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CoolStuff
Newsletter Article
Vol. 19, April 2005
Electricity
Part II: Getting Connected |
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You might assume that most of your students are
capable of lighting a light bulb given a battery, a bulb, and a
single strand of wire. In actuality, when faced with this challenge,
some students may succeed at the task immediately while others may
never be able to make the bulb light without assistance. According
to a study done by Harvard Astronomy professor Phillip Sadler, only
half of the students taking part in the survey could make a light bulb light up when given a
bulb, battery and a piece of wire. What can pre-college
teachers do to improve this statistic?
Research tells us that students learn best when they are allowed to
ask questions of nature through exploration and experimentation. With
that in mind, this edition of CoolStuff offers several activities
that invite students to ask questions and find answers regarding
electrical phenomena.
Chris Chiaverina
Philip M. Sadler, assistant professor of
education at the Harvard Graduate School of Education (HGSE) and
director of the
Science
Education Department at the Harvard-Smithsonian Center for
Astrophysics (CFA), The full text of the study is available from
the Publications Department of the Harvard-Smithsonian Center for
Astrophysics. |
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This issue of CoolStuff brings you a
dramatic video of the largest known example of Jacob's Ladder!
Click here for the complete story |
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Detecting Electrical Currents
Key Concept: The flow
of charge produces a magnetic field that may be detected with a
compass. A galvanometer, a device capable of detecting rather small
currents, can be constructed by wrapping several turns of wire
around a compass.
Place a wire just above and parallel to the needle in a compass.
Observe what happens when a closed circuit is formed by connecting
the two ends of the wire to opposite ends of a battery. What do you
observe? Now reverse the polarity of the battery by reversing the
direction of the battery in the circuit. What happens to the compass
needle now? How can you explain your observations?
The response of the compass when the wire is connected to the
battery indicates that something is happening in the wire. When the
polarity of the battery is reversed, the compass moves in the
opposite direction. The behavior of the compass suggests, but does
not prove, that something is moving. A variety of
experiments tell us that electrical charges are indeed moving down
the line.
A number of activities in this edition of CoolStuff require the use
of a galvanometer. If you do not have a commercial galvanometer, you
may make one by wrapping thin copper wire tightly around a compass.
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Click here for information on Galvanometers |
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Sources of electromotive force (emf) |
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Key Concept: The
electromotive force (emf) is not a force, but the potential
difference, or voltage that can be used to supply energy to an
electrical circuit. Sources of emf include batteries, electrical
generators, and solar cells. Strictly speaking, the emf is defined
as the potential difference measured across the terminals of a
source when no current is passing through the source. |
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Homemade Sources of emf |
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“The Eleven-Cent Cell”
There are
numerous electrical cells that can be made from materials found
around the house. The simplest perhaps is the “eleven-cent cell.” It
may be constructed by placing a piece of paper toweling that has
been soaked in salt solution or vinegar between a dime and a penny.
When the leads of galvanometer are attached to the coins, the meter
will register a small voltage. When a number of these cells are
connected in series, that is, end to end with dimes touching
pennies, a larger voltage may be obtained. |
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“The Lemon Battery”
To the surprise of many, a lemon
and a couple of metallic electrodes are all you need to produce a
little “juice.” Any two types of metal may be used as electrodes. In
fact, you can use a dime and a penny again. To produce a bit more
voltage (~ 0.9 v) try using a galvanized nail and a penny.
Unfortunately, a cell using these metals does not produce enough
voltage to light a mini bulb, but don’t despair. By
connecting three or four of these lemon cells in series, you should
be able to light up a light emitting diode (LED) or even power a
liquid crystal clock. If you find that your galvanized nail/penny
lemon cell is not capable of lighting an LED, you may want to try
samples of copper and zinc from the science storeroom.
Click here for details on various electrodes |
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“The Hand Battery”
The hand
battery takes advantage of the naturally-occurring electrolytes
found on the skin to produce a potential difference. When one hand
is placed on a copper plate and the other hand on an aluminum plate,
a current is produced through a galvanometer connected to the
plates. Sweaty hands will increase the meter reading as will
pressing down harder on the plates, thus increasing the area of
contact between skin and metal. |
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“Thermoelectricity”
If a closed loop is formed by joining
the ends of two wires made of dissimilar metals and the two
junctions of the metals are at different temperatures, an
electromotive force, or voltage, will be produced that is
proportional to the temperature difference between the junctions.
