10,000. These materials would be extremely valuable if they had better dielectric strength.

## Working voltage

If the voltage across a capacitor is increased until charges jump from one plate to the other, the capacitor will probably fail, either momentarily or permanently. Capacitors are rated to specify the maximum continuous voltage that can be applied across the dielectric before the capacitor will fail.

## Capacitors as a cause of electronics equipment failures

Failed capacitors are a common cause of electronic-equipment breakdowns. When a capacitors dielectric is destroyed the resulting short circuit may cause other components to fail. Capacitors also develop open circuits, causing the loss of the capacitance.

Electrolytic capacitors are generally less reliable than other types, a tradeoff made to secure very high capacitance in a small package. They tend to fail if stored without a voltage across their terminals. The electrolytic paste may dry in time, causing a loss of capacitance. Experienced electronic technicians consider electrolytic capacitor failures as a likely cause of an equipment fault that is not otherwise immediately obvious.

## The significance of capacitance

Capacitance, inductance, and resistance are the passive electrical properties in electrical circuits. Understanding capacitance is an essential part of the study of electricity and electronics.

## Resources

### BOOKS

McWhorter, Gene, and Alvis J. Evans. Basic Electronics. Lincolnwood, IL: Master Publishing, Inc., 2004.

Myers, Rusty. The Basics of Physics. San Francisco: Greenwood Press, 2005.

Donald Beaty

# Capacitance

views updated May 17 2018

# Capacitance

Capacitance is an electrical effect that opposes change in voltage between conducting surfaces separated by an insulator. Capacitance stores electrical energy when electrons are attracted to nearby but separate surfaces. The voltage across an unchanging capacitance value will stay constant unless the quantity of charge stored is changed.

## The Farad, the unit of capacitance

The unit of capacitance is the Farad, in honor of Michael Faraday's work with electrostatics. When a 1-Farad capacitance store 1 Coulomb the result will be 1 volt. The Coulomb is the basic unit of electrical charge, equal to 6.2422 × 1018 charges the size carried by an electron or by a proton .

An electrical component that introduces capacitance is called a capacitor . Practical capacitors may have as small a value as a few trillionths of a Farad or as large as several Farads.

## Energy storage in capacitors

Work is performed to accumulate charge in a capacitor. Each additional electron stored must overcome the repelling force caused by the charge previously stored. Energy storage increases as the square of the voltage across a capacitor. This often considerable energy can be used later.

Capacitors used as energy reservoirs can deliver powerful pulses of energy. A capacitor can discharge quickly then slowly recharge until the next power demand. The power source needs only to be large enough to supply the average energy. Inexpensive audio amplifiers often use large capacitors to provide high power peaks required by occasional loud sounds. Quiet intervals allow the capacitor to recharge before the next power burst.

## Capacitance and alternating current

A capacitor effectively conducts alternating current even though electrons do not cross from one plate to other plate. Alternating current that appears to pass through a capacitor is actually, the charge and discharge current resulting from the constantly-changing voltage across the capacitor.

An uncharged capacitor always appears as a short circuit because its voltage must equal zero when its stored charge is zero. A capacitor carrying an alternating current continually charges and discharges, spending much of the time in a near-zero charge state. The resulting low voltage across its terminals means that it is often less significant in limiting circuit than other components in the circuit.

A capacitor's opposition to alternating current is called reactance. Higher capacitance introduces less reactance and higher frequencies result in lower reactance.

## Capacitance and direct current

In a direct-current circuit a series capacitor will permit only a single pulse of charging current when the circuit voltage is changed. The charging current in quickly falls to almost zero as a capacitor charges from a constant-voltage source. Capacitors are sometimes used in circuits to oppose direct current. They may block direct current while simultaneously passing a superimposed alternating currents. A blocking capacitor is commonly used to separate alternating and direct current components.

## Dielectrics

Dielectrics are the insulating materials used between the conducting plates of capacitors. Dielectrics increase capacitance or provide better insulation between the plates. Dielectrics materials exhibit very little ability to conduct electric charge . Mylar, paper , mica, and ceramics are commonly-used dielectrics. When extremely-high capacitance is required, a thin film of aluminum oxide on etched aluminum plates is used as a dielectric.

