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Spring 2002 w Answers - Practice Final PHY 232 Spring 2002...

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Practice Final: PHY 232 Spring 2002 1 1. Three charges (q,q,-2q) are placed at the corners of an equilateral triangle. Consider the point which is equidistant from the three sides of the triangle. At that point which one of the following statements is true? (Remember: E is a vector, V is a scalar) A) Both the electric field and the electric potential are zero. B) The electric field is zero but the electric potential is not. C) The electric potential is zero but the electric field is not. D) Both the electric field and the electric potential are non-zero. E) The net force on a negative charge is zero but on a positive charge it is non-zero. 2. Six identical capacitors (capacitance C) are connected as shown in the diagram to the right. What is the total capacitance of the combination? 3. When a fully discharged C = 3.2 μ F capacitor is charged through a R = 100 resistor, how long does it take to reach 0.63% less than its final voltage? 4. A 2.0 μ F capacitor and a 3.0 mH inductor are connected in series with an AC power supply. At what frequency will the inductive reactance be three times the capacitive reactance? 5. What is the distance between two q 1 = q 2 = 5 μ C charges if the Coulomb force between them is F = 4.6 × 10 3 N ? A) 14.2 m B) 7.0 × 10 - 3 m C) 9.8 × 10 - 3 m D) 9.8 m E) 3.5 × 10 - 2 m q q -2q x For two C’s in series, C eq = C/2. For three C eq in parallel, C total = 3C eq = 3C/2. Time constant τ = RC = 3.2 × 10 -4 s. Also t t / 3 3 / final 10 3 . 6 10 3 . 6 1 ) 1 ( / ) ( t t e e V t V - - - - = × × - = - = . Then s. 10 62 . 1 / 07 . 5 ) 10 3 . 6 ln( 3 3 - - × = - = - = × t t t Given ( 29 LC f fC fL X X C L 3 2 2 3 2 3 2 = = = = p p p Then Hz 10 6 . 3 3 2 1 3 × = = LC f p m 10 0 . 7 3 2 1 2 2 1 - × = = = F q q k r r q q k F e e
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