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Lecture 11.2 :!Magnetic Force on Moving Charge

Lecture Outline:!More on Magnetic Force on Moving Charge!

The Hall Effect!Magnetic Force on Current-Carrying Wires!

Forces/Torques on Current Loops!!

March 26, 2015

Textbook Reading:!Ch. 32.7 - 32.9

Announcements

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•Homework #8 due at 9am on Tuesday, March 31 in Mastering Physics. !

•Quiz #5 (last quiz of the semester) next Thursday in class. Will cover Ch. 32 material.

Last Lecture…

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•Independent of the shape of the curve around the current.!•Independent of where the current passes through curve.!•Depends only on the total current passing through the area enclosed by the integration path.

Ampere’s Law

Last Lecture…

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What is the magnetic field inside the solenoid?

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?!!

Magnetic Force on Moving Charge

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Cyclotron Motion is result of a particle moving perpendicular to a uniform magnetic field.

Magnetic Force on Moving Charge

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184-inch cyclotron

Modern Spin on Particle Accelerator

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Magnetic Force on a Moving Charge

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•Modern cyclotrons can produce radioactive tracers used in medical applications, and can also be used directly to treat tumors.!•Technetium-99 is the most common isotope used in nuclear medicine. !•Technetium produced by radioactive decay of a rare isotope of Molybdenum (Mo-99...which comes from fission of U-235) or ~20 MeV proton bombardment of a more common isotope (Mo-100).

Technetium is radioactive and decays, producing easily detectable photons. Allows function of kidneys

and other organs to be studied.

Protons accelerated in cyclotrons can be targeted at tumors within the body (right), causing less collateral

damage than X-ray methods (left).

The Hall Effect

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We know charges moving through a magnetic field experience a force. Consider charges moving through a

conductor located in a magnetic field:

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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Magnetic field exerts a force on a current!!Turning off the current in either wire will make the

attractive/repulsive force vanish.!

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.

Reminders

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!•Homework #8 is due Tuesday (March 31).!!