Class notes by RE-SEED Leader - Alex Vanderburgh

RESEED Notes Class 13 2 Oct 2001 Framingham Magnetism 13-1
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Magnetism
Like electricity, it is a wonder we ever discovered
magnetism! It does occur naturally in the form of "lodestone", an
ore that is magnetized enough in its natural state to serve as a
magnetic compass, but no other use comes to mind. "Magnet" comes
from the Greek word "Magnesia" a place in Thessaly where
lodestones were found. "Lode" is an Anglo-Saxon word meaning
"leading" or attracting. Magnets were not used for compasses by
the Greeks and Romans. The earliest literature on this point
comes from China. Compasses were in use in Europe in the twelfth
century.

The connection with electricity was not known until the
1800's! Since then, a lot has been discovered. There are 30
pages on Magnetism in the Encyclopaedia Brittanica, 1947.

Both electricity and magnetism have "fields of force". A
charged comb will attract small particles and similar charges
repel each other. Magnets will attract and repel eachother. In
the time of the ancient Greeks, the forces were very small. There
was no way to control them, and no useful work could be done. In
fact no one knew that electricity and magnetism were related!

Then came Volta's battery and it was possible to create
steady currents. At first, it was thought that there was no
magnetic field associated with a current. It was almost by
accident that the field was found - in a plane perpendicular to
the plane of the circuit wires! With a steady current, you could
make a controllable "electro" magnet, and with some ingenuity a
"Motor". It was also discovered that the reverse was true. You
could generate electricity by moving a conductor through a
magnetic field!

There is one remaining piece of the story. Iron is good at
concentrating a magnetic field. A coil of wire makes a good
magnet, but if it is wound around a core of iron, it is hundreds
of times stronger! [An opportunity to coin a word!
"Ferromagnetism". Ferro is latin for iron.] The affinity of a
material for magnetism is called its "magnetic permeability". A
similar word for the electric field is "electric permitivity". It
can be shown that the reciprocal of the product of their free
space values is proportional to the square of the velocity of
light! Something to look forward to - "Electro-Magnetic
Radiation" - radio waves, radiant heat, light, X-rays etc.]
Another genius - James Clerk Maxwell -applied higher mathematics
to the subject and produced "Maxwells Equations"
- the foundation of Electro-Magnetic theory. He forcast the
existance of radio waves long before they were created by the
engineers.
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Demo 1 Bar Magnet [10 disk magnets will do. Radio Shack has
them.]
Demo 2 Floating disk magnets [Put them on a post - with
alternate polarity.
Demo 3 Magnetic Compasses [kit compass, ring on a string,
drinking straw & ring magnets, floating needle]
Demo 4 Electromagnetism - Current and compass.
Demo 5 Electromagnet - wire and nail
Demo 6 Telegraph
Demo 7 Various devices - bell, motor, etc.
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RESEED Notes Class 13 2 Oct 2001 Framingham Magnetism 13-2
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Equipment required:
magnets Nuts and Bolts compasses
wire and nail needle motor
battery generator meter
straws thread copper wire
plastic cup
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A Little Philosophy

When we introduce Electricity and Magnetism we are at a
disadvantage for human beings can not experience them directly.
We knew what Force, Weight, Time, and Length were before the
Science Teacher began to talk about them, but Voltage, Current,
Charge, Resistance, "fields" etc. are all new ideas to
middleschool students. So you can expect them to drift away
easily. The waterwheel, pressure, pump etc. analogy is useful as
a start, but it should be replaced quickly via real circuits with
a voltmeter, an ammeter, light bulbs, electro-magnets, and small
motors. The idea is to give them a lot of "electro - magnetic"
hands-on experience. The bulbs and forces will be familiar. The
idea of a "circuit" becomes natural - alomst obvious. [Nothing
works without a completed circuit!]

Some teaching includes statements that are to be believed
without justification. We have done some of it om previous
classes - e.g. "There are 94 basic elements, and about a dozen
more that have been made in the laboratory." We believe this
statement and assert that our students will discover it to be true
later in their education. But I feel that where ever possible, we
should not ask students to take "facts" on faith alone. Mass,
Time, Force, and Length are assumed to be understood from
experience. We defined their units of measure, and observed some
interesting derived ideas, such as "Work", "Momentum", etc. The
Physics of forces, velocities, and such is easy to accept as long
as we are content to stay within human experience. We did give
the warning that the game changes if velocities approach the
"speed of light".

But electricity and magnetism are not as well known from
experience. Our students know it from static discharge in the
winter, and from power shortages that cancel school and force us
to light candles. We can show them interesting sparks and
invisible (small) forces from rubbing insulators such as wool and
plastic. Unlike the Ancient Greeks, we can also show our students
a flashlight battery. [Probably with a story about Galvani and
Volta to explain where they came from.]

Meanwhile, "magnets" were a curiosity that occurred in nature
and were used to make direction finders - i.e. magnetic compasses.
No one knew that magnetism was related to electricity.

RESEED Notes Class 13 2 Oct 2001 Framingham Magnetism 13-3
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Coulomb, Volta, Ampere, and Ohm worked independently on
electricity using the "Voltaic Pile" or battery, and four common
units used to measure electricity were named in their honor.

