Download Preclass video slides - University of Toronto Physics

Physics
ys cs 132:
3 Lecture
ectu e 18
8
Elements of Physics II
A
Agenda
d for
f T
Today
d



Magnets and the Magnetic Field
Magnetic fields caused by charged particles
 B-field from a currentcurrent-carrying wire
Magnetic fields
f
and forces
f
 B-fields
 Magnetic forces on moving particles
Physics 202: Lecture 8, Pg 1
Magnetism




Tape a bar magnet to a
piece of cork and allow it to
float in a dish of water.
It always
y turns to align
g itself
in an approximate northsouth direction.
The end of a magnet that points north is called
the north-seeking
gp
pole,, or simply
p y the north
pole.
The end of a magnet that points south is called
the south pole.
Physics 202: Lecture 8, Pg 2
Magnetism

Like poles repel, opposite poles attract
Physics 202: Lecture 8, Pg 3
Magnetism



Magnets can pick up some
objects,
j
, such as p
paper
p clips,
p ,
but not all.
If an object is attracted to
one end of a magnet,
magnet it is
also attracted to the other
end.
Most materials,
materials including
copper (a penny),
aluminum, glass, and
plastic experience no force
plastic,
from a magnet.
Physics 202: Lecture 8, Pg 4
Magnetism


Cutting a bar magnet in half produces two
p
magnets,
g
each with a
weaker but still complete
north pole and a south pole.
No matter how small the magnets are cut, even
down to microscopic sizes
sizes, each piece remains a
complete magnet with two poles.
Physics 202: Lecture 8, Pg 5
Magnetism
1.
2.
3
3.
4.
5.
Magnetism is not the same as electricity.
Magnetism is a long range force.
All magnets have two poles
poles, called north and
south poles. Two like poles exert repulsive
forces on each other; two opposite poles
attract.
attract
The poles of a bar magnet can be identified by
using it as a compass. The north pole tends to
rotate
t t to
t point
i t approximately
i t l north.
th
Materials that are attracted to a magnet are
called magnetic
g
materials. The most common
magnetic material is iron.
Physics 202: Lecture 8, Pg 6
Mapping Magnetic Field
Physics 202: Lecture 8, Pg 7
B-field

Similar to electric charges a magnet will create a
magnetic field in the area around it

Due to convention we will call the magnetic field a B-field

The force magnets apply on each other can be described
as an interaction between a magnet and another
magnet’s B-field
Physics 202: Lecture 8, Pg 8
B-field lines

We will also draw magnetic field lines

( ) The
(1)
ed
direction
ec o o
of a magnetic
ag e c field
e d is
s tangent
a ge to
o
a magnetic field line at any point

(2) The number of lines per area is proportional
to the magnitude of the B-field
Physics 202: Lecture 8, Pg 9
B-Field Lines
Physics 202: Lecture 8, Pg 10
Earth’s Magnetic Field

The Earth’s magnetic
field resembles that
achieved by burying a
huge bar magnet deep
in the Earth’s interior
Physics 202: Lecture 8, Pg 11
B-field of CurrentCurrent-Carrying Wire

In 1819 Hans Christian Oersted discovered that
an electric current in a wire causes a compass to
turn.
Physics 202: Lecture 8, Pg 12
B-field of CurrentCurrent-Carrying
y g Wire

The right-hand rule determines the orientation of the compass
needles to the direction of the current.
Physics 202: Lecture 8, Pg 13
B-field of CurrentCurrent-Carrying Wire
 Magnetic field lines are
imaginary lines drawn
through a region of space
so that:
th t
 A tangent to a field line is
in the direction of the
magnetic field.
 The field lines are closer
t
together
th where
h
the
th
magnetic field strength is
larger.
Physics 202: Lecture 8, Pg 14
RHR
Physics 202: Lecture 8, Pg 15
The Source of the Magnetic Field: Moving
Charges

The magnetic field of a charged
particle q moving with velocity v
is given by the Biot-Savart law:
Physics 202: Lecture 8, Pg 16
The Magnetic Field
The constant 0 in the Biot-Savart law is called the
permeability constant:
0 = 4 × 10-7 T m/A = 1.257 × 10-6 T m/A
The SI unit of
magnetic field
strength is the tesla,
abbreviated as T:
1 tesla = 1 T = 1 N/A m
Physics 202: Lecture 8, Pg 17
Currents Create B-Fields
Magnitude:
 0I
B
2r
Current I OUT
B
r
0  4   10 77 Tm / A
•
r = distance from wire
Direction of b-field from current
carrying wire:
Curly right hand rule
Lines of B
Thumb along current I,
direction of B along curled fingers.
Physics 202: Lecture 8, Pg 18
Give it a try:
Compared to the magnetic field at point A, the magnetic
field at p
point B is
A. Half as strong, same direction.
B. Half as strong, opposite direction.
C. One-quarter as strong, same
di ti
direction.
D. One-quarter as strong, opposite
direction.
direction
E. Can’t compare without knowing I.
Physics 202: Lecture 8, Pg 19
Give it a try:
What is the direction of the magnetic field at the
position of the dot?
A.
A
B.
C
C.
D.
E
E.
Into the screen.
screen
Out of the screen.
Up
Up.
Down.
Left
Left.
Physics 202: Lecture 8, Pg 20