The thermoelectric effect can be
demonstrated by making a device called a “thermocouple.” You will
need two copper wires and a length of steel wire, such as a
straightened paper clip. Twist one end of each of the copper wires
around the ends of the paper clip. Connect the free ends of the
copper wires to a galvanometer. Observe the galvanometer as you
place one copper-paper clip junction in ice water and the other end
in a flame. |
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NewThermoelectric power source |
Today thermoelectric generators rely on semiconductors rather than
junctions consisting of dissimilar metals. Some of the current
applications of these solid state devices are quite intriguing. In
Northern Sweden, stove top generators are used to produce sufficient
electrical power to operate small appliances. A particularly
ingenious use of thermoelectricity has been developed by the
watchmaker Seiko. They have managed to eliminate the need for a
watch battery by tapping into body heat. |
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Shake It Up!
Key Concept: An
electric motor may also function as an electric generator. A
galvanometer consists of a magnet surrounded by a coil of wire, the
essential components in both electric motors and generators.
Use two lengths of wire to connect two galvanometers. Shake one of
the galvanometers while watching the needle of the other meter. What
do you observe? Now shake the other galvanometer. Is it possible to
say which galvanometer is agent and which is victim?
The motor/generator effect may be dramatically illustrated with two
Genecon generators. After connecting the two generator’s leads
together, turn the crank of one of the Genecons and observe the crank
of the other. The first Genecon functions as a generator while other
unit acts as a motor. Now reverse the roles of the two Genecons.
Click here for details on Genecon hand generators |
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People Circuits
Key Concept: The human
body is a conductor of electricity.
Chris demonstrating the "People Circuit" at the
National Science Teachers Association Conference during Arbor Scientifics' Best Physics Demo
Show
Although the resistance of dry skin is
very high (~ 500,000 Ω), a tiny, yet detectable current (about a
millionth of an amp) will flow through the body with the application
of only a few volts. The Energy Ball is a device capable of
detecting such a current.
Place two fingers on the two metallic
contacts on an Energy Ball. Notice that the ball flashes and a tone
is produced. This indicates that a current is flowing through your
fingers and a portion of your hand. If either finger is removed, the
circuit will be broken and the Energy Ball will cease to function.
The Energy Ball may be used to
demonstrate what is known as series circuit. While holding hands
with a friend, touch one contact on the Energy Ball with a finger.
When your friend’s finger touches the other contact, the device will
become activated. If the circuit is broken, for example if you let
go of your friend’s hand, the Ball will stop functioning. Try
forming a series circuit consisting of more people. See if there is
a limit to the number of people you can include in your circuit.
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A parallel circuit provides more than
one pathway for the current. How would you include the Energy Ball
in such a circuit? Once you
have established a parallel circuit that activates the Energy Ball,
see what happens when one of the pathways is broken. Does the Energy
Ball stay on?
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Click here for details on the Energy Ball |
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Click here for details on the Visual Electricity Demonstrator |
Virtual Electricity Demonstrator
Key Concept: Light emitting diodes in the
Visual Electricity Demonstrator are used to represent charged
particles,
their motion and the flow of charge through a circuit.
Turn on the Virtual
Electricity Demonstrator. (VED) You should see stationary red LED’s.
These lights are meant to represent charged particles. With no source of emf
connected to the device, there is no current, yet there are still
charges present, here represented by stationary LEDs. Now form a
circuit containing the VED, a 1.5 volt battery, and connecting
wires. Describe the LEDs now. What happens when you reverse the
polarity of the battery? What does this tell you about the direction
of charge flow? |
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Connect the leads of
a Genecon generator to the VED. Observe the LEDs as you slowly turn
the generator’s crank. What happens as you turn the crank more
quickly? (Caution: To avoid damage to the VED, only turn crank at a
moderate rate.) Now reverse the direction of rotation and observe
the motion of the LEDs. What does this indicate about the direction
of motion of the charged particles in the circuit?
