Transcript Physics B (AP)

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
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
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?
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
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:
Low Use conservation of energy. Set up the problem as follows:
voltage
System
– electron and vertical plates
here
Initial state – electron with 0 velocity at left plate
a, v v at right plate
Final state – electron with speed
0
Ext forces – none (gravity ignored)
Wext = DEsys
0 = DK + DUel
1.Is DK positive or negative and why?
2.Is DUel positive or negative and why?
3.How is DUel 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?
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