Unformatted text preview: PHY2054 Spring 2008 15. The potential difference between a pair of oppositely charged parallel plates is 450V when
the plate separation is 1 pm. If the spacing between the plates is tripled without altering the
charge on the plates, what is the new potential difference (in volts) between the plates? @ 1350
(3) so (4) 450
(S) 4050 Since the charge is unchanged, the E ﬁeld is unchanged. Using V = Ed, the voltage is tripled to
I 35 0 V. 16. Refer to the previous problem. If the plate separation is doubled to 2 pm while the potential
difference between the plates is kept constant, what is the ratio of the ﬁnal charge on one of the pgtes to the original charge? @1/2
(4)4 (5) 3 If the plate separation is doubled, the capacitance is halved. Thus if the voltage is unchanged,
then by Q = CV the charge on the plates is halved. 17. Refer to the previous problem with the capacitor having its original plate separation otlﬂn.
After being charged to a potential 0&0], the capacitor is disgopngcted from the circuit and has
a slabof dielectric (x = 2.7) carefully inserted between the plates without touching them. The
new pom @None of these
(2) 320 (3) 120 (4)0 (5) 60 The charge is unchanged. Placing a dielectric inside has the eﬂect of reducing the voltage by K‘.
Thus the new voltage is 120 /2. 7 = 44.4 V. l.‘ ...
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- Spring '09