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Phys. 122 Midterm 1 Name: l k ,1, Read each question carefully and make certain that you understand it. The numbers in the
brackets indicate the marks for each question. Marks are awarded both for the correctness of your answers and for the methods used to obtain
them. It is in your best interest to be very detailed and clear in your reasoning. Sketch a
diagram whenever applicable. The correct answer to a question, without an appropriate
method, is worth at most 1/2 of the allotted mark. Watch sig. figs. A formula sheet is appended. You may detach it for your convenience. BOX OR HIGHLIGT THE FINAL ANSWERS. All vector quantities present in components unless otherwise indicated. PART I  Multiple Choice Questions. Each question is worth 1 mark. Circle the
letter corresponding to the right answer in each question. Sketching a diagram may help
to solve some problems. 1. Which of the following is inotyy‘ue? The electric force a) decreases with the inverse of the square of the distance between two charged
particles.
b) between electron and proton is much stronger than the gravitational force between
them. between two protons separated by distance d is larger than between two electrons
separated by the same distance d.
d) may be either attractive or repulsive. 2. A material that permits an electric charge to move through is called a (n) a) insulator.
Lb} conductor.
c) capacutor.
d) inductor. 3. A charge q is placed a distance r from the origin and a charge 2q is placed a distance
2r. There is charge 0 at the origin. if all charges are positive, which charge is on a
higher potential? E) q ) 2Q c) the two charges have the same potential. d) it depends whether 0 is greater or smaller than q. Which statement (5) is (are) true? The electric potential energy of a system of charges
is (a); equal to the amount of work required to assemble the system by bringing the charges from infinity. b) proportional to the square of the electric field generated by the charges. 0) both of the above. d) neither of the above. In order to increase energy stored in a parallelplate capacitor when the capacitor is
connected to a battery, we should: a) insert a dielectric between the plates.
b) decrease the separation between the plates.
9) increase the area of the plates.
id)» all of the above.
e) none of the above. Two identical resistors are connected first in series and then in parallel. Which
combination has the larger net resistance? a) the pair in parallel. 5):): the pair in series. 0 the two combinations have the same resistance.
d) it depends how big is the voltage applied. The resistance of a resistor increases when a) the applied voltage increases. b) the applied current decreases. the temperature increases. 6) all of the above. e) none of the above. A parallel plate capacitor has a capacitance Co. The capacitor is then connected to a battery. While the capacitor remains connected to the battem, a dielectric with m 2.0
is pushed between the plates of the capacitor. Which statement below is not true? a) The capacitance will double.
b) The charge on the capacitor will double.
c) The energy stored in the capacitor will double. The potential difference between the plates will double. 1. Part II  Solve the problems below. Box the final answers! Show your work.
Numbers in brackets indicate the marks allotted for each problem. 1. Three point charges are placed as shown in the accompanying diagram. Q = 50.0 nC is
located at (x, y) = (40.0, 0.00) m, Q2 = 50.0 nC is located at (x, y) = (0.00, 40.0) m and Q3 = . —50.0 nC is located (x, y) = (30.0, 40.0) m. a) Calculate the net electric ﬁeld at the origin due to the three point charges. Give your answer in
i . E3 vectors in the diagram. [4 marks] the component notation. Show E1, E2, .. k 50).!1096 4
/5/' (“to”); = 0.22/N/C E, :(—0.zs9/, 0) U/o
 x q
15;” k (bash/:2 0.: 0.294%; E; __ ( 0 I _a_29) U/C
x q =’ _
lE3/ : W . p/gou/c ES —(0./0€/ o/qq) 1% = [90 7:0ng E): (“Q/73/ £33 , 0180 959 : 0.1?0103 1)) Compute the net electrical potential V at the origin due to the three point charges. [4 marks] V _ __ 0 0', 7L ('5 Wm Q0“ 60” V=—7.00l/ 0 kiJIkié/ _ k '%~CI&C€€:—G mm? 7
0,_.(»5’.‘?9(/) tic k:g.9a«:0 Ca 2. A parallel plate capacitor, with a capacitance C is connected to a battery that supplies a voltage V. When the capacitor is fully charged, a charge on the capacitor is Q and energy stored in
the capacitor is E. Ar (9
Genoa (£77 After the capacitor is fully charged, it is disconnected from the battery and then the distance
between the plates is increased 3 times (d1 = 3 d) Find C], Q), V1 and energy E, for the new separation of the plates in terms of C, Q, V and E.
[4] 3. A small spherical insulator of mass 8.00 x 10'2 kg and charge +0600 pC is hung by a thin wire of
negligible mass. A charge of —0.900 ,uC is held 0.150 m away from the sphere and directly to the right of it, so the
wire makes an angle 9 with the vertical (see the drawing). Find ’ ‘ ‘ “ "' ‘ I?
+0600 pC 0.900 11C a) Draw a RED. for the spherical insulator [2] b) Find the angle 0 [3] and the tension in the wire [3] Z—Fax : 0 {Eq‘O TT SILHQ =+Fq
2%} =0 T5+wj°=o T0538 = Mj
+0M61 = F“ “3
: 0.2?5  1’11  W = 08:50
I: was ‘ an 15%" 4. t with their assigned directions
a) Calculate the unknown currents 11, I; and 13. Use the curren s as shown in the diagram. [6 marks] 40], +20I2a0 _’ 3
3 +101} +513  30 /.“J C7,) IMBZ "Ij :Df,=—’—3"IZ @ —
(a? [571‘212:.6 13=€'2fe
 v /. / :25
..ZO(g2,I‘2) +30I2 :0 I3 6 9( 7) }Z
wry120 I,» 2.57/.7/:oye4
Ia. =/.7I/4 b) The potential difference between points a and b in the diagram. State clearly which point has
the higher potential. [2] vain),3 . —5_/'3 +20 : —5(:z.s;) +20 ¥./5{/ Point [5 4 has a
higher potential. b) BONUS You will get marks for the BONUS only if you attempted all problems above. If you left
any problem blank (or almost blank), the mark for the bonus will not be given. ' For the circuit shown: l \ 3J7.
SD 0 8.0 Q __ W“
’L J) 2. J7.
l :7 m
A I A B
‘ “‘"—‘ I
IN! l 2 v
Find the equivalent resistance. [9} Z LCD
.L~ J’ + ——L + 1—
a%~‘/3 2 I?
J— o. 652 Find the current through 5.0 Q. W Z VA_6= \IV I" at l5~9=lzv 12V , : 0. A
15%} qz I I '
Find the voltage on 2.09 resistor. W 2/ V:\/ = m Am) 19. \/ ...
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 Winter '08
 Wrzesneski
 Physics, Electric Potential, Electric charge, Parallel Plate Capacitor, NC

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