March 2 Physics 54 Lecture Professor Henry Greenside

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March 2 Physics 54 LectureProfessor Henry
Greenside
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Outline

• Hello some bio, some research interests.
• The big picture where we are heading and why.
• Chapter 28
• - Magnetic fields of straight-wire currents
• - Forces between straight-wire currents
• - Amperes law how B is related to currents
• - Solenoids and toroids.
• - Note we are skipping Section 28-6 (no
  Biot-Savart!)
• 4. Application of what you have learned
  creating a star to save the Earth (tokamaks and
  stellarators).

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PRS Answers Posted Later

1. Will add answers to PRS questions a day or so
   after the lectures files are posted online.
2. Answers will be in the Notes section of the
   PowerPoint slide tell PowerPoint to go to the
   Normal View and then notes are available in
   thin panel under the PRS slide.

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Key Ideas and Formulas From Previous Lecture
Magnets and coils produce similar fields.
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PRS Question Nearby Current Loops
Two circular current loops with current
going in same direction float side by side in the
same tub of water. Then they will
1. attract each other. 2.
repel each other. 3. do
nothing. 4. I have no idea (I
thought this course was
Polysci 91).
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At the White Board

1. Quantitative description of B field produced by
   straight wire carrying constant current. B is
   radially oriented and becomes weaker with
   distance from wire.
2. Calculation of force/length between two parallel
   wires carrying constant currents same attract,
   opposite repel (opposite of charges).
3. Various worked examples, various PRS examples,
   related to interaction of straight wires or
   superpositions of straight wires.

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Revisit Nearby Current Loops
Knowing that the magnetic field becomes weaker
with distance from a wire and using the Lorenz
force Fq (v x B), or the fact that a current in
a B field feels a force, we can understand this
problem more directly
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PRS Question Crossed Wires
(1)
(2)
(3)
Two straight wires are carrying equal currents I
in the directions shown (the wires are close but
do not touch each other). Then the diagram that
shows where the total magnetic field can become
zero (dotted line) is (1)
(2) (3) 4 Some other diagram.