Dielectrics have a property called polarizability. A dielectric placed within an electric field appears to have electric charge on its surfaces even though the insulator remains electrically neutral. Each of the dielectric's molecules is stretched when the electric field causes its negative charges to be pulled toward the positive-charged capacitor plate and the molecule's positive charges are pulled toward the negative plate. This polarization strain causes each dielectric molecule to act as a voltage source. These voltages add in series aiding as do the voltage from several cells making up the battery in a flashlight. A phantom charge appears on each surface of the dielectric canceling much of the electric field produced by the real charges. The greater the polarization developed by a dielectric the larger the quantity of real charge the capacitor must store to develop a given voltage. The capacitance appears to increase as a result of dielectric polarization.

The capacitance multiplier for any dielectric is called its dielectric constant. The dielectric constant of a perfect vacuum is defined as exactly 1. Common dielectrics have dielectrics constants in the range of 2-4. Using a higher quality dielectric increases the capacitance by a factor equal to the dielectric constant.

## Dielectric strength

Dielectric strength is the measure of a dielectric's ability to resist electric stress without losing its insulating capabilities. A high dielectric constant does not always correspond to high dielectric strength. Distilled water has a fairly high dielectric constant but it has poor dielectric strength. Water, therefore, is not a useful dielectric for capacitors because it breaks down too easily. Some ceramics have dielectric constants as high as 10,000. These materials would be extremely valuable if they had better dielectric strength.

## Working voltage

If the voltage across a capacitor is increased until charges jump from one plate to the other, the capacitor will probably fail, either momentarily or permanently. Capacitors are rated to specify the maximum continuous voltage that can be applied across the dielectric before the capacitor will fail.

## Capacitors as a cause of electronics equipment failures

Failed capacitors are a common cause of electronic-equipment breakdowns. When a capacitor's dielectric is destroyed the resulting short circuit may cause other components to fail. Capacitors also develop open circuits, causing the loss of the capacitance.

Electrolytic capacitors are generally less reliable than other types, a tradeoff made to secure very-high capacitance in a small package. They tend to fail if stored without a voltage across their terminals. The electrolytic paste may dry in time, causing a loss of capacitance. Experienced electronic technicians consider electrolytic capacitor failures as a likely cause of an equipment fault that is not otherwise immediately obvious.

## The significance of capacitance

Capacitance, inductance, and resistance are the passive electrical properties affecting electrical circuits. Understanding capacitance is an essential part of the study of electricity and electronics .

## Resources

### books

Asimov, Isaac. Understanding Physics: Light, Magnetism, andElectricity. Vol. II. Signet Books, The New American Library.

Bord, Donald J., and Vern J. Ostdiek. Inquiry Into Physics. 3rd ed. West Publishing Company, 1995.

Sear, Zemansky, and Young. College Physics. 6th ed. Addison-Wesley Publishing Company, 1985.

Donald Beaty

## KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alternating current

—Electric current that flows first in one direction, then in the other; abbreviated AC.

Direct current (DC)

—Electrical current that always flows in the same direction.

Electric field

—The concept used to describe how one electric charge exerts force on another, distant electric charge.

Electron

—A negatively charged particle, ordinarily occurring as part of an atom. The atom's electrons form a sort of cloud about the nucleus.

—The unit of capacitance, equal to 1 Volt per Coulomb.

Neutral

—No net charge, when positive and negative charges cancel.

Open circuit

—A physical break in a circuit path that stops the current.

Polarizability

—Possible asymmetrical charge distribution in a molecule.

Power supply

—A source of electrical energy used to supply a circuit.

Proton

—The positively-charged particle in atoms.

Short circuit

—Unwanted bypass of the expected current path in a circuit.

Voltage

—Ratio of electrical potential energy to the quantity of charge.

# capacitance

views updated May 14 2018

ca·pac·i·tance / kəˈpasitəns/ • n. Physics the ability of a system to store an electric charge. ∎  the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. (Symbol: C)

# capacitance

views updated May 21 2018

capacitance (symbol C) Property of an electrical circuit or component that describes its ability to store charge in its capacitor. Capacitance is measured in farads: one farad is a capacitance needing a charge of 1 coulomb to raise its potential by 1 volt. Most capacitances are small enough to be measured in microfarads (one millionth of a farad).