# TAMU PHYS 208 - 208Exam3spring14A (8 pages)

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## 208Exam3spring14A

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- Pages:
- 8
- School:
- Texas A&M University
- Course:
- Phys 208 - Electricity And Optics

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Physics 208 Spring 2014 Exam 3 A Name Last First ID Section You have 75 minutes to complete the exam Formulae are provided on a separate colored sheet You may NOT use any other formula sheet You may use only a simple calculator one without memory or with a memory demonstrated to be cleared When calculating numerical values be sure to keep track of units Results must include proper units Be alert to the number of significant figures in the information given Results must have the correct number of significant figures If you are unable to solve a problem whose solution is needed in another problem then assign a symbol for the solution of the first problem and use that symbol in solving the second problem If you need additional space to answer a problem use the back of the sheet it is written on Show your work Without supporting work the answer alone is worth nothing Mark your answers clearly by drawing boxes around them Please write clearly You may gain marks for a partially correct calculation if your work can be deciphered 208 Mid Term 1 POINTS TABLE Multiple Choice out of 20 Problem 1 out of 20 Problem 2 out of 20 Problem 3 out of 20 Problem 4 out of 20 TOTAL SCORE out of 100 MC1 5 marks In the figure a straight wire carries a steady current I perpendicular to the plane of the page A bar is in contact with a pair of circular rails and rotates about the straight wire with angular velocity in the direction indicated by the arrow The direction of the induced current through the resistor R is A B C D E from a to b from a to b but only if is increasing from b to a from b to a but only if is increasing There is no induced current through the resistor MC2 5 marks A very long wire of circular cross section with radius a is placed so that its center runs along the z axis This wire carries a net current I in the upward z direction the current is uniformly distributed across the wire Meanwhile a very long thin wire is placed along the line x y 4a 0 and carries the same current I but in the downward z direction Both wires have length L which is so much larger than the dimension a that it may be considered infinite Compare the magnitudes of the magnetic field at the point A located at the origin point B located at x y z 2a 0 0 i e halfway between the wire centers and at point C located at x y z 2a 0 0 A B C D E F G H BA BB BC BA BB BC BB BC BA BA BB BC BA BC BB BB BC BA BB BA BC BC BB BA MC3 5 marks A square wire loop of side s lies in the x y plane and carries a current of I flowing in the counter clockwise direction as viewed from z 0 A constant uniform magnetic field of magnitude B points in the x direction as shown in the figure d B y a c I S z x 45 b The torque vector caused by the magnetic field acting on this loop is in which direction A B C D E F G zero x x y y z z MC4 5 marks Which of the following graphs best depicts the time dependence of the energy UL stored in the inductor after the switch is thrown to connect position Z to B 1 20 marks A doubly charged helium atom whose mass is 6 6 10 27 kg is accelerated by a voltage of 3 4 kV a What is its resultant velocity b What will be its radius of curvature if it moves in a plane perpendicular to a uniform 0 570 T field after exiting the electric field region c What is its period of revolution 2 20 marks A segment of wire of length carries a current I as shown in the figure a What is the expression or value for the magnetic field magnitude and direction at any point such as Q along the positive x axis the axis of the wire b What is the expression or value for the magnetic field magnitude and direction at any point such as P along the positive y axis y P I Q x 3 20 marks A single rectangular loop of wire of dimensions a 180 cm and b 620 cm is situated as shown in the figure with part inside a region with uniform magnetic field of 0 920 T and part outside the field The total resistance in the loop is 0 150 The loop is pulled to the right Neglect any effects of gravity a Give the direction of the current flow in the loop clockwise or counterclockwise and explain your reasoning b What force is required to pull the loop at a constant velocity of 5 20 m s a X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X B b F 4 20 marks An inductor and a resistor appear in series in a circuit see figure At one instant the potential difference Vab across the pair is 3 05 V while the current is 450 mA and is increasing at a rate of 200 mA s At a later instant the potential difference is 2 15 V while the current is 400 mA and is decreasing at a rate of 260 mA s Determine the inductance L of the coil and the value R of the resistance i a b L R

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