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# HW16Sol - Platt David Homework 16 Due 4:00 am Inst Ken Shih...

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Platt, David – Homework 16 – Due: Oct 14 2005, 4:00 am – Inst: Ken Shih 1 This print-out should have 11 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. The due time is Central time. 001 (part 1 of 2) 10 points An unusual message delivery system is pic- tured in the figure. A 28 cm length of con- ductor (that is free to move) is held in place between two thin conductors. All of the wires are in a magnetic field. When a 11 . 2 A cur- rent is directed as shown in the figure, the wire segment moves upward at a constant velocity. The acceleration of gravity is 9 . 8 m / s 2 . 0 . 28 m 11 . 2 A 11 . 2 A 11 . 2 A Assuming the wire slides without friction on the two vertical conductors and has a mass of 39 g, find the magnitude of the minimum magnetic field that is required to move the wire. Correct answer: 0 . 121875 T. Explanation: Let : = 28 cm = 0 . 28 m , I = 11 . 2 A , m = 39 g = 0 . 039 kg , and g = 9 . 8 m / s 2 . For the wire to move upward at constant speed, the net force must be 0, so F mag = W B I ‘ sin θ = m g since θ = 90 , B = m g I ‘ = (0 . 039 kg) ( 9 . 8 m / s 2 ) (11 . 2 A) (0 . 28 m) = 0 . 121875 T 002 (part 2 of 2) 10 points What is its direction? 1. Into page 2. Out of page correct 3. Toward the top edge of the page 4. None of these 5. Toward the right edge of the page 6. Toward the left edge of the page 7. Toward the bottom edge of the page Explanation: Apply the right-hand rule: Force directed out of the palm of the hand, fingers in the direction of the field, thumb in the direction of the current. The thumb points to the left, the palm faces toward the top of the page, so the fingers point out of the page. 003 (part 1 of 2) 10 points The gravitational force on loop is downward. A rectangular loop with dimensions (hor- izontal = 0 . 48 m) × (vertical= 1 . 104 m), is suspended by a string, and the lower hori- zontal section of the loop is immersed in a magnetic field.

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Platt, David – Homework 16 – Due: Oct 14 2005, 4:00 am – Inst: Ken Shih 2 How must the current flow in the loop so that the tension in the supporting string is increased? The gravitational force on loop is down- ward. 1. either way 2. counter clockwise 3. clockwise correct Explanation: To have a tensile force in the supporting string, the loop must produce a force that pulls downward on the supporting string. The downward force in the loop comes from the ~ I × ~ B ‘ hor , force on the lower leg of the loop ( hor is the length of the horizontal leg), which would be downward if this current is directed to the left on this lower leg. Hence, as this
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