March 2 Physics 54 LectureProfessor Henry Greenside

Outline

1. Hello: some bio, some research interests.

2. The big picture: where we are heading and why.

3. Chapter 28: - Magnetic fields of straight-wire currents - Forces between straight-wire currents - Ampere’s 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).

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.

Key Ideas and Formulas From Previous Lecture

BU

B

ANI

BEqF

BlIdFd

)v(

Magnets and coils produce similar fields.

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).

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.

Revisit: Nearby Current Loops

Knowing that the magnetic field becomes weaker with distance from a wire and using the Lorenz force F=q (v x B), or the fact that a current in a B field feels a force, we can understand this problem more directly…

PRS Question: Crossed Wires

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.

(1) (2) (3)