Monday, May 11, 2020
Unizor - Physics4Teens - Electromagnetism - Magnetic Field - Inside Magnet
Notes to a video lecture on http://www.unizor.com
Magnetism - Internal Structure
Let's look inside a permanent bar magnet with two poles, North and South.
We model its magnetic properties as a result of a cumulative properties
of individual electrons rotating along parallel axes within parallel
planes in the same direction.
Each such rotating electron represent a tiny magnetic dipole with
its own North and South poles with attracting force between opposite
poles (North and South) and repelling force between the same poles
(North to North or South to South).
The attraction between two rotating electrons that face each other by
opposite poles we have explained by the fact that in this case electrons
rotate in the same direction and "help" each other. The repelling of
two rotating electrons that face each other by the same poles is
explained by the fact that they rotate in opposite directions and
"disturb" each other.
Since we are talking about permanent magnet, all axes of rotation of
electrons are always parallel to each other and planes of rotation are
always parallel as well.
Consider a situation of two electrons rotating on parallel planes around the same axis on the same radius.
In this case the magnetic properties of the South pole of the upper (on
this picture) electron are neutralized by properties of the North pole
of an electron under it.
So, the magnetic field of a pair of electrons in this position is the
same as for one electron with poles located on a greater distance from
Now expand this logic to a full size of a bar magnet. The result is that
all internal connections between South and North poles will neutralize
each other and the only significant magnetic properties are of those
electrons concentrated on two opposite surfaces of a magnet where its
North and South poles are located.
This looks like some magnetic charges of opposite types, that we called North and South, are concentrated on two opposite ends of a magnet.
These magnetic charges behave similarly to electric charges,
except magnetic ones always come in pairs. We can even think about
magnetic equivalent of the Coulomb Law. The only complication is that we
always have a superposition of two magnetic fields coming from two ends
of magnetic dipole.
This is the Gilbert model of magnetic properties, attributed to
William Gilbert, an English physician (including a physician for English
royalty), who published in 1600 a six volume treatise that contained
all the information about electricity and magnetism known at that time.
Gilbert was the one who discovered magnetic properties of Earth and came
up with formulation of properties of magnets and terminology that
describes them (like magnetic poles).
Consider a different approach - two electrons rotated within the same
plane around parallel axes and immediately near each other. The common
plane of rotation is, of course, perpendicular to the magnet's
North-South axis and axes of rotation of these electrons are parallel to
the magnet's North-South axis.
Electrons moving near each other are moving in opposite directions and
neutralize each other, as if there is no current there at all. So,
within every plane perpendicular to the North-South axis of a magnet all
inner currents are neutralized, and the only really present current is
around the outer boundary of a magnet.
This is the Ampere model of magnetism. It makes the magnetic
properties of permanent magnet equivalent to properties of an electric
current in a loop around the side surface of a magnet with each electron
moving within a plane perpendicular to a magnet's North-South axis.
This model of magnetism is extremely important, as it connects the
magnetic properties to those of properties of electric current and shows
inherent connection between electricity and magnetism.
It also opens the door to electromagnetism - generating magnetic field using electricity.
A loop of electric current acts similar to each electron inside a
permanent magnet, just on a larger scale. A number of electric current
loops of the same radius around the same axis parallel to each other
makes the magnetic field even stronger.
If we make a loop of electric current and put an iron cylinder (which by
itself does not have magnetic properties) inside this loop, the iron
cylinder will become magnetic, and the more loops the electric current
makes around this cylinder - the stronger the magnetic properties of an
iron cylinder will be, and it will act exactly as the permanent magnet,
But, as soon as we stop the flow of electric current around this cylinder, it will lose its magnetic properties.
Another important feature of the Ampere model is that it allows
to measure the strength of the magnetic field produced by an
electromagnet by such known physical quantities as amperage of the current circulating in the wire loops, producing the magnetic field, and some geometric properties of the wire loops.