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p235w11_practice_final_2

# p235w11_practice_final_2 - Physics 235 Winter 2011 Practice...

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Physics 235 Winter 2011: Practice Final Exam Version 2 Please print your name:_____________________________________________________ Instructions 1. Fill in your name above 2. This will be a 1 hour 50 minute , closed book exam. There are 20 short questions. 3. You may use a calculator, please do not share calculators 4. For this exam, you can use four notecards or one sheet of paper.

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1: (5 points) The figure below shows eight point charges +q and –q arranged at the corners of a cube with sides of length d to represent a model of an ionic crystal like NaCl. Calculate the potential energy of this assembly, with the usual assumption that PE = 0 for d = . a) ቀ12 െ ଵଶ √ଶ √ଷ ସగఌ b) ቀെ12 ൅ ଵଶ √ଶ √ଷ ସగఌ c) ଵଶ௤ ସగఌ d) ቀെ12 ൅ ଵଶ √ଶ √ଷ ସగఌ e) ଼௤ ସగఌ 2: (5 points) Two infinite planes are placed parallel to each other, separated by a distance d. The lower plane has a uniform positive surface charge density + σ , and the upper plane has a negative surface charge density of – σ . The electric fields above the upper plane, between the planes, and below the lower plane, respectively, are: a) E above = 0 E between = σ /2d ε 0 up E below = 0 b) E above = /2 0 down E between = σ / ε 0 up E below = /2 0 down c) E above = 0 E between = σ / ε 0 E below = 0 d) E above = 0 E between = 0 E below = 0 e) E above = /2 0 up E between = σ /2d ε 0 up E below = /2 0 up
3: (5 points) A magnetic “quadrupole” made of two bar magnets on a rigid frame sits in a uniform magnetic field pointing in the positive x-direction. Sketch how the potential energy of this arrangement changes while you rotate it one full turn clockwise. Draw the potential energy as a function of angle on the figure provided below. 4: (5 points) A fuel gauge uses a capacitor to determine the height of the fuel in a tank by measuring its capacitance, which changes as the level of fuel changes. The capacitance at any moment can be described by an effective dielectric constant D eff defined by C measured = D eff C empty .

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p235w11_practice_final_2 - Physics 235 Winter 2011 Practice...

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