Capacitors and Dielectrics (from lecture 15)

what forms a capacitor?

the uncharged state of a capacitor

• has no net charge separation between its two conductor plates

• stores no electric charge, voltage, or electric potential energy U

• the two plates are at the same electric potential

• this is the state after the capacitor has been fully discharged or before any battery is connected

the charging state of a capacitor

• a charge Q is transferred from one conductor plate to the other, givomh pme cpmductor a charge +Q and the other charge -Q

• a potential difference deltaV is created, where the positively charged has a higher potential than the negatively charged plate

• *the net charge on the entire capacitor is always zero whether if its charged or not*

symbols for capacitor

amount of charge Q stored in a capacitor (formula)

• by convention, Vab = Va - Vb

• capacitance C is measured in farads (F)

parallel plate capacitor

how to find the capacitance from a parallel plate capacitor

A = the area of each conductor plate in the capacitor (only factor in the area that overlaps) m²

d = the perpen dist between the conductor plates in the capacitor (m)

most common way to charge a capacitor

by connecting its plates to the terminals of a battery

combining capacitors connected in series

combining capacitors connected in parallel

electrical potential energy U of a parallel plates capacitor system

electrical potential energy U of a parallel plates capacitor system iin terms of electric field E)

A*d is the volume between the plates

formula for electric energy density (electric potential energy divided by area or volume)

take note of the bottom eqn

how the dielectric constant K is related to capacitance C

• where C₀ is the initial capacitance (where the space between the conductor plates is air or vacuum) and C is the final capacitance after the addition of dielectrics

how the dielectric constant K is related to electric potential V