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Phy122_0916

# Phy122_0916 - Physics 122 Sept 16 2010 The Physics 121-122...

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Physics 122 Sept. 16, 2010 The Physics 121-122 help room (Physics A-131) is mostly staffed by students. They understand the need to be present when scheduled, but ask for you to understand that sometimes they are held late at another class etc another class, etc. If you find no TA in the help room during a time when it is s h d l d l k f i ffi (B 134) I it is scheduled, look for me in my office (B-134). hope to be able to help you immediately. If I’m not there, send an email. Question 1:

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Distribution of scores for HW #16.
Question 2 In Lab 1 you took data that allowed you to draw electric field lines for two parallel plates and an electric dipole. F th f i l For the new case of a circular metal shell at + 5V near a metal plate at 0 V, which picture corresponds to the correct electric field lines: a, b, c , or d ?

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Question #3 . A 4 μ F capacitor and an 8 μ F capacitor are connected in series, and then to a 12 Volt battery. What is the charge in each of the capacitors?
Question #3. A 4 μ F capacitor and an 8 μ F capacitor are connected in series, and then to a 12 Volt battery. What is the charge in each of the capacitors? Connected in series, they have the same charge, which is Voltage x C eq . Equivalent capacitance ( μ F): 1/C eq = 1/4 + 1/8 = 3/8, so C = 8/3 μF eq = 8/3 μF. Q = 12 V x 8/3 μF = 32 μ C Each capacitor has a charge of 32 μ C.

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Leftover from Sept. 14. Dielectric materials. M l l i i t i l d t t l Molecules or ions in a material respond to an external electric field. This partly cancels the electric field from the plates, in a proportion called the dielectric constant, κ (Greek letter kappa). Ranges from 1.0006 (air) to several hundred (metal oxides) Tabulated in your book oxides). Tabulated in your book. Put a dielectric between the plates of a capacitor. For a given Q , this reduces E to E vac / κ (and therefore V Æ V vac / κ ). That increases C (= Q / V ) to κ C vac .
Conductor: charges are “free” to move. Let them move, e.g. , along a wire. Current – amount of charge that passes a given point per unit time.

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Phy122_0916 - Physics 122 Sept 16 2010 The Physics 121-122...

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