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# Prep101comsolutions 23 prep101 vd phys1026 april exam

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Unformatted text preview: he potential energy. CAPACITANCE PRACTICE QUESTIONS Question 1: Solution: The electric field is non-vanishing only in the region obtain Using Gauss’ Law, we Therefore, the potential difference between the two conducting shells is: Answer 6.2 Capacitance increases when connected in parallel, decreases in series. To get the maximum we close all 1 to 4 to put the three in parallel, and put 5 down to avoid putting them in series. To get the minimum we close 3 and 2, open 1 and 4, put 5 up, to put all four capacitors in series to get the minimum. Solutions will be posted at www.prep101.com/solutions 22 ©Prep101 Phys1026 April Exam Solutions CURRENT AND RESISTANCE PRACTICE QUESTIONS Question 1: Solution: For A: V 2 (120 V ) P= = R 144 Ω 2 P = 100 W For B: The voltage drop across each bulb is 60 V, since they have equal resistances. V 2 (60 V ) P= = R 144 Ω P = 25 W 2 For C: In the parallel configuration, the voltage drop across each bulb is 120 V, so the power drawn by each bulb is 100 W - as in part a. Hence, the bulbs are much brighter. Question 2: Solution: For A: dQ 90 C = dt 3600 s i = 2.5 × 10-2 A i= For B: The electron density in m-3 is: n = 5.8 × 10 22 j≡ electrons 10 6 cm 3 electrons × = 5.8 × 10 28 3 3 cm 1m m3 i = nqv d A q = -e = -1.60 × 10-19 C where Note that since we are not worried about the direction of the current, we can ignore the minus sign in the charge. j≡ i 2.5 × 10 −2 A A = = 3.18 × 10 4 2 2 −3 A π × 1.0×10 m m 2 ( ) Solutions will be posted at www.prep101.com/solutions 23 ©Prep101 vd ≡ Phys1026 April Exam Solutions j 3.18 × 10 4 A ⋅ m −2 = nq (5.8 × 10 28 m −3 )× (1.60 × 10 −19 C ) vd = 3.4 × 10-6 m/s 7.3 Answer Strategy -draw a diagram of the situation -consider the internal resistance to be a single extra resistor in series -find the effective resistance of the resistors in parallel (Reff) The terminal voltage is half the emf – so fully half the voltage is dropped across the internal resistance. Since the internal resistance and the rest of the circuit are in series, this means that the internal resistance and the effective resistance of the rest of the circuit are the same, ∴ Ri = Reff = 0.50Ω Solutions will be posted at www.prep101.com/solutions 24 ©Prep101 Phys1026 April Exam Solutions Answer 7.4 Strategy -find R1 using Ohm’s Law -find V2, V3 using loop rule -IT splits into two equal parts -find R2 & R3 using Ohm’s Law again First, all current goes through R1, so R1 = V1 / IT = (10.0V)/(10.0A) = 1Ω VT = V1 + V2 Find V3: VT = V1 + V3 Find V2: 40.0V = 10.0V + V3 40.0V = 10.0V + V2 so V3 = 30.0V so V2 = 30.0V Now, I2 = I3 = ½ IT = 5.0A Finally, combine the above to solve for resistances (by symmetry, R2 = R3) R2 = V2/I2 = (30.0V)/(5.0A) = 6.0 Ω … = R3 *We could check this answer by finding Rtotal and applying it to R = V/I CIRCUITS PRACTICE QUESTIONS Question 1: Solution : This is a question where Kirchhoff’s rules need to be used. There will be a fair amount of substitution of equations into each other, so they will be numbered where necessary. Al...
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