University of California, San Diego
Physics 1b - Thermal Physics & Electromagnetism

H. E. Smith   Spring 2000

Physics 1B - Tutorial #8

  1. The figure at the right represents the circuit of an emergency highway flasher. The flash lamp itself has a very high resistance (~107 ) at voltages below 110V. At voltages greater than 110V, gas inside the flasher becomes ionized and conducts electricity; the resistance is then 10. Explain how the flasher works. What determines the time between flashes? How long is it? What is the duration of the flash?

    Until the capacitor voltage reaches 110V, the high resistance of the flash lamp ensures that it draws no current; the capacitor charges like the RC circuit example in class. When the capacitor voltage reaches 110V, the flasher resistance becomes small compared with that of the resistor and the capacitor discharges through the lamp.

  2. Practice the right hand rule for vector cross-products; write on your fingers the appropriate vectors for Force, Velocity, B-field. If you don't wash your hands before Friday, you can use your fingers as a reference for the quiz.

  3. An electron is moving in the earth's magnetic field with velocity, v = 1.4 m/s. The particle's components of velocity are 1 m/s parallel to the field and 1 m/s perpendicular to the field, which has a magnitude of 10-4T. Describe quantitatively the motion of the particle.
    The electron will move with a helical pattern - a combination of linear motion along the field line with v = 1m/s, and circular motion with R = mv/Bq = 5.6 x 10-8m.

  4. The figure below shows three configurations of a set of parallel wires. Wire 3 is always equidistant from Wires 1 and 2. The magnitudes of the currents in the wires is the same with "" indicating current directed into the page and "" indicating current out of the page. Find the direction of the net force on Wire 3 in each case. In figure (c) the distance between nearby wires is 10cm and the current is 1A in each wire. Calculate the force on Wire 3.

    Problem 4
    In (c), F = F1 + F2 = 2 (µ0I/2 r) = 4 x 10-6 N

  5. A Square loop of wire is placed in the magnetic field as shown.

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Gene Smith

Last modified: Tues., 23 May 2000