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 ifyou were stranded on a desert island and had no

compass?

A cylindrical ferromagnet

How 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.)

Magnetic field lines as shown by iron filingsThe lines converge on the poles.

The field is strongest where the lines are most dense.

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

.

Like poles facing

A model of the Earth’s magnetic fieldA model of the Earth’s magnetic field

Where are the poles?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 poleB. Toward the Earth’s geographic south poleC. Toward the Earth’s magnetic north poleD. 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 voltageto accelerate electrons

Provides the currentto produce a magnetic field

High voltageapplied

here

Circular coils produce the magnetic field

Low voltage

here

Evacuatedtube

Fluorescent screenshows 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?

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 v0 in terms of e, m, and V1, 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 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 platesInitial state – electron with 0 velocity at left plateFinal state – electron with speed v0 at right plateExt forces – none (gravity ignored)

Wext = Esys

0 = K + Uel

•Is K positive or negative and why?•Is Uel positive or negative and why?•How is Uel 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 v0 within the region of electric and magnetic fields, determine an equation for e/m in terms of B, V1, V2, and d.