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 ⋅= ∫∫ EA G G w points from inside to outside d A G moving from to b ab b a a VV V ∆= = Es G G d 0 d −⋅= G G v many point charges 1 4 N i i oi q V πε = = rr GG V Q C = C Q V = parallelplate o A C d = U = 1 2 C V 2 = Q 2 2C series 1 2 11 CC =+ 1 C parallel 1 2 C where is the resistivity ρρ = EJ 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 Pi V i R R =∆ = = 2 ˆ 4 o q c r µ π × = << vr Bv G G G 2 ˆ r 4 o I d d × = B sr G G ˆ points source observer rf r o mt o through contour o dI Bs G G v where I through is the current flowing through any open surface bounded by the contour: through open surface I d = JA G G d s right-handed with respect to d A sgl loop d d N dt dt Φ = Ed s BdA v ext q Fv B G G G ext d Fs B G 2 cent. Fm v = r = perpendicular to loop, right-handed with respect to IA I μn n G ± ± = × τ μ B G G G z zz dB F dz = Cross-products of unit vectors: ˆˆ ˆˆˆˆ 0 ×=×= × = ii jjkk ˆˆ ˆ × = i j k ˆ × = j ki ˆ × = j Some potentially useful numbers 2 97 2 0 1N m 91 0 4 1 0 4C eo k µπ == × = × 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
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MIT PHYSICS DEPARTMENT p. 3 of 11 Question C (3 points out of 21 points): Path 1 Path 2 Consider a current configuration consisting of 4 wires each carrying the same current I .
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This note was uploaded on 04/07/2008 for the course 8 8.02 taught by Professor Hudson during the Fall '07 term at MIT.

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Exam2_2005Fall_Solutions - Physics 8.02 Exam Two Mashup...

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