ohw21-solutions - Reilly(mar3978 ohw21 turner(56725 This...

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Reilly (mar3978) – ohw21 – turner – (56725) 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 10.0 points A circular coil is made oF N turns oF copper wire as shown in the fgure. A resistor R is in- serted in the copper wire. Initially, a uniForm magnetic feld 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 ±ield B ( t ) During a time interval t the feld uniFormly changes at a constant rate, until a reversed feld is reached equal in magnitude to the initial feld. The electrons in the resistor, R , shown in the fgure 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 feld decreases (and eventually ²ipping sign and increasing in magnitude) it Follows From Lenz’s law (op- position to the change in magnetic feld will tend to keep the current constant and ²owing in the same direction) that the induced emF will produce an leFt-to-right magnetic feld arising From induced currents in the coil. By the right hand rule, the induced current ²ows 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 ²ow through the resistor in the leFt-to- right direction, which means that electrons are ²owing right-to-leFt in the resistor. That is, the electrons ²ow 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 fgure is placed in a spatially uniForm magnetic feld such that the plane oF the circular loop is per- pendicular to the direction For the magnetic feld as shown in the fgure. The magnetic feld v B ( t ) varies with time, with the time de- pendence given by B ( t ) = a + b t, 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 ²ux through the loop as a Function oF time? 1. Φ B ( t ) = 2 ( a + b t ) π r 2. Φ B ( t ) = ( a + b ) π r 2 3. Φ B ( t ) = p a t + b P π r 2 4. Φ B ( t ) = 2 a π r 5. Φ B ( t ) = a π r 2 6. Φ B ( t ) = 2 ( a + b ) π r 7. Φ B ( t ) = ( a + b t ) π r 2 correct
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Reilly (mar3978) – ohw21 – turner – (56725) 2 8. Φ B ( t ) = 2 b π r 9. Φ B ( t ) = 2 p a t + b P π r 10. Φ B ( t ) = b π r 2 Explanation: Faraday’s Law of Induction E ind = - d Φ B dt = - ΔΦ B Δ t . Lenz’ Law is used to ±nd direction of E ind . Magnetic ²ux is de±ned by Φ B i S v B · d v A = v B · v A .
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ohw21-solutions - Reilly(mar3978 ohw21 turner(56725 This...

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