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Exam2_2005Fall_Solutions

# Exam2_2005Fall_Solutions - Physics 8.02 Exam Two Mashup...

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Physics 8.02 Exam Two Mashup Solutions Fall 2005 Some (possibly useful) Relations: inside closedsurface o Q d κ ε = ∫∫ E A G G w points from inside to outside d A G moving from to b a b b a a V V V = = − E s G G d 0 d = E s G G v many point charges 1 4 N i i o i q V πε = = r r G G V Q C = C Q V = parallelplate o A C d ε = U = 1 2 C V 2 = Q 2 2C series 1 2 1 1 C C = + 1 C parallel 1 2 C C = + C where is the resistivity ρ ρ = E J G G V = i R L R A ρ = 1 R parallel = 1 R 1 + 1 R 2 R series = R 1 + R 2 2 2 ohmic heating V P i V i R R = ∆ = = 2 ˆ 4 o q c r µ π × = << v r B v G G G 2 ˆ r 4 o I d d µ π × = B s r G G ˆ points source observer r from to through contour o d I µ = B s G G v where I through is the current flowing through any open surface bounded by the contour: through open surface I d = J A G G d s right-handed with respect to d A sgl loop d d N dt dt ε Φ = − = − E ds B dA v ext q = × F v B G G G ext d I d = × F s B G G G 2 cent. F mv = r = perpendicular to loop, right-handed with respect to I A I μ n n G ± ± = × τ μ B G G G z z z dB F dz µ = Cross-products of unit vectors: ˆ ˆ ˆ ˆ ˆ ˆ 0 × = × = × = i i j j k k ˆ ˆ ˆ × = i j k ˆ ˆ ˆ × = j k i ˆ ˆ ˆ × = k i j Some potentially useful numbers 2 9 7 2 0 1 N m 9 10 4 10 4 C e o k µ π πε = = × = × T m A Breakdown of air E ~ 3 x 10 6 V/m Earth’s B Field B ~ 5 x 10 -5 T = 0.5 Gauss Speed of light c = 3 x 10 8 m/s Electron charge e = 1.6 x 10 -19 C Avogadro’s number N A = 6.02 x 10 23 mol -1

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MIT PHYSICS DEPARTMENT p. 2 of 11 Problem 1: Seven Short Questions. Circle your choice for the correct answer. Question A (3 points out of 21 points): A B Both of the above pictures show an iron filings representation of the magnetic fields generated by a loop of current held above a permanent magnet. In both pictures the coil has a current which is counterclockwise when viewed from above. The magnet, however, has been flipped over between A & B. What is true about the direction of the magnet and the force it exerts on the coil? 1) In A its North pole is up and it attracts the coil. 2) In B its North pole is up and it attracts the coil. 3) In A its North pole is up and it repels the coil. 4) In B its North pole is up and it repels the coil. Question B (3 points out of 21 points): You are given two long straight pieces of wire and a semicircle of wire and asked to construct a shape that, when a current is passed through it, creates a large B field at the center of the semi-circle (that is, the center of the circle from which the semi-circle is cut). You construct: The B field created at the center of the semi-circle in the above circuits is: 1) Largest in A 2) Largest in B 3) Largest in C 4) Largest in A & C (they are equal) 5) Largest in B & C (they are equal) 6) The same in all 3 circuits (A) (B) (C) 8.02 Exam #2 Mashup Solutions Fall 2005