Transcript Magnetism2014 timson

Do Now (1/23/14):
Do not touch the materials on your desk until
instructed!
1. What is a magnet?
2. What are some properties of magnets?
3. How do you use magnets in your life?
4. Are some magnets stronger than others?
Investigate:
• Work with your group to complete the
activity. You have ten minutes!!!
Like repels
like…
Opposites attract!
Magnetism Resources:
• http://coe.kean.edu/~afonarev/Physics/Mag
netism/Magnetic%20Fields%20and%20For
ces-eL.htm
•Magnets have been known for centuries.
•The ancient Greeks used a stone substance
called “magnetite.” They discovered that
the stone always pointed in the same
direction.
•Later, stones of magnetite called
“lodestones” were used in navigation.
What is Magnetism?
force of attraction
or repulsion of a
magnet due to the
arrangement of its
atoms, particularly
its electrons.
•Magnetic effect is strongest at
the poles (ends)
•Each magnet has 2 poles – 1
north, 1 south.
Poles of a magnet
always come in
pairs!
If you cut a magnet in half,
you get 2 magnets!
Magnetic Fields
The region where the magnetic forces
act is called the “magnetic field”
Atoms themselves have magnetic properties due
to the spin of the atom’s electrons.
Groups of atoms join so that their magnetic
fields are all going in the same direction
These areas of atoms are called “domains”
When an unmagnetized substance is placed in a magnetic
field, the substance can become magnetized.
This happens when the spinning electrons line up in the
same direction.
An unmagnetized
substance looks like
this…
While a magnetized
substance looks
like this…
How to break a magnet:
1. Drop it
2. Heat it
This causes the
domains to become
random again!
The Earth is a magnet:
surrounded by
a magnetic
field that is
strongest near
the North and
South
magnetic poles
Magnetic South Pole
Geographic South Pole
Geographic North Pole
Magnetic North Pole
Sometimes,
the Earth’s
magnetic
poles flip.
This happens
every halfmillion years
or so.
Magnetic North Pole
Magnetic South Pole
Use the Earth’s magnetic field to find direction.
The needle of a compass always points
toward the magnetic south pole.
We call this direction “North” (remember, opposites attract)
Do Now (1/24/14):
(Do not touch the materials on
your desk until instructed!)
Write down three things
you learned yesterday
about magnets
Magnetic Field
1) Review:
a) Natural permanent magnets
– Like poles repel, unlike attract
– come in pairs (no monopoles)
– Interact with earth;
define N (or north-seeking) pole as pole
attracted to North pole of earth
b) Magnetic field direction:
- direction of force on N pole
B
Review
• What is an electric dipole?
Review:
• (in your notes) Draw the electric field of an
electric dipole
Hypothesize (2 min):
• What do you think the magnetic
field of a bar magnet would look
like? Draw it in your notes.
• Discuss with your elbow partner
Investigation
• Work with your table to complete the
Investigation. You have twenty minutes to
complete the activity.
Results:
• Go to the board and draw the field lines that
you discovered. If yours looks like another
group’s, put a check mark next to it.
Field of a magnetic dipole
d) Magnetostatics for poles
(identical to electrostatics for charges)
–
–
–
–
–
2 types: N, S vs +,Unlike attract, like repel
Inverse square law
Force along joining line
Magnetic Field:

F
B
qM
Why study magnetism?
– No monopoles (yet)
– Poles (dipoles) produced by moving charges
– Charges affected by magnetic field
The fundamental unit is still charge
– magnetic fields can be created by charge
– The force on a charge can be due to magnetic field
Magnetic fields do not
interact with
stationary charges!!!!!
Magnetic Field
• Represented by B
• Units: Tesla
• Brainstorm: how big do you think
Earth’s magnetic field is?
-5
10
T!!!!!
Force on a moving charge in a
magnetic field
• Proportional to component of v perp to B
F  qvBsin 
(Alternative definition of B)
• Perpendicular to B
• Perpendicular to v
Force on a moving charge
what would result in a force of zero?
F  qvBsin 
• charge traveling parallel to B
• charge at rest
Force on a moving charge
what would result in a maximum force?
F  qvBsin 
• charge traveling @ 90° to B
Example:
What is the force on an
electron moving at 43 m/s in
a magnetic field of 0.5 T?
Practice:
• Work on the Magnetic Force Worksheet
Do Now (1/27/14):
(On your Do Now sheet from last
week)
• How fast is a proton moving in a
magnetic field of 0.8 T if the
magnetic force exerted on it is
0.9 N?
Finding Directions of B-Fields
• Consider an arrow
OUT OF THE PAGE
INTO THE PAGE
The Right Hand Rule!
• Follow along on your paper
• Three different methods – find the one that
works for you!
#1 (ON YOUR PAPER)
• Direction of v:
To the right
• Direction of B:
Out of the page
• Direction of F: DOWN
(TOWARDS THE
BOTTOM OF THE PAGE)
Practice:
• Complete the Right Hand Rule Worksheet
by the end of class.
• If you finish early, please continue working
on your homework (Magnetic Force)
Do Now (1/28/14):
1. Draw the following on your paper:
a. A magnetic field pointing to the right
b. A proton traveling towards the top of the page
c. What is the direction of the force exerted on the
charge?
2. If the proton travels at 3000 m/s and the magnetic
field has a magnitude of 4 T, what is the force
exerted on the proton?
3. What is the proton’s acceleration?
RHR: Electron vs. Proton
• What if an electron travels through the field
instead of a proton?
2) Magnetic field due to current (direction)
• Oersted (1820)
Right Hand Rule #2:
I
B
r
3) Magnetic force on current
a) Orthogonal case
Force per unit length
F
 IB
defines B
Direction from RHR1: B fingers, I thumb, F palm
Practice:
• Use the rest of class to work on your HW
(Magnetic Force and/or Force on a CurrentCarrying Wire)
Do Now (1/29/14):
• What is the force on a 15 cm wire carrying
a 10 A current surrounded by a 0.2 T?
Force on a current carrying wire
• Look on your homework paper.
Example:
• A current in the +x direction and a
magnetic field in the –y direction
Investigate!
• Work on ONE of the investigations for full
credit.
• Work on both for extra credit!
Do Now (1/30/14):
• Come in quietly, pass in your Do Now’s
and Homework, then wait for further
instructions.
Units:

