Capacitance Capacitor A capacitor is a device that

Capacitance

Capacitor • A capacitor is a device that stores electric charge.

Capacitor applications: Computer RAM memory and keyboards.

Capacitor applications: Electronic flashes for cameras.

Capacitor applications: Electric power surge protectors.

Capacitor applications: Radios and electronic circuits.

Capacitor applications: Electric Power Supplies

Capacitor applications: Timing Circuits

Capacitor applications: Voltage Storage Device For Car Stereos

Capacitors A basic capacitor has two parallel plates separated by an insulating material l A capacitor stores an electrical charge between the two plates l The unit of capacitance is Farads (F) l Capacitance values are normally smaller, such as µF, n. F or p. F l

Capacitors l Basic capacitor construction Plate 2 Dielectric material Plate 1 The dielectric material is an insulator therefore no current flows through the capacitor

Capacitors Storing a charge between the plates l Electrons on the left plate are attracted toward the positive terminal of the voltage source l This leaves an excess of positively charged holes l The electrons are pushed toward the right plate l Excess electrons leave a negative charge + + + _ _ -

Capacitors Types of capacitors l The dielectric material determines the type of capacitor l Common types of capacitors are: l Mica l Ceramic l Plastic film

Capacitors l l Some capacitors are polarised, they can only be connected one way around Electrolytic capacitors are polarised

Capacitors l l l Variable capacitors are used in communication equipment, radios, televisions and VCRs They can be adjusted by consumers by tuning controls Trimmers are internal adjusted capacitors that a consumer cannot adjust

Capacitors l l These variable capacitors would be difficult to squeeze into your mobile phone and i. Pod Current technology uses semi-conductor variable capacitors called varactors (varicaps)

Capacitor A capacitor consists of two conductors separated by an insulator, which could be air or even a vacuum. Metal Plates Insulating Material (Dielectric)

Some Capacitors conductor insulator

Capacitance : Definition l Take two chunks of conductor l l l Apply a potential V between them Charge (Q) will appear on the conductors, l l l Separated by insulator Q+ = +CV Q- = -CV C depends upon both: l l “geometry” Insulator material (dielectric) Q+ = +CV +++++++++++ V V -- --0 - -------V C = Q

+ e e - e ee e V e e eeee e

Types of Capacitors Parallel-Plate Capacitor Cylindrical Capacitor A cylindrical capacitor is a parallel-plate capacitor that has been rolled up with an insulating layer between the plates.

Dielectrics - Insulator Air - Often used in radio tuning circuits

Dielectrics Ceramic - Used for high frequency purposes like antennas, X-ray and MRI machines

Dielectrics Mylar - Most commonly used for timer circuits like clocks, alarms and counters

Dielectrics l Glass - Good for high voltage applications

Dielectrics Super capacitor - Powers electric and hybrid cars

Example 1 : Parallel-Plate Capacitor 1. 2. 3. Calculate field strength E as a function of charge ±Q on the plates Integrate field to calculate potential V between the plates Q=CV, C = V/Q Area A +Q Dielectric constant e Separation d Area A -Q E V

Example 1 : Parallel-Plate Capacitor +Q Area A e d Area A dl âz E -Q

RC Time Constant T = RC

RC Time Constant The time it takes for a capacitor to charge to 63. 2% or discharge to 36. 8% of the maximum voltage.

Capacitance in AC Circuits

Capacitive Reactance

Find the current flowing in a circuit when a 4 u. F capacitor is connected across a 880 v, 60 Hz supply.