Capacitor & Dielectrics 3
Consider a parallel-plate capacitor
· Initially the plates are separated by a vacuum and connected to a battery, giving the charge on the plates +Q and –Q
· The battery is now removed and the charge on the plates remains constant
· The electric field between the plates is uniform and has a magnitude of E0
· Meanwhile the separation between plates is d
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· When a dielectric is placed in the electric field between the plates, the molecules of the dielectric tend to become oriented with their positive ends pointing toward the negatively charged plate and vice versa
· The result is a buildup of positive charge on one surface of the dielectric and of negative charge on the other
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· The number of field lines within the dielectric is reduced thus the applied electric field E0 is partially canceled
· The new electric field, E is electric field between the plates is uniform and has a magnitude of E0
· The new electric field strength (E < E0) is less, then the potential difference, V across the plates is less
V = Ed
· Since V is smaller while Q remains the same the capacitance
C = Q/V
is increased by the dielectric
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