MARCH 4, 2014 MAGNETISM

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

A cylindrical ferromagnet How would you determine which pole was N if you were stranded on a desert island and had no compass?

A cylindrical ferromagnet How would you determine which pole was N if you 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.)

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

Opposite poles facing Field lines go from magnetic north to south poles..

Like poles facing

A model of the Earth’s magnetic field Where are the poles?

Finding your way Which way does the N pole of a compass needle point? (There are 2 correct answers.) A. Toward the Earth’s geographic north pole B. Toward the Earth’s geographic south pole C. Toward the Earth’s magnetic north pole D. Toward the Earth’s magnetic south pole

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

A coil with 2 turns At the top of the coil, does the current come toward you or go away from you?

What’s going on here?

No magnet

With magnet

Just the magnet

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

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

Low voltage here a, v coils

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

Which plate is at higher potential?

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

Low voltage here a, v coils Considering just the initial acceleration of the electrons between the two vertical plates, determine an equation for v 0 in terms of e, m, and V 1, where m is the mass of an electron.

Low voltage here a, v Considering just the initial acceleration of the electrons between the two vertical plates, determine an equation for v 0 in terms of e, m, and V 1, 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 v 0 at right plate Ext forces – none (gravity ignored) W ext =  E sys 0 =  K +  U el 1.Is  K positive or negative and why? 2.Is  U el positive or negative and why? 3.How is  U el calculated?

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

Low voltage here a, v coils Assuming that the electrons move with constant velocity v 0 within the region of electric and magnetic fields, determine an equation for e/m in terms of B, V 1, V 2, and d.