midterm practice exam 1 solution

# Physics for Scientists and Engineers (3rd Edition)

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PHY132 Midterm-I Practice Exam Solution Problems: 1) A parallel plate capacitor has plates 10 cm x 10 cm, with a spacing of 0.885 mm. a) What is the capacitance? pf 100 F 10 m 10 885 . 0 m 01 . 0 Nm C 10 85 . 8 1 d A K C 10 3 2 2 2 12 0 = = × × × × = ε = b) The plates are charged with a 10 V battery. How much charge is on the capacitor? nC 1 C 10 V 10 F 10 CV Q 9 10 = = × = = c) The battery is now disconnected, and the plates are pulled apart to double the spacing to 2 x 0.885 mm. What is the new voltage on the capacitor? V 20 V 2 C / Q 2 C / Q V 2 / C C . const Q = = = = = = d) Compare the energy stored on the capacitor after the plates are pulled apart with the initial stored energy, and explain why they are the same or different. 2 CV 2 1 V 4 2 C 2 1 V C 2 1 U 2 2 2 × = = = The stored energy doubles because work needs to be done to pull the capacitor plates apart. 2) In a certain region of space, the electric potential is V = axy + by 2 + cy, where a = 3 V/m 2 , b = -2 V/m 2 , and c = 5 V/m. a) Calculate E x , E y , and E z . ) ay ( x V E x = = ) c by 2 ax ( y V E y + + = = 0 z V E z = = b) At what points is the electric field zero? always 0 E m 3 / 5 a / c x 0 c ax c by 2 ax 0 E 0 y 0 E z y x = = = = + = + + = = = E is zero along a line parallel to the z axis at x = -5/3 m and y = 0. 3) What is the equivalent capacitance of the network of capacitors shown below?

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2 µ F 2 µ 1 µ 1/2 µ 1 µ The three capacitors on the right (2, 2, and 1 microF) are in series. Their equivalent capacitance
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midterm practice exam 1 solution - PHY132 Midterm-I...

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