Download Document

Units
Tesla (SI)
N/(Cm/s)
N/(Am)
Gauss
1 T = 104 gauss
Magnetic
Dipole
Magnetic
Field (B)
Magnetic Monopoles
Do not exist!
In this way they differ
from electric dipoles,
which can be separated
into electric monopoles.
Magnetic Force on
Charged Particle
 magnitude: F = qvBsin
–q: charge in Coulombs
–v: speed in meters/second
–B: magnetic field in Tesla
–: angle between v and B
 direction:
Right Hand Rule
Magnetic Force
Calculate the magnitude and
direction of the magnetic force.
v = 300,000 m/s
34o
q = 3.0mC
B = 200 mT
Magnetic Fields
Formed
by moving
charge
Affect moving charge
Magnetic Forces can...
 accelerate
charged
particles by changing their
direction
 cause charged particles to
move in circular or helical
paths
Magnetic Forces cannot...
 change
the speed or
kinetic energy of charged
particles
 do work on charged
particles
When v and B are at right angles
to each other...
qvBsin = mv2/r
qB = mv/r
q/m
=
v/(rB)
V
B
F
V
F
F
F
V
V
Electric and Magnetic Fields
Together
B
q
E
v = E/B
Magnetic Force on
Current-carrying Wire
F = I L B sin
–I: current in Amps
–L: length in meters
–B: magnetic field in Tesla
–: angle between current
and field
Magnetic Field for
Long Straight Wire
 B = moI/(2r)
• mo: 4  10-7Tm/A
•magnetic permeability of
free space
• I: current (A)
• r: radial distance from
center of wire (m)
Hand Rule
i
•

r




Curve your fingers
Place your thumb (which is
presumably pretty straight) in
direction of current.
Curved fingers represent
curve of magnetic field.
Field vector at any point is
tangent to field line.
For
straight
currents
I
Principle of Superposition
When there are two or more
currents forming a magnetic
field, calculate B due to
each current separately and
then add them together
using vector addition.
Beyond
the 4th
Grade
I
Magnetic Field Inside a
Solenoid
 B = monI
• mo: 4  10-7Tm/A
• n: number of coils per unit
length
• I: current (A)
Lab: Magnetic Field Map
Using a compass, map the magnetic
field inside and outside your
solenoid. Show the following:
a) Tracing of solenoid (true size)
b) Current through solenoid
c) Connection to DC outlet
d) Field lines mapped with compass
e) Edge effects (What happens to
those field lines farther away from
the solenoid? Can you explain it?)
f) North and South Poles of solenoid
g) The Compass Rose
h) One drawing per group (Write all
names on paper)
I
Lab Evaluation
Magnetic Field Map
A) Are field lines visible in the core of the
solenoid?
B) Are the directions of the field lines in the
core consistent with the Right Hand Rule?
C) Are the North and South Poles correctly
identified?
Magnetic Flux


The product of magnetic
field and area.
Can be thought of as a
total magnetic “effect” on
a coil of wire of a given
area.
Magnetic Flux

FB = BAcos
 FB: magnetic flux in
Webers (Tesla meters2)
 B: magnetic field in Tesla
 A: area in meters2.
 : the angle between the
area and the magnetic
field.
Magnetic Flux


A system will respond so
as to oppose changes in
magnetic flux.
Changing the magnetic
flux can generate
electrical current.
Faraday’s Law of
Induction
 e = -NDFB/Dt
 e: induced potential (V)
 N: # loops
 FB: magnetic flux
(Webers, Wb)
 t: time (s)
A closer look …
 e = -DFB/Dt
 e = -D(BAcos)/Dt
To generate voltage
Change B
Change A
Change 
Lenz’s Law


The current will flow in a
direction so as to oppose
the change in flux.
Use in combination with
hand rule to predict
current direction.
Announcements
– AP Review every morning at
7:00 AM.
– Lunch Bunch tomorrow
(come with pretest
completed).
– Exam Thursday
– 1988 AP Free Response:
Thursday and Friday, after
school or at lunch
Challenge Problem #3
How large a force is needed to
move the rod at a constant speed
of 2 m/s? How much power is
dissipated in the resistor?




























3
W


50
 cm

















B = 0.15
T





v = 2m/s








