17-7 Capacitance A device that stores electric charge Two plates that are separated by an insulator Used in electronic circuits Store charge that can later be released Camera flash, energy back up

Symbols or capacitor and battery

If voltage is applied across the capacitor plates the plates gain a charge The charges on the plates are equal and opposite It is found that the charge, Q, gained by each plate is proportional to the magnitude of the potential difference, V It becomes equal with the constant of proportionality, C, called capacitance Q=CV

The units of capacitance is a farad, F Sometimes pF, picofarad, Or  F, microfarad, 10 -6

Capacitance, C doesn’t depend upon Q or V but the size, shape and position of the two conductors ϵ o =8.85 x C 2 /Nm 2

17-8 Dielectrics An insulator that is placed between the plates of the capacitor Several reasons to use a dielectric material So the capacitor retains the charge longer Plates can be placed closer together increasing C by decreasing d Increases the capacitance by a factor of K The dielectric constant

For a parallel-plate capacitor

17-9 Storage of Electric Energy The energy stored in a capacitor is equal to the work done to charge it. Very little work is required to first move the charge, but as charge builds it requires more work. Similar to stretching a spring W=½QV f

PE= work done=energy=½QV Or Q=CV PE =½QV= =½CV 2 = =½Q 2 /C

Energy Stored in Terms of Electric Field V=Ed C= ϵ o A/d