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Lecture 11.2 :"Magnetic Force

Lecture Outline:"Forces on Current-Carrying Wires"

Forces on Current Loops"Magnetic Properties of Matter"

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March 27, 2014

Textbook Reading:"Ch. 32.7 - 32.10

Announcements

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•Homework #9 due on Monday, April 1 in Mastering Physics."

•Note physics dept. colloquium today (3:45pm) is relevant to MRIs and medical imaging:"

•“Big Data in Biomedicine: Multidimensional Analysis of Spatio-Temporal Image Patterns”,

Prof. Axel Wismueller from the University of Rochester.

Last Lecture...

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Magnetic Force depends on how moving charge’s velocity is oriented relative to a magnetic field.

Clicker Question #1

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Which magnetic field (if it’s the correct strength) allows the electron to pass through the charged electrodes without being deflected?"!

The Hall Effect

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Sign of Charge carrier matters here. “Hall” voltage is established across conductor.

Hall probes can measure magnetic field strengths. How else might we measure magnetic fields?

Forces on Current-Carrying Wires

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�Fon q = q�v � �B

�Fwire = I�l � �B

q = I�t = Il

v

|⇥Fwire| = IlB sin� Direction from right-hand rule.

Clicker Question #2

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The horizontal wire can be levitated – held up against the force of gravity – if the current in the wire is

A. Right to left."B. Left to right. "C. It can’t be done with

this magnetic field.

Forces on Current-Carrying Wires

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Example: What magnetic field strength and direction will levitate the wire?

Forces on Current-Carrying Wires

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http://www.youtube.com/watch?v=wa_vuX5_oAk

Forces on Current-Carrying Wires

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Forces on Current Loops

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Recall the current loop and its magnetic field.

Forces on Current Loops

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Two neighboring current loops will attract/repel each other depending on the orientation of their current.

Clicker Question #3

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If released from rest, the current loop will

A. Move upward."B. Move downward. "C. Rotate clockwise."D. Rotate counterclockwise."E. Do something not listed here.

Net torque but no net force

Forces on Current Loops

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A current loop placed in an external magnetic field will rotate.

Forces on Current Loops

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This is the basis for a simple electric motor.

Magnetic Properties of Matter

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We now understand how moving charges create magnetic fields, but what about materials that seem to have inherent magnetism?

Atoms, like hydrogen, can have a magnetic moment due to the orbiting electron “current”.

Electrons have their own intrinsic magnetic moment! Unpaired electrons in an atom contribute to the atoms magnetic moment.

Magnetic Properties of Matter

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In the semiclassical Bohr model of the hydrogen atom, the electron moves in a circular orbit of radius 0.53x10-10 m with speed = 2.2x106 m/s. According to this model, what is the magnetic field at the center of a hydrogen atom? (Problem 32.73)

Magnetic Properties of Matter

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Even though atoms may have a magnetic moment, the atoms in a material can be randomly arranged, canceling each other out.

Ferromagnets are materials where the atomic magnetic

moments line up.

Magnetic Properties of Matter

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Moving domain walls in a grain of silicon steel caused by an increasing external magnetic field in the "downward" direction. White areas are domains with magnetization directed up, dark areas are domains with magnetization directed down. - Source: Wikipedia

Magnetic domains in ferromagnets can be altered by external field.

Magnetic Properties of Matter

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A slice of ferromanganese crust from the Pacific Ocean was analyzed using a SQUID microscope at Vanderbilt University. The top image shows a portion of the image of the slice taken with an electron microprobe. The second image shows the magnetized regions in the slice, with red areas showing one direction of magnetization and blue the opposite direction.

http://web.mit.edu/newsoffice/2011/squid-geomagnetism-0304.html

Magnetic Properties of Matter

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Probably all of you are carrying an example of a magnetic material where the arrangement of domains matters.

Reminders

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"•Homework #9 is due Tuesday (April 1)."•Quiz #5 next week (April 3). Covers Ch. 32."!