CAPACITORS & CAPACITANCE

End of the lesson, students should be ; Understand capacitors and capacitance Understand capacitance equivalent circuits for series and parallel connections Understand circuits with capacitive load Understand the process of charging and discharging in a capacitor

The Capacitor Capacitors are one of the fundamental passive components. In its most basic form, it is composed of two plates separated by a dielectric. The ability to store charge is the definition of capacitance.

Capacitance Rearranging, the amount of charge on a capacitor is determined by the size of the capacitor (C) and the voltage (V). Q for charge = unit Coulomb, C for Capacitor = unit Farad V for voltage = unit Volts

Construction of capacitor

Construction of capacitor Conductors Dielectric

Types of capacitors Fixed Variable

Fixed Capacitor Is a capacitor with a fixed value of capacitance which cannot be adjusted. Is classified according to the type of material used as its dielectric, such as paper, oil, mica or electrolyte.

Example of Fixed Capacitor Mica Mica capacitors are small with high working voltage. The working voltage is the voltage limit that cannot be exceeded.

Ceramic disk Ceramic disks are small nonpolarized capacitors They have relatively high capacitance due to high er.

Plastic Film Plastic film capacitors are small and nonpolarized. They have relatively high capacitance due to larger plate area.

Electrolytic (two types) Electrolytic capacitors have very high capacitance but they are not as precise as other types and tend to have more leakage current. Electrolytic types are polarized. Symbol for any electrolytic capacitor

Variable Capacitor Is a capacitor that is constructed with an adjustable value of capacitance. E.g : Example :rotor-stator type, trimmer capacitor

Capacitance A capacitor stores energy in the form of an electric field that is established by the opposite charges on the two plates. The energy of a charged capacitor is given by the equation where W = the energy in joules C = the capacitance in farads V = the voltage in volts

Example If a 0.001 mF capacitor is connected in series with an 800 pF capacitor, the total capacitance is Answer : 444.44pF

Example If a 0.001 mF capacitor is connected in parallel with an 800 pF capacitor, the total capacitance is Answer : 1.8 x10-9 F

Series-parallel Capacitors CT = C1 Siri (C2 // C3) CT = C1 Siri (C2 + C3) 1 1 1 ---- = ----- + --------------- CT C1 (C2 + C3)

When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electrons are therefore the charge carrier in a typical solid conductor. For an electric current of 1 ampere, 1 coulomb of electric charge (which consists of about 6.242 × 1018 elementary charges) drifts every second through any plane through which the conductor passes.

For a steady flow of charge through a surface, the current I (in amperes) can be calculated with the following equation: where Q is the electric charge transferred through the surface over some time t. If Q and t are measured in coulombs and seconds respectively, I is in amperes. More generally, electric current can be represented as the rate at which charge flows through a given surface as: Current is the time rate of change of charge,

Electric flux,  Flux, a scientific term describing the rate of flow of something through a surface. Electric flux, a measure of quantity of electricity. In electromagnetism, electric flux is the flux of the electric field. Electric flux is proportional to the number of electric field lines going through a virtual surface. is measured in coulombs.

Electric flux density, D is the amount of flux passing through a defined area A that is perpendicular to the direction of the flux: also called charge density, σ

Electric field strength,E also called potential gradient Electric field strength is a quantitative expression of the intensity of an electric field at a particular location. The standard unit is the volt per meter (v/m or v · m -1 ). A field strength of 1 v/m represents a potential difference of one volt between points separated by one meter. Any electrically charged object produces an electric field. This field has an effect on other charged objects in the vicinity. The field strength at a particular distance from an object is directly proportional to the electric charge, in coulomb s, on that object. The field strength is inversely proportional to the distance from a charged object. E = V/d

Dielectric The insulating medium separating charged surfaces is called a dielectric. Compared with conductors, dielectric materials have very high resistivities. They are therefore used to separate conductors at different potentials, such as capacitor plates or electric power lines.

Factors affecting capacitance..