[geocentrism] Re: magnetism and inverse sq.
- From: "Robert Bennett" <robert.bennett@xxxxxxx>
- To: <geocentrism@xxxxxxxxxxxxx>
- Date: Mon, 30 Apr 2007 19:49:31 -0400
Maybe you didn?t get it, because it isn?t right.
From Wiki:
Inverse Squared Law of Magnetic Fields at close Distances
Close to one pole of a magnet, field strength diminish as the inverse square
of the distance. This is because it behaves as a "unipolar magnetic field"
(that is, the close pole seems much stronger than the far pole, so the far
pole can be ignored). Gravity is also a unipolar field, and it also
diminishes as the inverse square of distance; but, unlike magnetic fields,
gravitational fields always obey the inverse squared law.
Inverse Cubed Law of Magnetic Fields at far Distances
Far from a magnet, both poles appear to be practically at the same point.
Mathematically, this "dipolar magnetic field" diminishes as the inverse cube
of distance. Hence, far from Earth, the geomagnetic field diminishes as the
inverse cube of distance.
Robert
-----Original Message-----
From: geocentrism-bounce@xxxxxxxxxxxxx
[mailto:geocentrism-bounce@xxxxxxxxxxxxx]On Behalf Of philip madsen
Sent: Monday, April 30, 2007 6:13 PM
To: geocentrism list
Subject: [geocentrism] magnetism and inverse sq.
I could not follow the math, algebra, but I gues they mean that dipoles
upset the inverse square law . Gravity does not have dipoles so everything
is ok there.
Is inverse sq designated by a minus 2 which becomes minus 4 with a magnetic
dipole.. Now does a -4 mean that with magnets the decrease in power much
more rapidly by the power of 4 of the distance .. If they had drawn the
graph, I might have gotten it.
Philip.
The Force between Two Magnets as a Function of Distance
The force between two magnets is not necessarily inverse square.
A digital scale can be used to measure the force between two magnets as a
function of distance.
In this drawing the magnets are black. The upper magnet is glued to a meter
stick.
Introduction
You can use a scale with a digital readout to measure the force between two
magnets as a function of distance. A plot reveals that the force does not
decrease as the inverse square of the distance.
Material
A scale to measure weight (with a digital readout.)
If your scale will read to +- 0.01 gram then:
two magnets, 1.25 inch diameter disk magnets from Radio Shack work well.
If your scale only reads to +- 0.1 gram then you will need to use neodymium
magnets at least 2 cm x 2 cm x1 cm which exert larger forces on each other.
Graph paper and a calculator on which to take logs or log-log paper.
a hot melt glue gun.
A meter stick and a way to mount the meter stick over the pan of the scale,
e.g. a ring stand with test tube clamps.
A metric ruler
Assembly
Tape one magnet on the pan of the scale.
Zero the scale.
Hot-melt-glue the second magnet to the end of the meter stick, measurements
will be easier if you glue it near the "0" end.
Mount the meter stick so that the magnet is exactly over the meter stick on
the pan of the scale. Also mount the meter stick so that it can be easily
slide up-and down, varying the distance between the two magnets. Create a
reference point next to the meter stick that the distance the meter stick is
moved can be measured.
To Do and Notice
Check that the scale reads zero when the upper magnet is removed. (Use the
"Tare" option on the scale.)
Mount the magnet on the meter stick as far from the magnet on the scale as
possible.
Note the reading on the scale, it should still be zero, or nearly zero.
Move the magnet on the meter stick closer to the magnet on the scale. The
reading should increase if the magnets are set to repel. If the reading
decreases remove the magnet from the scale and turn it over. Then start over
again.
Move the magnet on the meter stick down until the reading on the scale
increases.
Use the ruler to measure the distance between the centers of the two
magnets. (This distance is equal to the distance between the tops of two
identical magnets.)
Measure the force on the scale and record your data.
Continue to move the magnets together recording the force of their repulsion
versus the distance between their centers. You should be able to devise a
way of doing this with the meter stick to which one magnet is glued.
Continue measuring until the two magnets are almost touching.
Math Root Make a Graph
Plot a graph of force versus distance on regular graph paper.
Notice how the force changes with distance. The force gets weaker as the
distance decreases.
Make a log-log plot of your data. ????
Plot a graph of force versus distance on log-log paper, or plot the log of
the force versus the log of the distance on regular graph paper.
Notice that over a range of distances the data lie on a straight line on the
log-log plot.
Measure the slope of the straight line. (Use a ruler to measure the rise
versus the run.) The slope of the line will be near minus four.
What's Going On?
On regular graph paper the graph of force versus distance is not a straight
line. However on a log-log plot it is.
A straight line on a log-log plot is produced when the dependent variable is
a power law of the dependent variable. i.e. when y = a xb.
The slope of the graph of y versus x on a log-log plot gives the power, b.
If the line has a slope of 2 then y = x2. If the line has a slope of -2 then
the force is y = x-2. If the force between the two magnets decreases as the
inverse square then a log-log plot of force versus distance would be a
straight line with a slope of -2. It is not. It is a line with a slope
of -4.
If you could have magnetic monopoles, just a north pole or just a south
pole, then the force between these monopoles would indeed decrease
proportional to the inverse square of the distance. However magnetic
monopoles have never been found in nature. Magnets always come with a pair
of poles, north and south together called a magnetic dipole.
The force between two pairs of poles falls off proportional to the inverse
fourth power of the distance between them. (The like poles repel and the
unlike poles attract. The nearer poles exert stronger forces on each other
than the more distant ones.)
So, at large distances, the force between these two magnets should fall off
proportional to the inverse fourth power of distance. (Where large distance
here means a distance large compared to the largest dimension of the
magnet.)
These donut magnets are not tiny dipoles. When they get closer, each dipole
on one magnet interacts with all the dipoles on the other, the dipoles range
over many different distances at the same time. The resulting dependence of
force on distance is complicated but when the distance between the magnets
get small compared to the diameter the force becomes more constant with
distance. The log-log plot rolls over and is no longer a straight line.
Other measurements
Measure the force between a magnet and a steel washer on the balance.
Notice that the dependence of force on distance is different. In particular
the force drops off more quickly for the steel washer than it does for the
magnet. The washer becomes magnetized when it is near a magnet. The
magnetization of the washer falls off in proportion to the strength of the
magnetic field from magnet. The magnetic field of the first magnet falls off
as the inverse third power of distance, combined with the inverse fourth
power fall off of the force between two magnets the resulting force falls
off as the inverse seventh power. (However if the steel washer is not a
perfect soft magnet, that is if it retains magnetization when removed from
the presence of a magnet then the force will fall off somewhat more slowly
than the inverse seventh power.)
Measure the forces between magnets with different shapes. Long rod shaped
magnets have their north and south poles separated by a large distance. Over
distances large compared to the diameter of these magnets and small compared
to the length the force between two poles will be nearly inverse square.
Etc
The electric force between two spheres of charge decrease with distance
inversely as the square of the distance. However When the charge is arranged
in two long lines then the force falls off inversely proportional to
distance (for distances small compared to the length of the lines.)
While if the charge is arranged in to sheets the force does not change with
distance.
The same proportionalities hold for lights as well The intensity of a point
of light decreases as the inverse square of distance, from a line of line
inversely proportional to distance, and from a large flat plane of light the
intensity does not decrease with distance.
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