Download Chapter 36: Magnetism

Chapter 20: Magnetism
• Purpose: To describe magnetic field
around a permanent magnet.
• Objectives:
• Describe a magnetic poles
• Describe magnetic field.
• Magnetic field lines.
Warm-up
• Where and how do we use magnets?
• What is an electro magnet and where is it
used?
Describing a magnet
• magnet is a material
or object that
produces a magnetic
field.
A bar magnet
• Horse shoe magnet
Magnetic poles
• Magnetic poles are regions that produce
magnetic forces
N
North pole
s
South pole
Like poles repel, unlike poles attract.
• South repels south, North Repels North,
and South attracts North.
Breaking the magnet
• Magnetic poles
cannot be isolated.
• When you break a
magnet into two you
end up with two
magnets.
• Keep breaking further
and further, you still
get the same results.
Magnetic field
• The magnetic field
(usually denoted B) is
a space around a
magnet where
magnetic force is
exerted.
• The SI units of
magnetic field (B) is
Tesla (T)
• Iron fillings trace out a pattern
of magnetic field lines in the
space around the
magnet.
Magnetic field lines
• Magnetic field is
represented by
magnetic field lines
• where lines are closer
together, the field
strength is greater.
• The direction of magnetic
field is, from North pole to
south pole.
Nature of magnetic field
• What is in a magnet that makes it a magnet?
 Permanent magnets, like all other substances
are composed of atoms that contain electrons.
These electrons are in a continuous motion.
 Electrons have two types of motion;
 a) spinning on their positions and
 b) Orbiting around the nucleus.
Magnetic Materials
source of all magnetism
• The source of all
magnetism is moving
electric charge
(motion of electrons)
– Every spinning and
orbiting electron is a
tiny magnet that
creates a magnetic
field around an atom.
Question;
– What is the source of
magnetic field on a bar
magnet?___________
__________________
Magnetic domains
• Magnetic domains are • Irregular shaped domains
with aligned dipoles
large cluster of
aligned atom.
• Each domain contains
dipoles (magnetized
atoms) that are
aligned.
Magnetic and non magnetic
• Magnetic materials
have electrons
spinning in the same
direction. This leads
to aligned atoms with
a strong magnetic
field around them.
– Examples include Iron,
Nickel, and Cobalt
• For non magnets,
the electron spins
opposite to one
another causing their
magnetic fields to
cancel each other.
Therefore, no net
magnetic field exists
EARTHS MAGNETIC FIELD
Electron Vs Earth
Magnetic declination
• Magnetic declination
is the discrepancy
between the
orientation of a
compass and true
North.
• That means that
compasses do not
point to true
Geographic North.
• Magnetic declination
Homework
1. Using double bubble Differentiate
between magnetic poles from electric
charges.
2. Textbook Pg 576 # 1 -6
Warm-up
• What is the source of all magnetic field?
Beam of electrons
• Since moving electrons create magnetism,
a beam of electrons will have a magnetic
field around it.
Electric currents produce Magnetic fields
• Straight conductors (wires) will have magnetic
fields around them. The magnetic field form
complete loops around the conductors.
Direction of magnetic field
• First Right hand rule
– The thumb indicate the direction of current.
– The four fingers indicate direction of magnetic field.
Right hand rule
• First Right hand rule
Magnetic field
Electric current
Question
– Consider a beam of electrons moving from
west to east, what is the direction of
magnetic field at point G?
N
W
E
e-
G•
S
Question
• What is the direction of a magnetic field at
the center of a current carrying loop with a
counter-clockwise flow of current?
answer
• Out of the page
Magnetic Field Produced by a Coil
• This is how magnetic field lines looks like in and
out of an electromagnet
Counterclockwise
current---North pole
Clockwise
current
flow--South Pole.
North and south poles of an
electromagnet.
• Second Right hand rule.
•Wrap the fingers
around the coil in the
direction of current.
•The thump will indicate
the North pole.
question
• What is the direction of the magnetic field
inside the coil shown on the right?
Class work
• Textbook Pg 576 # 7-16
Warm-up
•
How come a magnet can attract a nail?
•
What kind of charge, negative or
positive, attracts a neutral object?
•
What happens to the electrons within a
iron nail when it gets magnetized?
Warm-up
•
What happens when you put a small
compass in a magnetic field say of a
straight conductor?
Lab
• Lab- magnetic field
Magnetic force on moving charge
• What happens when a charge/current
moves near a magnetic field?
B
Magnetic field
A
Current
carrying
conductor
N
S
Direction of current and magnetic field
• Maximum force is
achieved when the
current is flowing
perpendicular to the
magnetic field.
• If the current is
flowing parallel to the
magnetic field, there
will be no interaction
and therefore no force
Direction of Force on a conductor
• The conductor is forced to move in a certain direction.
• The Third right hand rule indicates the direction of the
force.
•
Concepts in Motion
What is the direction of the magnetic force
on the current in each of the six cases below
Magnitude force on a current carrying wire.
Magnetic force
• Magnetic force
formula is just the
magnetic field formula
rearranged as shown;
• When Current (I) and
the length (L) of
conductor is given we
use the 2nd formula.
F  vq  B
F  I  L B
The magnetic field formula
F
B
v q
• Where;
• B = magnetic field,
• V = speed of the charged
particle, and
• q = charge of the particle,
• F = force of the field
Example
• A proton speeding at 3.0 x107 m/s
experiences a magnetic field of 4.0
Teslas. What is the magnetic force
pulling on the proton?
Class work
• Textbook page 577, # 1-7
Path of a charged particle
• The path of a
charged particle in
a uniform magnetic
field is a circle.
(Just as the force
of gravity causes
the moon to move
in a circle around
the earth).
• Force on the
particle, and
particle’s motion
are perpendicular
to each other.
Path of a charged particle
• Thus, a charged particle moves in a
circular path with constant centripetal
acceleration.
2
mv
• Therefore;
qvB 
r
mv
r
qB
Example;
• An electron travels at 2.0 x 107 m/s in a
plane perpendicular to a uniform 0.01 T
magnetic field. Describe the motion of the
electron quantitatively.
Helical path of a charged particle
• If the velocity of
the charged
particle is not
perpendicular to
the magnetic
field, the path of
the charged
particle
becomes
Helical.
Why Helical path
• The velocity component
perpendicular to the field
results in circular motion
about the field lines.
However, the velocity
component parallel to the
field lines results in no
force. The particle
continues to move in the
direction of that vector.
Direction of charge
• Electrons (negative charges) move in the
opposite direction of positive charges and
current.
e

