Physics 10
Lecture 24A
"When you feel bad about yourself you
reverse your magnet and repel people."
--S A Grafio
History of Magnets
Magnets were discovered by early man.
Asia Minor has region known as Magnesia
where rocks would attract each other.
Early researchers found that if you put a
magnet on a string it would want to face a
certain direction (i.e. towards the North Pole).
Thus, magnets were used as compasses.
Later on it was discovered that magnets will
only attract iron or other very select metals
(cobalt, nickel, etc.).
History of Magnets
Researchers also found that magnets “always”
came with two sides a north pole and a south pole.
North poles attract south poles. But north poles
repel north poles and south poles repel south
poles.
With the aid of compasses,
magnetic field lines were
mapped for simple shapes of
magnets.
It was defined that magnetic
field lines exited the north end
and entered the south end.
History of Magnets
Magnetic field lines are actually easier to visualize
than electric field lines.
Just put some iron filings next to a magnet and
they will align themselves with the magnetic field.
History of Magnets
But it was later discovered that inside the physical
magnet the magnetic field lines actually moved
from south to north.
This means that
magnetic field lines
(unlike electric field
lines) formed an
endless loop.
If a particle were to
follow a magnetic field
line it would
eventually come back
to the same location.
History of Magnets
Many similarities between magnetism (MG) and
electrostatics (ES) were found:
(ES): Two types of charges, + and –.
(MG): Two types of poles (north and south).
(ES): Like charges repel, opposite charges attract.
(MG): Like poles repel, opposite poles attract.
(ES): Can move objects at a distance.
(MG): Can also move objects at a distance.
History of Magnets
There are also many differences between
magnetism (MG) and electrostatics (ES):
(ES): Can separate a + away from a –.
(MG): Can’t isolate a north pole from a south pole.
S
N
S
N
S
N
S
N S
(ES): Field lines always start on + and end on -.
(MG): Field lines have no end; endless loops.
N
History of Magnets
In 1820, it was discovered
that electric current
produced a magnetic field.
That led to an interesting
question: What did
moving electrical charges
have to do with rocks in
Magnesia?
Everything as it turns out,
since all magnetism is
fundamentally caused by
moving charges.
History of Magnets
In iron, for example, the magnetic properties are
caused by moving electrons.
The moving electrons act as current, creating
magnetic fields. The iron atoms act like tiny magnets
that appear to have north and south poles.
There are not really
north and south poles
there, only locations that
appear to act that way.
Poles are a man-made
creation to explain
magnetism.
Magnetic Fields
The proper way to handle magnetic forces is
to concentrate on magnetic fields.
A magnetic field is located in a region of space
surrounding a moving charge.
This charge will also have an electric field
surrounding it.
A magnetic field is a vector quantity given by:
The SI unit of the magnetic field, B is the Tesla.
Earth’s magnetic field is: 0.5x10–4 Teslas.
Magnetic Fields
We will treat magnetic forces and fields how
we treated electric forces and fields.
We will use a two-step process:
step (i): Moving charges, I, creates a B field.
step (ii): Moving charge, q, experiences
magnetic force from the field.
exerts
force on
creates
other moving
Moving charges
B field
charges
(current I)
q
[Amps]
[Teslas]
[Coul]
via RHR1
via RHR2
Magnetic Fields
Note that a moving charge will not create a
magnetic field, B, that will exert a force on
itself.
That violates Newton’s Third
Law (you need two objects
to have a force).
Let’s examine the simplest
case of a long, straight
wire with current I.
This current will create a
magnetic field, B,
surrounding it.
Magnetic Fields
The direction of the magnetic field from the
wire will be given by Right Hand Rule 1.
Grasp the wire in your
right hand.
Point your thumb in the
direction of the current I
(positive charge flow).
Your fingers will curl in
the direction of the
magnetic field.
Step 1
Many times you will be asked
for the magnetic field at a
given point.
I
Here you will just apply
RHR1, but take the tangent
of the curve at the given
point for your answer.
For example, if we were standing below the wire
we would observe the magnetic field moving to
the right.
But if we were standing above the wire we would
observe the magnetic field moving to the left.
Step 2
Then to find the direction
of the magnetic force on a
moving charge, q, due to
magnetic field, B use RHR2.
Note: this works for a
positive charge, q, only.
So, if the moving charge that is experiencing the
force is a negative, just change the direction of
the resulting magnetic force by 180o.
Basically, just flip the final force direction.
Note: A charged particle at rest will not interact
with a magnetic field.
Force on a Wire
If the current in a wire is moving
downward and the magnetic field is
into the board apply Right Hand
Rule 2:
Your positive current velocity points
down (thumb)
Your magnetic field points into the
board (forefinger).
Middle finger then points to the
right.
Force is to the right since current is
defined as positive charge moving.
Force on a Beam of Charges
If a moving charged particle (either positive or
negative) is placed in an external magnetic field
it will feel a force.
The force that this charged particle feels will
be perpendicular to both the magnetic field
and the particle’s velocity.
As in the picture, via RHR2 the electron would
feel an upwards force.
If it were a proton
the force would
instead be down.
Clicker Question 24A-1
Two current-carrying wires are exactly parallel to one
another and both carry 1 Amp of current. The
current in wire 1 moves up and the current in wire 2
moves up, as well. What is the direction of the
magnetic field caused by wire 1 at the location of
wire 2?
A) Into the page.
B) Out of the page.
C) Up.
D) To the left.
E) To the right.
Wire 1
I
I Wire 2
For Next Time (FNT)
Start reading Chapter 25.
Start the homework for
Chapters 23 and 24.