Class notes by RE-SEED Leader - Alex Vanderburgh

RESEED Notes - Class 12 -2 Oct 2001 -Framingham -Electricity 12-1
------------------------------------------------------------------
Electricity:
How did we ever discover Electricity? Is there some easy way to show that it exists and is as useful as we all know it is? Lightning and certain eels display it worldwide. Colder climates have "static" electricity. Combs attract bits of paper. Clothing "clings". Static electricity has been good for fun and games since the Greeks gave it a name, but is "static" electricity the same as "real" electricity? Our parents keep us from experimenting with the "real" stuff, but we can learn a lot from the fun and games. The idea of current can be demonstrated with static
electricity since it is related to insulators and conductors.
[Well a continuous current is hard to get...] We have known about electricity and magnetism for a over 2000 years, but we did not know they were related!

Can you think of anything in nature that shows, or even hints that there is a relationship between Electricity and Magnetism! It was the discovery of a chemical battery by Volta (c. 1800) that made it possible to generate continuous currents. We did not notice their connection with magnets and force, until about 1812 or so with the work of Oersted and Faraday. The discovery that the magnetic field is in the plane perpendicular to the current was accidentally discovered and came as a surprise!

Controlled currents became possible with Volta's chemical batteries. [By stacking a large number in series, they showed that it was the same thing as "static" electricity.] Then Ampere showed that a flowing current was accompanied by a magnetic force, and Faraday showed that a magnetic force could do the opposite. [i.e. That we could make electricity by forcing a wire to move in a magnetic field.] THEN, we were off to the races - generators, motors, voltmeters, ammeters, et al.

In this lesson, we can investigate the fun and games of static electricity. We can make some speculations about "charge" and "electrons". We can make simple batteries [e.g. lemons and potatos], and do some battery and light bulb circuit experiments. The concepts are a bit of a jump for our students. We can feel electricity, but we can't see it or "measure" it. Well, next week we can measure the force that an electromagnet creates, and both voltmeters and ammeters do just that (the old ones), but the difference between "voltage" and "amperage", requires a level of
abstraction that will be very new to most students. Some teachers
bring in Ohm's law, Electrons, Atomic Models, and even Electromagnetic Radiation! The problem this gives me, and perhaps the students, is that distinction between fact and theory is lost.

There are times when I can picture a battery as a source of "charge". I see the charge under pressure called "voltage", and that it wants to have somewhere to go. I think of it as positive charge. We have "conductors" (wires) that take it to a "load" perhaps a light or a motor. The rate of charge movement per second is called "current". So "current" is a flow of "charge". (The language inspires thoughts of water flow.

The ability to conduct is called "conductivity". Its opposite - the ability to limit current - is called "resistance". (Very large resistance is called "insulation".) The "circuit" has to be "complete". It won't work unless the charge can get back home to the "battery". You can think of "voltage" as the pressure that pushes the "coulombs" of "charge" along.

The trouble is, that you can also think of a current as "causing" a voltage to appear across a resister. A complete "dual" system is just as logical as the familiar voltage based circuit. [Transistor circuits often use this approach.] In this case a battery can be thought of as a "current source" with an "internal conductance".

So we get to measure these things - especially, current, voltage, and resistance - and we do just what we did with length, mass, and time. We define units: "coulombs" for charge, "Amperes" (coulombs per sec) for current, "Volts" for voltage, and "Ohms" for resistance. Only the "coulomb" is called "basic". Units for current, voltage, and resistance can be defined in terms of the "coulomb", and the other "basic" units. [length, time, and mass.] For example, an "Ampere" is a Coulomb per Second.

Then we look for experiments that show how they are related.

RESEED Notes -Class 12 -2 Oct 2001 -Framingham -Electricity 12-2
-----------------------------------------------------------------

But it is harder this time. Back in the length, time, and
mass discussion we all "knew" what they were. Voltage, Current,
Resistance, and Charge are complete mysteries at this point.
We will define them next week. [Note that as usual we will define
the UNITS. Please do NOT expect a definition of Voltage, Current,
and Resistance.]

