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Unformatted text preview: Reilly (mar3978) ohw21 turner (56725) 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points A circular coil is made of N turns of copper wire as shown in the figure. A resistor R is in- serted in the copper wire. Initially, a uniform magnetic field of magnitude B i points hor- izontally from left-to-right through the per- pendicular plane of the coil. When viewed from the right the coil is wound counter-clockwise. R Magnetic Field B ( t ) During a time interval t the field uniformly changes at a constant rate, until a reversed field is reached equal in magnitude to the initial field. The electrons in the resistor, R, shown in the figure 1. do not move in a preferred direction, since they have thermal kinetic energy. 2. move in a direction that is undetermi- nated from the information given. 3. move left-to-right. 4. move right-to-left. correct Explanation: As the left-to-right magnetic field decreases (and eventually flipping sign and increasing in magnitude) it follows from Lenzs law (op- position to the change in magnetic field will tend to keep the current constant and flowing in the same direction) that the induced emf will produce an left-to-right magnetic field arising from induced currents in the coil. By the right hand rule, the induced current flows counter-clockwise when viewed from the right. The coils are wound counter-clockwise as the wire goes from the right to left termi- nals. The current must enter the loop from the right terminal and exit at the left termi- nal. Since the current is continuous, the current must flow through the resistor in the left-to- right direction, which means that electrons are flowing right-to-left in the resistor. That is, the electrons flow in the opposite direction from the current since they have a negative charge. 002 (part 1 of 4) 10.0 points The circular loop of wire shown in the figure is placed in a spatially uniform magnetic field such that the plane of the circular loop is per- pendicular to the direction for the magnetic field as shown in the figure. The magnetic field vector B ( t ) varies with time, with the time de- pendence given by B ( t ) = a + bt, where a = 0 . 36 T and b = 0 . 044 T / s. The acceleration due to gravity is 9 . 8 m / s 2 . r = 5 . 5 cm radius B ( t ) What is the magnetic flux through the loop as a function of time? 1. B ( t ) = 2 ( a + bt ) r 2. B ( t ) = ( a + b ) r 2 3. B ( t ) = parenleftBig a t + b parenrightBig r 2 4. B ( t ) = 2 a r 5. B ( t ) = a r 2 6. B ( t ) = 2 ( a + b ) r 7. B ( t ) = ( a + bt ) r 2 correct Reilly (mar3978) ohw21 turner (56725) 2 8. B ( t ) = 2 b r 9. B ( t ) = 2 parenleftBig a t + b parenrightBig r 10. B ( t ) = b r 2 Explanation: Faradays Law of Induction E ind =- d B dt =- B t ....
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