UnitExam1Csolutions

# UnitExam1Csolutions - ID C UCSD Physics 2B Summer Session...

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ID: C 5 UCSD Physics 2B Summer Session Unit Exam 1C Charges, Force & Fields Answer Section MULTIPLE CHOICE 1. ANS: E Fundamental properties of conductors : Electric field is zero inside Electric field is perpendicular at surface Excess charge must reside at surface only TOP: CONDUCTOR FIELD CONCEPT 2. ANS: A This is analogous to the case inside the non-conducting sphere. Instead of using the "canned" formula E = 2 k λ R for which we'd need to calculate the linear charge density, let's construct a Gaussian Cylinder with radius R = 4.9 cm and arbitrary length L . The surface area A = 2 π RL while the volume VOL = R 2 L . The enclosed charge is Q ENC = R 2 L ρ Now apply Gauss's Law: E = Q ENC ε 0 A = R 2 L 0 2 RL () = R 2 0 = 4.9 × 10 2 m Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 0.200 × 6 C / m 3 Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 28 .8 5 × 12 C 2 / N m 2 Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ = 554 N C Note: the outside radius of the cylinder is not used. TOP: ELECTRIC FIELD INSIDE CYLINDER 3. ANS: C The charge for each species is the number of particles times the charge per particle. The total net charge is the sum of these: Q net = n p q p + n e q e = n p e + Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ + n e e Ê Ë Á Á ˆ ¯ ˜ ˜ = n p n e Ê Ë Á Á ˆ ¯ ˜ ˜ e + = 2.90 × 22 9.01 × 23 Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ + 1.60 × 19 C Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ =− 8.72 × Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ + 1.60 × C Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 1.40 × 5 C TOP: ELEMENTARY CHARGE 4. ANS: D Φ E = E A = EA cos θ = 73.1 × 6 N / C Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 26.2 m 2 cos 61.1 ° = 2.43 × 2 N m 2 C

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## This note was uploaded on 08/23/2008 for the course PHYS 2b taught by Professor Schuller during the Summer '08 term at UCSD.

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UnitExam1Csolutions - ID C UCSD Physics 2B Summer Session...

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