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Lecture Outline: !
More on Magnetic Force on Moving Charge !
The Hall Effect !
Magnetic Force on Current-Carrying Wires !
Forces/Torques on Current Loops !
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Textbook Reading: !
Ch. 32.7 - 32.9
March 26, 2015
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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.
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Last Lecture…
Ampere’s Law
• Independent of the shape of the curve around the current.
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• Independent of where the current passes through curve.
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• Depends only on the total current passing through the area enclosed by the integration path.
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Last Lecture…
What is the magnetic field inside the solenoid?
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Last Lecture…
Magnetic Force depends on how moving charge’s velocity is oriented relative to a magnetic field.
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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?
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Magnetic Force on Moving Charge
Cyclotron Motion is result of a particle moving perpendicular to a uniform magnetic field.
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Magnetic Force on Moving Charge
184-inch cyclotron
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Modern Spin on Particle Accelerator
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Magnetic Force on a Moving Charge
• Modern cyclotrons can produce radioactive tracers used in medical applications, and can also be used directly to treat tumors.
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• 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.
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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
We know charges moving through a magnetic field experience a force. Consider charges moving through a conductor located in a magnetic field:
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The Hall Effect
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?
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Forces on Current-Carrying Wires
Magnetic field exerts a force on a current!
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Turning off the current in either wire will make the attractive/repulsive force vanish.
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Forces on Current-Carrying Wires
F on q
= qv B l q = I t = I v
F wire
= Il B
|
⇥ wire
| = IlB sin
Direction from right-hand rule.
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Clicker Question #2
The horizontal wire can be levitated – held up against the force of gravity – if the current in the wire is
A.
Right to left.
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B.
Left to right. !
C.
It can’t be done with this magnetic field.
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Forces on Current-Carrying Wires
Example: What magnetic field strength and direction will levitate the wire?
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Forces on Current-Carrying Wires http://www.youtube.com/watch?v=wa_vuX5_oAk
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Forces on Current-Carrying Wires
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Forces on Current Loops
Recall the current loop and its magnetic field.
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Forces on Current Loops
Two neighboring current loops will attract/repel each other depending on the orientation of their current.
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Clicker Question #3
If released from rest, the current loop will
A.
Move upward.
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B.
Move downward. !
C.
Rotate clockwise.
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D.
Rotate counterclockwise.
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E.
Do something not listed here.
Net torque but no net force
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Forces on Current Loops
A current loop placed in an external magnetic field will rotate.
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Forces on Current Loops
This is the basis for a simple electric motor.
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Reminders
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• Homework #8 is due Tuesday (March 31).
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