Students need time to play with bulbs, batteries, and wires.
They should play long enough to become familiar with simple
circuits. They will notice that bulbs get hot, and that short
circuits get hot too - until the battery dies. Even a "dead"
battery can be interesting. I say let them play for a while
before bringing in voltmeters, ammeters, and ohmmeters. Bulbs
have resistance, but their resistance is complex. (It changes
with temperature, and that changes with current...) On the
other hand wires have resistance too, but at low currents the wire
will not get hot enough to change its resistance. You can show
that the resistance of a wire is doubled if its length is doubled.
The thought that a "resister" can be useful is not obvious, but
when you find out that it can control how bright a bulb is....
They should try bulbs "in series", and "in parallel" - two more
new words! And ask them to try batteries in "series and
parallel". Then, if they remember doing rate/distance problems,
you can bring up "Ohm's Law". Ohm's law is not more complicated
than D = RT ( distance equals Rate times Time), but V = IR deals
with three unfamiliar things and does not make sense by itself.
[You may find some students that are stopped by wondering why use
"I" for current! It comes from the French word. ] Next week we
start with an electromagnet and perhaps using it with a very high
resistor to make a voltmeter, and a very low one to make an
ammeter will give some sort of insight into what we mean by
"Voltage", "Current", and "Resistance". Since an electromagnet
produces a force, it makes an easy introduction to making a motor.
If you can believe that moving a wire in a magnetic field produces
a voltage, you can see that maybe turning a motor will generate
electricity!
------------------------------------------------------------------

Demo 1. Triboelectric Series [rabbit fur,GLASS,MICA,wool,cat
fur,silk,cotton, WOOD, PLASTIC, METALS]
Demo 2. Conductors and Insulators
Demo 3. An Electrophorus.
Demo 4. Making a battery [Potato,lemon, etc and two different
electrodes]
Demo 5. What is inside a light bulb?

Demo 6. Electric current will melt a wire. [Note that real
fuses are made of a metal alloy that melts at a low
temperature but is a good conductor too.]

------------------------------------------------------------------
Equipment needed:
straws cans elastics
foil thread scotch tape
glass rod plastic rod dowel rod
metal rod silk cotton
fur wool pie plate
styrofoam cup duct tape plexiglass
wood blocks wire bulbs
batteries nails lemon, potato
voltmeter hammer newspaper
balloons + hair Fluorescent tube
------------------------------------------------------------------

Just for fun, I've been thinking of "Mho's" law. V=I/G where G
is the conductance, I is current, and v is voltage. Consider a
circuit with a battery and a light bulb. Naturally, a battery is
a device that creates a current. We assume our bulb has a
conductance of G since it is the only obvious conductance. [We
assume our wires have conductances so big that they can be
ignored.]
If you have an ammeter you can read the short circuit current,
and with a voltmeter you can read the open circuit voltage. So it
is obvious, is it not, the battery is not just a constant current
source! It can be simulated as a constant current source with the
short circuit current as its constant current. Since we know the
open circuit voltage, the "battery" must contain a internal
conductance across the current source of the right value [G = I/v]
to produce the open circuit voltage we measured. What will happen
when you put two bulbs in series? More conductance? How about in
parallel?

Well was it fun? Not for me, I LIKE VOLTAGE SOURCES, thank
you! But it gives a flavor of what the kids are hearing! New
words, new rules, units for these unknown things, etc....How about
a "battery" that "contains" either a constant current source and
internal shunt conductance, or a constant voltage source and an
internal series resistor, - But none of these can be found
inside!? I prefer to say it's our circuit diagram for a battery
that "contains" these things - NOT the battery.

It gets to be fun if you can PLAY with the meters, resistors,
and circuits. Ordinary drycells with holders and good clip leads
are essential. Use pictorial symbols in your circuit diagrams.
The standard circuit symbols are second nature to us, but
meaningless at first and can spoil the fun for many students.

Short circuits will make wires hot, but are not likely to
burn anyone. On the other hand, a car battery can easily melt a
ring. Even a nickle-cadmium rechargeable battery can heat a wire
hot enough to glow red! I had a "D" size ni-cad in my pocket with
my keys. It got hot enough to feel very uncomfortable!

When we do introduce circuit symbols, I am much more
comfortable picturing a battery as a constant voltage source with
a series resistor. (Open circuit voltage is the source size,
short circuit current gives us the resistor size from Ohm's Law.)

What is "really" inside? Well we can carefully cut open an
oldfashioned battery. You find a carbon post down the middle,
some black moist powder, and a zinc can around it all. The whole
thing used to be covered with cardboard, but now it is steel. A
"battery" houses a chemical reaction. [The chemicals won't hurt
you if you wash them off. They do destroy your clothes and do a
job on your skin if you give them time.] You won't find the
internal resistor, the power source of either kind, or the
internal conductance. BUT be careful. Do NOT open an alkaline
battery! The chemicals in "alkaline" batteries are much more
dangerous. Get an old zinc battery with a cardboard cover. [A
9v battery is interesting - it will have 6 little batteries
connected in series.]

Chemical systems like the battery have a great story to tell
in our search for the elements of the periodic table. Look up the
story of Aluminum for example. Before electricity, Aluminum was
more precious a metal than Gold or Silver!