Exam__1_Solutions_9_27_07

# Exam__1_Solutions_9_27_07 - PHYSICS 222 FALL 2007 EXAM#1...

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PHYSICS 222, FALL 2007 EXAM #1 Solutions SEPTEMBER 27, 2007 Note: i ˆ , j ˆ , and k ˆ are unit vectors directed in the +x, +y, and +z directions, respectively, of a (right-handed) Cartesian coordinate system. Problems 1-15 are worth 2 points each. 1. A particle with positive charge q is moving at speed v in a magnetic field of magnitude B. If the angle between the particle velocity and the magnetic field is 30 degrees, the magnitude of the magnetic force on the particle is ____ . A. qvB / 2 B. 2qvB C. (qvB) 2 D. 2 qvB E. qvB / 2 F = |q| v B sin φ = qvB sin(30˚) = qvB / 2. 2. A flat surface with area A is in a uniform magnetic field of magnitude B. The magnetic flux through the flat surface is ____ . A. 0 B. AB C. 2AB D. AB/2 E. The answer cannot be determined from the information given. Φ B = B A cos φ , but φ is not given. 3. Which one of the following five statements is false? A. Magnetic field lines can never cross. B. A circular current carrying coil does not have a North pole or a South pole. C. The magnetic flux through any closed surface is zero. D. Magnetic monopoles do not exist. E. The magnetic field lines due to a uniform current in a long straight wire are circles centered on the axis of the wire.

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4. Two long thin parallel wires carry equal currents I but in opposite directions. The magnitude of the magnetic field at a point midway between the axes of the wires is ____ . (d is the distance between the two wires) A. 0 B. µ o I 2 π d C. µ o I 4 π d D. 2 µ o I π d E. 4 µ o I π d From the circular right-hand rule, the magnetic fields at the midpoint P from each of the two wires are in the same direction, so the magnitude of the field at P is twice the field due to either one, where r = d/2: B = 2 µ o I 2 π r = µ o I π (d/2) = 2 µ o I π d 5. A long straight wire of radius R carries a uniform current. Which one of the following four plots (A., B., C., or D.) best describes the magnitude B(r) of the magnetic field produced by the current, as a function of the distance r from the axis of the wire? B(r) r R 0 0 A. r R 0 0 B. r R 0 0 D. r R 0 0 C. The answer is C. See Fig. 28.19 in the text.
6. Lenz's law must be true in order that the law of conservation of _____ is not violated. A. linear momentum B. angular momentum C. charge D. energy E. current This problem is the same as Assigned Homework Problem 3.2(d). There it was found that if Lenz's law did not hold, energy would continuously be generated by a slidewire generator with no input of energy, thus violating the law of conservation of energy. 7. A circular coil lies in the x-y plane as shown in the figure and its two ends are connected together. The coil is bathed in a uniform magnetic field directed in the +z direction and the magnetic field magnitude is decreasing with time. The current induced in the coil is in the _____ direction as viewed from the positive z-axis. x

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Exam__1_Solutions_9_27_07 - PHYSICS 222 FALL 2007 EXAM#1...

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