Capacitance

Device that stores electric charge.

Construction:

A capacitor is two conducting plates separated by a finite distance

Typically separated by an insulator or dielectric.

Capacitor plates carry charges of the same magnitude but opposite charges.

Capacitor

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Capacitance (C)

Proportionality constant that relates the charge (q) on a capacitor plate to the potential difference (V) between the plates of a capacitor.

Units: Coulomb/Volt = farad (F)

Typical values are in micro-farads (μF)

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Copywrited by Holt, Rinehart, & Winston

Charging a Capacitor

When the switch is thrown, a current will flow for a short time and stop.

The current will flow out of the positive battery terminal with electrons flowing in the opposite direction.

Negative charge will accumulate on the left plate, while the same amount of positive charge will accumulate on the right plate. Charge will accumulate until the capacitor reaches its capacitance.

Copywrited by Holt, Rinehart, & Winston

Dielectric

A dielectric is an insulator; when placed between the plates of a capacitor it gives a lower potential difference with the same charge, due to the polarization of the material. This increases the capacitance.

Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

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Dielectric Constant (κ)

The dielectric constant is a property of the material.

E0 electric field without the dielectricE electric field with the dielectric

E

E0

Dielectric Breakdown

occurs when the electric field becomes too large tearing electrons off the atoms allowing the material to conduct.

Dielectric strength – electric field at which dielectric breakdown occurs.

Parallel Plate Capacitor

Relationship:

Capacitance depends on

Area of the plate (A) – as the area the capacitance

Separation between plates (d) – as separation the capacitance

Dielectric constant of the material () – as dielectric constant the capacitance

Copyright ©2007 Pearson Prentice Hall, Inc.

Energy Storage

Main function of a capacitor

Electrical energy stored is found by

Electrical energy density (uE)

Application: RAM storage

In a RAM chip a capacitor is connected to a transistor switch with two inputs - an address line and a data line.

A pulse on the address line turns on the transistor switch.

With the switch on, a pulse on the data line will charge the capacitor. Charged capactior = 1, Uncharged capacitor = 0

Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

Capacitors in Circuits

Parallel Circuits• Voltage is the same

across each capacitor.• Capacitance adds up

Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.Copyright ©2007 Pearson Prentice Hall, Inc.

Capacitors in Circuits

Series Circuits• Each capacitor carries the same

charge.• The total voltage is the sum of the

voltages across each capacitor• Total capacitance is found by

summing the inverse of each individual capacitance.

Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

Copyright ©2007 Pearson Prentice Hall, Inc.