March 4, 2014

1. March 4, 2014 magnetism

2. Announcements & Reminders • To the teacher: Turn on the recording! • To students: • WebAssign quiz on Chs. 22,23 is Wednesday, March 12. Submit C11 by this Friday. • Tonight, we will... • Review ferromagnetism • Review magnetic fields and forces • Examine an application to an experiment to determine the charge-to-mass ratio of the electron

3. A cylindrical ferromagnetHow would you determine which pole was N ifyou were stranded on a desert island and had no compass?

4. A cylindrical ferromagnetHow would you determine which pole was N ifyou were stranded on a desert island and had no compass?Tie a hair around the middle of the magnet and suspend it by the hair. The magnet will rotate until it aligns itself with the Earth’s magnetic field. The end of the magnet pointing in the general direction of geographic north is the magnetic north pole of the magnet. (You can do try this using your string and bar magnet.)

5. Magnetic field lines as shown by iron filingsThe lines converge on the poles. The field is strongest where the lines are most dense.

6. Opposite poles facingField lines go from magnetic north to south poles..

7. Like poles facing

8. A model of the Earth’s magnetic fieldWhere are the poles?

9. Finding your wayWhich way does the N pole of a compass needle point? (There are 2 correct answers.)A. Toward the Earth’s geographic north poleB. Toward the Earth’s geographic south poleC. Toward the Earth’s magnetic north poleD. Toward the Earth’s magnetic south pole

10. Magnetic field around a current-carrying wireIs the current going into the page or coming out of it? (Green = North)

11. A coil with 2 turnsAt the top of the coil, does the current come toward you or go away from you?

12. What’s going on here?

13. No magnet

14. With magnet

15. Just the magnet

16. An experiment to determine the charge-to-mass ratio of the electron Provides the high voltage to accelerate electrons Provides the current to produce a magnetic field

17. Evacuated tube Circular coils produce the magnetic field Low voltage here a, v Charged plates produce a vertical electrical field High voltage applied here Fluorescent screen shows electron path

18. Low voltage here a, v coils

19. With no current in the coils, what is the direction of the electric force on the electrons?

20. Which plate is at higher potential?

21. What is the direction of the electric field between the plates?

22. What is the direction of the electric field between the plates?

23. Considering just the initial acceleration of the electrons between the two vertical plates, determine an equation for v0 in terms of e, m, and V1, where m is the mass of an electron. Low voltage here a, v coils

24. Considering just the initial acceleration of the electrons between the two vertical plates, determine an equation for v0 in terms of e, m, and V1, where m is the mass of an electron. • Strategy: Use conservation of energy. Set up the problem as follows: • System – electron and vertical plates • Initial state – electron with 0 velocity at left plate • Final state – electron with speed v0 at right plate • Ext forces – none (gravity ignored) • Wext = DEsys • 0 = DK + DUel • Is DK positive or negative and why? • Is DUel positive or negative and why? • How is DUel calculated? Low voltage here a, v

25. With current in the coils to produce a magnetic field, the path tends to straighten out. What is the direction of the magnetic field?

26. Assuming that the electrons move with constant velocity v0 within the region of electric and magnetic fields, determine an equation for e/m in terms of B, V1, V2, and d. Low voltage here a, v coils