# Magnets by fionan

VIEWS: 9 PAGES: 24

• pg 1
```									                 Magnets
• Magnetism is closely related to electricity
• Magnets are materials in which some
electric charges are in orderly orbits
• We shall learn about how moving charges
create magnetism
Magnets
Every magnet has two
special ends called poles.
We designate these as the
north pole and the south
pole for historical reasons.
Like poles repel and unlike
poles attract.
Magnets
If we saw a magnet in
half, we create a new
north pole and a new
south pole.
Poles always occur in
pairs. No magnetic
monopoles have ever
been observed.
Magnetic Fields
• When we dealt with electric charges, it was
convenient to define the interaction between
charges as taking place due to each charge
creating an electric field.
• Similarly, we can define the concept of a
magnetic field established by the two poles
of a magnet.
Magnetic Fields
• The field lines originate at a north pole and
terminate at a south pole
Magnetic Fields
• As before, the number of lines per unit area
is proportional to the strength of the field
• The direction of the field is tangent to a line
at any point in space.
• We will define the strength of a magnetic
field in terms of the torque the field applies
to a compass needle.
Magnetic Fields
The magnetic field
strength B is
proportional to the
torque applied to the
compass needle.
We will define it
precisely later.
Earth’s Magnetic Field
The field doesn’t point to
true north. The
difference is called the
declination. Also, the
field is not exactly
tangent to the earth’s
surface at all points. The
deviation is called the
angle of dip.
Earth’s Magnetic Field
Can be lots of confusion.
The magnetic pole in the
Arctic is really a south
magnetic pole. The
magnetic pole in the
Antarctic is really a north
magnetic pole. Note how
the compass needle is
labeled.
Magnetic Fields
• The simplest magnetic field occurs between
the poles of two very large magnet pole
faces. Near the center the field will be
simple and straight.
Magnetism and Currents
• It was found long ago that a stationary
electric field and a stationary magnetic field
do not interact.
• Oersted found that if you pass a current
through a wire, a magnetic field is created
around the wire!!!
• The charges have to be moving!!!
Magnetism and Currents
The magnetic field lines are
circles which close on
themselves. Note that there is no
north pole or south pole. The
field lines simply are arranged as
circles around the wire.
We can find the direction by
using the right-hand rule.
Right Hand Rule
Point the thumb of your right
hand in the direction of the
conventional current (+ to -) and
your fingers will curl in the
direction of the magnetic field
lines.
Circular Loop of Wire
If you have a circular loop of
wire carrying a current, the
magnetic field lines run
perpendicular to the plane of the
loop. We can use another right-
hand rule to determine the
direction.
Circular Loop of Wire
Definition of B
• We know that a current-carrying wire exerts
a force on a compass needle (a magnet)
• By Newton’s Third Law, the compass
needle must exert a force on a current-
carrying wire
• Experiment confirms this to be true
Definition of B
The current flows from front
to back. The magnetic field
goes from left to right. The
foce is downward!!!
Isn’t this a strange result.
Test by reversing the
current.
Definition of B
Now the current goes from
back to front, while the
magnetic field still goes from
left to right. Now the force is
in the opposite direction, that
is down to up!
We need a rule to help us
remember.
Definition of B
Definition of B
• Each component is perpendicular to the
other
• I perpendicular to B and to F
• B perpendicular to I and to F
• F perpendicular to B and to I
• WEIRD!!!
• Just remember the right hand rule
Definition of B
• Now that we have the directions straight,
we have to get the quantitative relationship
• As you might guess from all the direction
business, this is a little messy
Definition of B
Here is a current-carrying wire in a
magnetic field. The force on the
wire, using the right hand rule, is
into the screen.
Force depends directly on length of
wire and size of current. Also
depends on the angle between the
current and the field. When angle is
zero, the force is zero. When angle
is 90o the force is a maximum.
F = I x length x B x sine(theta)
Definition of B

F  I Bsin 
• The unit for B is the tesla
• :You may encounter the unit gauss
• One gauss is 10-4 tesla
Definition of B
Can precisely measure the field
with the loop shown here. Forces
on the vertical portions of the
wire cancel out. The force on the
left side wire is to the left. The
force on the right side wire is to
the right. Leaves only the force
on the horizontal wire which is
down! Hang from a balance.

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