Brief Discussion of Midterm Problem 7: Potential on Axis of Shell

1. Brief Discussion of Midterm Problem 7: Potential on Axis of Shell

2. Moving charges create magnetic fields. Magnetic fields can be understood as consequence of Coulomb's law, charge invariance, and special relativity. (Good way to convince you that relativistic length contraction is a physical effect.) Repeat argument for parallel wires with opposite or same current directions

3. Magnets attract some materials but not all. Not all substances can be magnetic.

4. Paper clips are enclosed in an aluminum box. Then (A) the magnet will pick up the clips. (B) the magnet will not be able to pick up the clips N S

5. Magnetic fields deflect moving charged particles Electron beam bent into circular or helical path by magnetic field due to nearby currents.

6. Magnetic fields exert forces on currents Show jumping wire experiment: large current through wire that is sitting between poles of strong magnet. http://www.youtube.com/watch?v=tUCtCYty-ns

7. Currents can Repel or Attract http://www.youtube.com/watch?v=43AeuDvWc0k

8. Biot-Savart Law: Magnetic Coulomb's Law v << c

9. Physics of the Biot-Savart Law

10. What is the direction of the magnetic field at the position of the dot? • Into the screen. • Out of the screen. • Up. • Down. • Left.

11. Biot-Savart Question At this moment in time, an electron (blue) moves to the right with speed v << c and a positron (red) moves to left with same speed as shown. Then magnetic field B at black point is • Zero • Finite and up • Finite and down • Finite and out of page • Finite and into page

12. Biot-Savart in Steady Current Form Current segments must be part of closed current loop Note: Unsteady currents generate electric fields and electromagnetic waves

13. B Field of Long Wire Carrying Constant I Derivation on board of B of infinite straight wire from Biot-Savart.

14. A long, straight wire extends into and out of the screen. The current in the wire is • Into the screen. • Out of the screen. • There is no current in the wire. • Not enough info to tell the direction.

15. Compared to the magnetic field at point A, the magnetic field at point B is • Half as strong, same direction. • Half as strong, opposite direction. • One-quarter as strong, same direction. • One-quarter as strong, opposite direction. • Can’t compare without knowing I.

16. B field of two wires

17. The magnet field at point P is • Into the screen. • Out of the screen. • To the left. • To the right. • Zero.

18. A proton is shot straight at the center of a long, straight wire carrying current into the screen. The proton will Go straight into the wire. Hit the wire in front of the screen. Hit the wire behind the screen. Be deflected over the wire. Be deflected under the wire.

19. B Field of Rings Qualitatively from Biot-Savart

20. Where is the north magnetic pole of this current loop? • Top side. • Bottom side. • Right side. • Left side. • Current loops don’t have north poles.

21. What is the current direction in the loop? Out at the top, in at the bottom. In at the top, out at the bottom. Either A or B would cause the current loop and the bar magnet to repel each other.

22. The current in this solenoid Enters on the left, leaves on the right. Enters on the right, leaves on the left. Either A or B would produce this field.

23. The line integral of Baround the loop is 0· 7.0 A. Current I3 is • 0 A. • 1 A out of the screen. • 1 A into the screen. • 5 A out of the screen. • 5 A into the screen.

24. For the path shown, • 0. • 0(I1I2). • 0(I2I1). • 0(I1I2).

25. Solenoid 2 has twice the diameter, twice the length, and twice as many turns as solenoid 1. How does the field B2 at the center of solenoid 2 compare to B1 at the center of solenoid 1? • B2B1/4. • B2B1/2. • B2B1. • B2 2B1. • B2 4B1.

26. Which magnetic field causes the observed force?

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

28. QuickCheck 32.19 The horizontal wire can be levitated – held up against the force of gravity – if the current in the wire is • Right to left. • Left to right. • It can’t be done with this magnetic field. Slide 32-141

29. If released from rest, the current loop will • Move upward. • Move downward. • Rotate clockwise. • Rotate counterclockwise. • Do something not listed here.

30. Shield Slowly Varying B Fields With MuMetal