p

Magnetic field at distance r from the wire
• Magnetic field around a
straight conductor is
directly proportional to the
current through the wire
and inversely proportional
to the radius or ( the
perpendicular distance
from the wire)
I
B~
r
Magnetic field at distance r from the wire
I
B~
r
0I
B
2r
0
where,
is the constant of proportionality
2
 0  4 x 10-7 T  m / A {permitivity of free space},
0
therefore;
 2.0  107 T  m / A
2
Example
• An electric wire in the wall of a building
carries a DC current of 25 A vertically
upward. What is the magnetic field due to
this current at point p 10 cm due north of
the wire?
Exercise
• Textbook pg 577 # 8-18
Warm-up:
• Can a stationary electron be set into
motion with a magnetic field? With an
electric field? Explain.
Class work
•
•
•
Workbook problems
Pg 226; #s 1- 4 and
Pg 231; #s A1- A5
• Worksheet- Magnetism
Force between two parallel wires
• Two parallel wires
carrying currents I1
and I2 separated by a
distance r, exerts a
force F on each other
given by the formula:
 0 I1 I 2
F2 
l2
2 r
Wire 1
• consider the field produced by wire 1 and the force it exerts
on wire 2 (call the force F 2 ). The field due to I 1 at a
distance r is given to be;
 0 I1
B1 
2 r
Force exerted of wire 2
• This field is uniform
along wire 2 and
perpendicular to it, and
so the force F 2 it exerts
on wire 2 is given by;
F2  I 2 B1l2
Force exerted on wire 2
• Substituting for B1
from;
 I
B1 
F2  0 I1 I 2

l
2 r
0 1
2 r
F2  I 2 B1l2
•We get the formula for
the force F2 as;
 0 I1 I 2
F2 
l2
2 r
or
F2  0 I1 I 2

l2
2 r
Direction of force
• When the currents in the two wire is in the
same direction, the force is attraction.
• When the currents are in opposite
direction, the force is repulsion.
Class work
• Textbook pg 578 # 26-32
Putting electricity to work
• This force can then be used for daily work
such as;
– sharpen pencils
– Cut down trees,
– Driving cars,
– In other words, we transform electrical energy
to mechanical energy in order to do work.
Simple Galvanometer
• This is a meter that
detects small electric
currents.
• When electricity is
present in the coil,
each loop produces
its own effect on the
needle causing it to
deflect.
Electric motor
• This is the
device used to
transform
electrical
energy to
mechanical
energy
• Concepts in
Motion
Direct Current Motors
• Direct Current Motors
Loud speakers
• Loud speakers convert
electric energy to
mechanical energy just
as Motors do.
• Changing current in a
magnetic field creates
varying forces on the
conductor, and varying
motions of the paper
cone which is attached
to the conductor.
Research Homework
• Explain how a motor works
• Explain how a speaker works