Science News from the "Outlet"
Electricity
& Magnetism Exploratory
The Chinese
philosopher Lao-tzu once said that a journey of a thousand miles begins with a
single step. This ancient proverb aptly applies to understanding science and
technology. Before students can understand the workings of smartphones, iPads,
and electric cars, they must first grasp the basic physical principles operating
at the heart of these modern-day marvels.
Electromagnetic
phenomena underlie every electronic device currently in use. For instance,
electric cars, energized by electrochemical cells, are propelled by electric
motors that also function as generators that also function as brakes. The
amazing versatility of an electric car’s motor is baffling until one understands
how the operation of motors, generators, and electric braking systems all rely
on the relationship between electricity and magnetism.
The experiments in
this edition of the Science Outlet newsletter will give your students first-hand
experiences with some basic electrical and magnetic phenomena. Through hands-on
activities students will learn about electric currents and magnetic fields, how
they are produced, and how they interact.
Enjoy,
Chris Chiaverina
Concept:
An
electrochemical cell, frequently referred to as a battery, is a device that
produces a potential difference as a result of a chemical reaction. Strictly
speaking, a battery consists of two
or more cells connected in series or parallel
Try It!
Following the steps below, a simple electrochemical cell can be
made with a penny, a piece of aluminum foil and a weak salt solution. The
potential difference established by the cell is revealed by a clicking or
scratchy sound in a pair of earphones.
-
Put on a
pair of earphones.
-
Moisten
the tips of your fingers on both hands.
-
Hold a
piece of aluminum foil in one hand and a penny in the other.
-
Listen as
you touch the tip of the earphone jack with the penny and the side with the
aluminum foil.
-
While
keeping the penny at the tip, move the aluminum foil up and down the length
of the jack.
-
Repeat
steps 2 through 5 using different metallic objects in place of the penny and
aluminum foil.
What’s Going
On?
An
electrochemical cell consists of a negative electrode, a positive electrode, and
an electrolyte, which conducts ions. In the simple cell produced in this
activity, the aluminum foil acted as the negative electrode, the copper penny as
the positive electrode, and a salt solution (water and salts found on the skin)
as the electrolyte.
A variety of
cells can be made from materials found around the house or lab. A “lemon cell”
may be constructed by sticking a penny and a galvanized nail into a lemon. An
“eleven-cent cell” may be made by placing a piece of paper toweling that has
been soaked in salt solution or vinegar between a dime and a penny. When a
number of these cells are connected in series, a larger voltage may be obtained.
In fact, it is possible to light an LED with a collection of these simple cells.
Both the
lemon cell and the eleven-cent cell should produce a sound when connected to a
pair of earphones. The potential difference produced by the cells may be
measured with a sensitive voltmeter or a multi-meter.
Concept: An electric motor converts electrical energy into mechanical
energy. Most electric motors operate through the interaction of magnetic fields
and current-carrying conductors to produce force.
Try It! A very simple motor, such as the one
in the photograph, can be constructed by following the steps below.
 1.
Obtain a battery, a small nail, a neodymium disc magnet, and a length of wire
made from a non-ferromagnetic material such as copper.
2.
Magnetically attach the head of the nail to one side end of the magnet.
3. While
holding the battery in one hand, with the positive terminal of the battery
pointing downward, bring the tip of the nail in contact with the positive
terminal of the battery. The nail and magnet should now be attached to and
hanging from the positive terminal of the battery.
4. Hold one
end of the wire to the negative terminal of the battery while touching the other
end of the wire to the side of the magnet.
What’s Going On?
The charge carriers moving radially across the
magnet in the vertically-oriented magnetic field experience a magnetic force
whose direction is tangent to the magnet (see figure). The direction of this
force defines the sense of rotation of the motor.
Concept: An electric generator is a device
that converts mechanical energy to electrical energy. The Hand Generator
provides students with direct experience in the production and transformation of
electrical energy, an immediacy not provided by conventional power supplies.
Try It! The Hand Generator can be used as a
source of electrical energy for a wide range of devices.
The generator will easily light one or more low
voltage bulbs obtained by cutting up an old string of holiday lights. One bulb
is brightly lit by the generator. Connecting the bulbs in series and parallel
teaches circuit fundamentals as well as something about the conservation of
energy. Students quickly realize that more work is needed to light additional
bulbs. You may wish to use the Science Outlets Series-Parallel Bulb Board to
demonstrate circuit fundamentals.
Batteries and transformers used with toys, such as
small-gauge electric trains, may be replaced with the Hand Generator. Students
delight in making an HO-gauge train circle a track using the Hand Generator.