Coulombs for charge in honor of Charles Agustin de Coulomb.
Volts for voltage in honor of Allesandro Volta.
Amperes for current in honor of Andre Ampere.
Ohns for resistance in honor of Georg Simon Ohm.

Electricity was thought to be a "flow of charge" through a
"conductor". An analogy with water flow is unavoidable, and
useful as an introduction to electric current. What can we say
about "voltage and current"? [Are they like water pressure and
flow? I would say the math is much easier for electricity!]

There is one more "parameter" - "resistance". Ohm is given
the honor for this one. He showed that the intensity of the
electric current was proportional to the voltage ["potential"] .

We don't know what we are talking about, but at least we have
the words! Voltage is "electric pressure", Current is "the flow
of Electric Charge", and Resistance is the factor of
proportionality between them. (V = RI or R = V/I or I = V/R)
[The letter "A" is often used for current, but I is the first
letter of the French word, and Ampere was a Frenchman. The letter
A will be used by electric power engineers - as in VA or KVA .]
With some effort, we can make measuring instruments for each of
these.

This is a bit arbitrary. We could have said that
"conductance" is the factor of proportionality between current and
voltage. We call it G. (I don't remember if G stands for
anything.) So G = I/V or I = VG, or V =I/G. Conductances in
"series" and "parallel" seem to behave strangely at first.

The same can be said for rubber bands. The force on a rubber
band is proportional to the distance stretched. What is the
constant of proportionality for combinations of different
rubberbands in series and in parallel? It is this kind of
thinking we are asking our students to do.

The force of an electromagnet is related to the intensity of
the current. We know how to measure forces. (Spring balances.)
A "Galvanometer" is a very sensitive electro-magnet designed with
a small spring balance to measure the magnetic force of a tiny
electric current. We want to measure Voltage and Current without
changing either. Our meter should have "no effect" on the
circuit. Typically the current measured by a galvanometer is
around 20 to 50 micro-amperes. How do we make a difference
between current and voltage?

1. To measure voltage, we connect the galvanometer through
a very large resistor. Only a tiny current can flow! If it
is small enough, we can say it has no effect and therefore we
can make a scale for the galvanometer that reads the VOLTAGE
in Volts. [We say a voltmeter looks like an OPEN circuit,
and therefore will not change the circuit.]

RESEED Notes Class 13 2 Oct 2001 Framingham Magnetism 13-4
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2. To measure current, we break the circuit, and install a
known very small resistance. ( Called a "shunt". ) We
assume the resistance is so small, we can neglect its affect
on the current. However, there is still a tiny voltage
across our shunt. We therefore connect the galvanometer
across the shunt and make up a special scale so that we
have a galvanometer that reads the CURRENT in Amperes. [We
say the Ammeter looks like a short circuit.

We can calibrate the voltmeter via a "standard cell". It
turns out that carefully made voltaic cells produce a reliably
identical voltage. (No doubt the chemists can explain that this
has to be so. They can probably give us a current standard too -
perhaps by timing the rate of silver-plating.)

3. We can also use the galvanometer to measure charge, but it is
not easy. Essentially you use a torsion balance, and measure the
distance an unknown charge will move it.

It is tempting at this point to introduce a simple
theoretical model - "electrons". You can say it is a little round
ball of stuff that carries the smallest charge we have ever
measured. You can say that current is a flow of these things.
But the danger is that you will not make it clear that this thing
is just a useful idea that will become so handy it will seem to be
real. I remember a movie that showed how a vaccum tube triode
worked. The cathode was a tree full of monkeys. Electrons were
coconuts. The Plate was another tree with monkeys wearing
baseball gloves. The grid was a shutter like a venetian blind.

I feel that models - such as "electrons" - should be the
result of an investigation, and perhaps a shorthand for
discussion. They should not be presented as a "reason" for the
experimental results. For example, finding a minimum charge leads
to the idea of "electrons". The idea did not cause the observed
fact.

Be careful when you introduce symbols. The symbols for
sources - "constant voltage source" and "constant current source"
do not reflect the real world. A constant voltage source has a
short circuit current of infinity. A constant current source has
an open circuit voltage also of infinity. We make them more
"real" by adding a series resistor to the voltage source and a
parallel conductance to the current source. If you put them in
boxes and bring out only the load wires, there is no test you can
make at the terminals that will tell you which is which! We use
the same symbol for conductance as for resistance. (The unit of
"conductance" is a "mho". That is Ohm spelled backwards. Most of
us prefer resistance.)

Symbols tend to move our thoughts away from the real world.
We find ourselves discussing the "circuit" and what math we have
to do, and we forget that the real world could be something else.

Now we can measure the "potential" and the "current". To measure
the resistance, we can meaure these two and calculate the ratio
V/R. We can make a "OHMMETER" by using a known voltage source
and a series resistor that would make the galvanometer read full
scale. Note that an additional series resistor would read half
scale. We can mark the galvanometer scale with resistance values,
and make an Ohmmeter! The scale will read backwards. This was a
big enough deal to get a Name - It is called OHM'S LAW.

Usually stated: I = E/R (Where I is current, E is Voltage,
and R is resistance.)

One more thing. Faraday discovered that there is a magnetic
field associated with current, and Oersted showed that a conductor
moving in a magnetic field creates a current! Forces are involved
in both! This led to motors and generators! It all started about
400 years ago with the development of the battery!