F
N
B 
 tesla (T)
I
Am
Bearth  .5 gauss  5 105 T
Bfridge magnet  .01T
Bsuper conducing  110 T
b) General case
Force per unit length
F
 IBsin 
L
4) Force between parallel wires
I1
B ;
d
F

F
 I2 B
I1I2
 k 
d
FE
+
v
FB
FB
+
v
FE
Attraction or repulsion?
Does it depend on reference frame?
-
-
+
+
-
-
+
+
-
-
+
v
+
v
F
I1I2
 k 
d
• Define Ampere as the quantity of current that
produces a force per unit length of 2 x 10-7 N/m
for separation of 1 m

• Then
(2 107 N/m)(1m)
7
2
k 
 2 10 N/A
2
(1A )
• This defines C and gives

k
1
40
 8.988 10 Nm /C
9
2
2
• Permeability of free space
0  2k  4  10 N/A
7

Then
F
0 I1I2

2 d
2
0
 k 
2
5) Field due to long straight wire (magnitude)
I
B
r

0 I
B
2 r
6) Force on a moving charge
• Zero at rest
• Zero parallel to B
• Max perpendicular to B
• Proportional to component of v perp to B
F  qvBsin 
(Alternative definition of B)
• Perpendicular to B
• Perpendicular to v
7) Motion of a charge in a magnetic field
a) Constant force
motion is parabolic
electric or gravitational field
not everywhere perp to velocity
not magnetic field
Mass spectrometer:
• Diagram:
b) Constant magnitude perpendicular to motion
radial field (circular motion)
mass on a string
motion is circular
magnetic field
produces circular
motion
(initial vel. perp. to B)
Force due to the field:
F  qvB
For circular motion:
2
mv

F  Fc 
r
So,

mv
r 
qB

mv 2
 qvB
r
r depends on v, B
v
qB
 
r
m
angular freq.
independent
of speed,
radius
Tracks in a bubble chamber
• electron-positron creation
• 1, 3 positive
• 2 negative
• energy: 3 > 2 > 1
• energy decreases by collisions
Example: Find speed and radius for proton
B = 0.10 T
V = 2100 V
c) Work done by magnetic field
Work by a force F
F


displacement, x
W  Fx cos 
For a magnetic field,   0

Work = 0

d) Velocity selector
Force due to E (down):
FE  qE
Force due to B (up):

FB  qvB
For zero deflection, FE = FB :


qE  qvB
E
v
B
e) Mass Spectrometer
Ion energy:
2qV
KE  mv  qV  v 
m
1
2

Radius of motion:
mv  m 2qV
r

qB
qB m
r


2
m
q
2V
B

m r 2B 2

q
2V
Additional Info about Magnets
William Gilbert, an
English physician,
predicted in 1600 that
the Earth would be
found to have
magnetic poles.
The sun has a magnetic field, too.
It extends far above the sun’s
surface.
Other planets in the solar
system also have these
magnetic fields
When a charged
particle enters a
magnetic field, an
electric force is
exerted on it.
If a charged
particle moves at
an angle to a
magnetic field,
the magnetic
force acting on it
will cause it to
move in a spiral
around the
magnetic field
lines.
The solar wind is constantly bombarding
the Earth’s magnetic field. Sometimes
these charged particles penetrate that field.
These particles are found in two large
regions known as the Van Allen Belts.
The Earth’s magnetic field extends far into
space. It is called the “magnetosphere.”
When the magnetic particles from the sun, called “solar
wind”, strike this magnetosphere, we see a phenomenon
called…
The Aurora Borealis in the Northern Hemisphere
And the Aurora Australis in the Southern Hemisphere