Get
details on Hand Generators
The conversion of mechanical energy into electrical
potential energy can be illustrated by charging a low-voltage, high-capacitance
(1-10 farad) electrolytic capacitor. Once charged, the capacitor can be used to
light a low-voltage bulb or power a motor such as the Hand Generator itself.
What’s Going On?
As a source of electrical power, the Hand Generator
can energize virtually any low-voltage electrical device.
Get details on Series Parallel Bulb Board
Concept: An electric motor may also function as an electric generator and
vice versa.
Try It!
The
motor/generator effect may be dramatically illustrated with two Hand Generators.
After connecting the two Hand Generators together using the leads attached
to both devices, turn the crank of one of them and observe the crank of the
other. The first device will function as a generator while other unit acts as a
motor. Now reverse the roles of the two devices.
What’s Going On?
Electricity and magnetism are closely related to one another. Moving electric
charges produce magnetic fields and changing magnetic fields produce forces on
electric charges. As a result, in principle, any electrical generator can also
serve as an electric motor, and vice versa. This reversibility is known as the
motor/generator effect.
Concept: Although the resistance of dry skin
is very high (up to 500,000 ohms), a tiny, yet detectable current (about a
millionth of an ampere) will flow through the body with the application of only
a few volts. When the Energy Ball’s two electrodes are touched simultaneously,
the ball flashes and makes a strange tone.
Try It! Place two fingers on the metallic
contacts on an Energy Ball. Notice that the ball flashes and produces a tone.
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 both
series and parallel circuits. Have two students hold hands, then have one of the
students touch a contact on the Energy Ball with a finger. When the second
student touches the other contact, the ball will flash and produce a sound. If
the circuit is broken, for example if the students stop holding hands, the Ball
will stop functioning. Now have students form a series circuit consisting of
more people. It’s fun to see if there is a limit to the number of people you can
include in your circuit. There doesn’t seem to be!
Get Details on the Energy Ball
 To demonstrate a parallel circuit with the Energy
Ball, have students get in a formation that looks like a ladder: two lines of
students holding hands connected by students between the lines. To activate the
Energy Ball, have two students at the end of formation touch the ball’s
contacts. This formation provides multiple pathways for the current. If all
pathways but one are removed, there will still be a complete circuit.
What’s Going On?
The Energy Ball contains circuitry capable of
detecting and amplifying very small electrical currents. Even the slightest
conduction between the two electrodes activates the Energy Ball. When the two
electrodes are touched simultaneously, a small current flows through a
transistor. This transistor serves as switch that controls a circuit containing
a battery, light bulb, and sound source.
Concept: The magnetic force on an electrolyte
can produce rotation.
Try It! Line the inside of a Petri dish with
a copper mesh or aluminum foil strip (see photograph below). This lining serves
as an electrode. Place a second, smaller circular electrode in the center of the
dish. Insert a strong neodymium magnet inside the center electrode. After
filling the dish with a weak salt solution, use two leads with alligator clips
at each end to connect the outer and inner electrodes to a 6-volt DC power
supply. (Note: We would like to thank Matt Lowry of Lake Forest High School in
Lake Forest, Illinois for sharing his device with us.)
What’s Going On?
The electric field between the electrodes cause the
positive sodium ions to move radially toward the negative electrode and the
negative chlorine ions to drift toward the positive electrode. The ions moving
perpendicular to the magnet’s magnetic field experience a magnetic force. Since
the positive and negative ions move in opposite directions, the magnetic force
acts on both types of ions in the same direction. As a result, the solution will
begin to rotate after a short time. Pepper sprinkled on the surface of the
liquid will make this rotation easier to see. The direction of rotation of the
liquid will reverse if the polarity of the electrodes is reversed or if the
magnet is inverted.
Concept: According to Lenz’s Law, when a
magnet moves past a conductor, a current is caused to circulate in the conductor
producing a magnetic field which opposes the motion of the magnet. The Science
Outlet’s Lenz’s Law Demo beautifully illustrates this principle.
Try This! First slide a standard steel marble
down the track. Students will observe how gravity accelerates the marble. When a
magnetic sphere is used, the sphere will lazily rolling down the track,
seemingly defying gravity.
What’s Going On?
According to Lenz's law, when a current is caused to
flow in an electrical conductor by a change in an external magnetic field
surrounding the conductor, the direction of the induced current is such as to
produce a magnetic field opposing the original change in the external magnetic
field. In the case of the falling magnetic sphere, the current induced in the
track produces a magnetic force that acts in a direction opposite to the motion
of the sphere.
Get
details on Lenz's Law Demo
Watch for the next issue of the Science
Outlet Newsletter!
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