You can produce an
alternating current (AC) by repeatedly changing the direction of
rotation of the Genecon’s crank. Notice that the red LEDs move in
one direction, then the other, but make no permanent headway. If a
light bulb is included in the circuit, it will light up, albeit with
a flicker, even though the charges are not moving around the
circuit. Hence, energy is still being delivered to the bulb even
though the charges are just moving back and forth.
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Did You Know?
An electric current may be thought of as the movement of electrical
charges through a conductor, such as a wire. Electric charges, such
as electrons, are not produced by a battery: they are already
present in conductors.
When a source of potential difference, say a battery, is connected
to a wire, electrons start to move. However, due to resistance in
the wire, electrons do not travel unimpeded. Instead, they
constantly suffer collisions that result in a start-stop type of
motion. This motion is described by a quantity known as the drift
velocity. A typical drift velocity may be of the order of a tenth of
a centimeter per second.
If the electrons move so slowly, why does a light seem to go on as soon
as we flip a switch? To understand this phenomenon, the distinction
must be made between the motion of the electrons and speed with
which the electromagnetic disturbance travels along a wire.
When a circuit is closed, a disturbance propagates down the wire at
nearly the speed of light. This disturbance results from one
electron pushing on the next. Just as with water in a hose, the
water from the spigot does not travel the length of the hose in an
instant, but the pressure exerted by water at the spigot does.
Metallic as well as nonmetallic conductors offer resistance to
charge flow. In metals, this resistance is caused by impurities and
imperfections in the metal as well as thermal vibrations of metal
ions. Electrical resistance gives rise to heat as is evident by
glowing toaster wires and electric range elements. In such household
applications, resistance is a good thing. However, in most
instances, the transformation of electrical energy into heat is
something to be minimized, or in a perfect world, eliminated
altogether.
It came as a real shock to Kamerlingh Onnes, a Dutch physicist, that
some materials lose all electrical resistance when cooled below a
certain temperature. In 1911, he found that once a current is
established in a super-cooled ring of mercury, it will continue
undiminished without any applied voltage. Superconductors, as he
called them, are finding applications in medicine, transportation,
and computing. |
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Very Cool
Product
Here's Something really new... magnetic
hookup wires! Each hookup wire is color coded black or red for
negative or positive connections. Then the safety coated wire is
terminated with a chrome plated magnetic terminal. These Magleads
make hooking up simple electric circuits a snap! Just touch the
magnetic terminal to the battery, switch terminal or bulb socket
terminal and your done. No need for screw drivers or baring wire
ends.
Click here for details on Magleads! |
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Interesting
Links:
Brian
Jones; Little Shop
of Physics University of Colorado: Traveling Physics demo show
http://littleshop.physics.colostate.edu/default.html
Thermoelectricity Past, Present, and
Future:
http://www.nanothermel.org/public_main.htm
The Exploratorium: Tools for
Teaching:
http://www.exploratorium.edu/educate/index.html
Museum of Science; Boston: The
Thomson Theater of Electricity
http://www.mos.org/doc/1327
Hunkins Experiments: Crazy stuff
kids can do at home
http://www.hunkinsexperiments.com/themes/themes_electricity.htm
Science Buddies.org Science
Buddies is a non-profit organization empowering students from all
walks of life to help themselves and each other develop a love of
science and an understanding of the scientific method.
http://www.sciencebuddies.org/mentoring/ |
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Next Issue:
Although a
school's science laboratory is the traditional venue for
exploration and experimentation, other locations exist that are
equally suited for doing science. One of these is the home. For
a number of years we have been encouraging our students to do
science experiments with family and friends. Needless to say,
parents love seeing what their children are doing in school. The
students derive satisfaction from demonstrating their knowledge
of science with others.
Quite often the materials needed to investigate physical
phenomena at home may be found in the kitchen or workshop. When
more specialized equipment is needed, we create a "lab in a bag"
by packing required materials in a plastic food-storage bag. In
the next edition of CoolStuff we will offer up a number of our
students' favorite "do at home" experiments. If you're like us,
you'll love getting families involved with science!
Regards,
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