Download CHAPTER 19 Magnetism

CHAPTER 19
Magnetism
eproton
There is a relationship between electricity and
magnetism.
A moving electric charge produces a magnetic field
(B-field).
Question:
All materials contain moving electric charges
(electrons). So why are not all materials magnetic?
Answer:
Their atoms are randomly orientated and the
B-fields (vectors) cancel each other out.
Question:
Why are there any magnetic materials?
Answer:
Some materials contain “domains” ( 10-4m across)
where the motion of electrons is roughly aligned.
When these domains are aligned, the material
becomes magnetic.
Domains Not Aligned
(No Magnetism)
Domains Roughly Aligned
(Some Magnetism)
Magnetic Materials (have domains)
Iron
Nickel
Cobalt
A compass needle is a magnet.
Magnets have “north” and “south” poles.
Opposites attract. Like poles repel.
The pointed end of the compass needle is a north
pole and it points to a south pole (magnetic) in the
picture and when used to find your way home.
Magnetic Field Lines
A model to help visualize the invisible.
Exit north pole of a magnet.
Enter south pole of a magnet.
Never cross.
Magnetic Fields
Created by Long Current Carrying Wires
I
r
I
B=
r
o I
B=
2r
B
=
o
2
o = permeability of free space
o = 4x10-7 Tm/A
o I
B=
2r
Magnitude of B-field
What about the B-field direction?
Use another Right-Hand-Rule
I
Grab wire with thumb pointing
in direction of current.
Fingers wrapping around wire
point in direction of B-field
Magnetic Forces Acting on Moving Charges
N
v
B-field
q+
S
A magnetic force, FB, is exerted on a charged
particle moving through a magnetic field.
q = electric charge of the particle (C)
v = velocity of particle (m/s)
B = strength of magnetic field (T)
T = Teslas
 = angle between velocity and Bfield vectors
where =90;  = 90 (often in AP Physics)
FB = q v B sin
FB = FB,max
FB = q v B
[B] =
[
[B] = [T]
[B] =
]
N
C  m/s
[
Wb
m2
]
Wb = Weber
FB = q v B = magnitude of the force
But what is its direction?
2nd Right Hand Rule (where v  B)
With right hand flat:
• Outstretched fingers point in direction of B-field.
• Thumb points in direction of velocity of a
positively charged particle. [Use opposite direction
or left hand for velocity of a negatively charged
particle]
• Palm points in direction of magnetic force acting
on the moving charge.
Example #1
B
v
B
Example #2
FB
q+
FBX
What is the direction of FB?
Note “x” denotes tail of arrow
pointed into paper
v
q-
B
What is the direction of B-field?
Note “ ” demotes tip of
arrow pointed out of paper.
B
F
v
Motion of a Charged Particle Moving in a
Magnetic Field
What do we call this
type of force?
Answer:
Centripetal (FC)
mv2
FC =
r
F C = FB
FB is the
centripetal force
mv2
=qvB
r
mv
r= qB
From Right Hand Rule
we see that FB is always
Often used formula
directed towards the
easily derived
center of circular path.
Magnetic Forces Acting on Current Carrying Wires
I
l
FB = q v B
L
FB = q
B
t
FB = I L B
L
v=
t
q =I
t
l = length of wire in
the magnetic field
Test Yourself
What will be the direction of the FB acting on
the wire?
Answer:
FB
Forces on Parallel Current Carrying Wires
I
I
A second current carrying wire is placed in this B-field.
What is the direction of FB on this second wire?
Use RHR.
FB on right wire points toward left wire.
Parallel wires with same direction current are
attracted to each other.
Prove left wire is attracted to right wire.
Prove if current are opposite directions, wires
repel each other.