Day 7

Friday, June 30, 2006

Announcements

We will use the clickers again today. If your clicker is not yet registered or having other technical difficulties, let me know by submitting a note with your name and an explanation of the problem.
If you have missed one or two WebAssign homework sets but have otherwise been keeping up with the assignments, let me know and I will extend your deadline on these assignments as long as you have not already viewed the answer key. For future reference, keep in mind that the "a" assignments are due at 8:00 am on the day that we start a new chapter, and the "b" assignments are due at midnight after we have completed a chapter. Do your best to keep up with the homework and avoid missing the submission deadlines!
Several students have asked to have the reading comprehension assignments due after the lectures for the chapters; however doing so would defeat the purpose of these assignments that are designed to motivate the class to read the chapters before coming to class. One year I did cave in to pressure from students on this issue and moved the deadlines for these assignments to after the corresponding lectures, and as a result (I believe) the class peformed worse on the exams than other years. Remember that you will have a chance to answer similar conceptual questions after the lectures - on the exams.

Assignments:

HW22b is now due Sunday at midnight.
RWP1 is due today.
HW23b is now due Tuesday at midnight.

Chapter 22: Magnetism

22-1 The Magnetic Field
    Magnetic fields are a consequence of moving charges.
    Definition of magnetic north and south poles. All magnets have two poles - no magnetic monopoles have been found.
    How does a compas work? What do you need to consider when using a compass?
    Demo: compass and bar magnet
22-2 The Magnetic Force on Moving Charges
    Lorentz force and right-hand rule (RHR): F = qvBsin(theta)
22-3 The Motion of Charged Particles in a Magnetic Field
    Comparison and contrast with force and motion in electric field
    A charged particle entering a uniform magnetic field may move with constant velocity or in a circular or helical path.
22-4: The Magnetic Force Exerted on a Current-Carrying Wire
    Since electric current consists of moving charges, there is a corresponding force: F = ILBsin(theta)
    Prob.22.30 - Find force on a horizontal current-carrying wire where B = 0.59 G at 72 deg. below horizontal if I = 110 A directed (a) east or (b) south.
22-5: Torque on a Current Loop in a Magnetic Field: tau = NIABsin(theta)
    Prob. 22.36 - What shape of a current loop provides more torque: a circle or square?
22-6: Ampere's Law
    Magnetic field around a long straight wire: B = mu(o)I/(2pi*r)
    Force between two parallel wires: F = mu(o)I1*I2*L/(2pi*d)
    Demo: A compass is placed 5 cm from a long vertical wire. What current is needed to make the compass needle rotate 45 degrees away from north?
    Prob.22.58 - Find the net force on the current loop shown in Fig. 22-44.
    Prob.22.42 - What current is needed in a wire to produce a magnetic field of 5.0 G to control a reed switch in a pacemaker that is 0.5 m away?
    Prob.22.54 - If the minimum magnetic field that can be measured using magnetoenephalography (MEG) is 1.0 fT, what current in a neuron will produce such a field at a distance of 5.0 cm?
22-7: Current Loops and Solenoids
    Magnetic field at center of current loop with N turns: B = Nmu(o)I/(2R)
    Magnetic field inside a solenoid: B = mu(o)I(N/L)
    The magnetic field RHR is used to find the direction of B given the direction of I.
    Prob.22.74 - Find the magnetic field at the center of a solenoid with n = 55/cm, r = 1.25 cm, and I = 0.150 A. Find and compare with the current needed to produce the same field at r = 1.25 cm from a long straight wire.
22-8: Magnetism in Matter
    Ferromagnetism - Material exhibits magnetic field in absence of external B. (i.e. permanent magnets)
    Paramagnetism - Material becomes magnetized in direction of external B.
    Diamagnetism - Material becomes magnetized in direction opposite to external B. (all materials to some extent)

Demo: Find the net force on a magnet in a uniform magnetic field of 0.5 T
Health effects due to magnetic fields.

Concept Tests (use phys25 username and password to access)