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PHY2049: Chapter 25 19 Dielectric Mechanism is Due to Polarization
E = 0, Dipoles randomly aligned E applied, partially aligns dipoles
Aligned dipoles induce surface charges
Surface charges partially cancel E field
PHY2049: Chapter 25 20 ConcepTest
Two identical capacitors are given the same charge Q,
then disconnected from a battery.
After C2 has been charged and disconnected it is filled
with a dielectric. Compare the voltages of the two
to V/κ (1) V1 > V2
(2) V1 < V2
(3) V1 = V2 C1 PHY2049: Chapter 25 C2 21 ConcepTest
When we fill the capacitor with the dielectric, what is the
amount of work required to fill the capacitor?
(1) W > 0
(2) W < 0
(3) W = 0 Energy lowered
C1 C2 If U is total energy in capacitor
Positive work: One “pushes in” dielectric
ΔU > 0
Negative work: Capacitor “sucks in” dielectric ΔU < 0
PHY2049: Chapter 25 22 Capacitors in Circuits
2 + 3 in series C o Combine 2 & 3 ⇒ C/2 Ceq 1 + (2+3) in parallel
Combine 1 & (2 & 3) ⇒ 3C/2 2
1C 3C o PHY2049: Chapter 25 23 Example: Find qi and Vi on All Capacitors
C1 is charged in position A, then S is thrown to B position
Initial voltage across C1:
Initial charge on C1: V0 = 12
q10 = 12 x 4 = 48μC...
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