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 Copyright ©2007 Pearson Prentice Hall, Inc.

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) Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. 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 5 th Ed. Copywrited by Holt, Rinehart, & Winston

Dielectric Constant (κ) The dielectric constant is a property of the material. E 0 electric field without the dielectric E electric field with the dielectric

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 (u E ) 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 5 th Ed.

Capacitors in Circuits Parallel Circuits Voltage is the same across each capacitor. Capacitance adds up Cutnell & Johnson, Wiley Publishing, Physics 5 th 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 5 th Ed. Copyright ©2007 Pearson Prentice Hall, Inc.