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© 2002 Flinn Scientific, Inc. All Rights Reserved.
Centripetal Acceleration
Spin a Penny on a Hanger
Publication No. 10308
Introduction
Take your students on an amusement park ride—for just a penny! Discuss how an object can be accelerating yet moving at
constant speed. Investigate how a change in direction (at constant speed) is acceleration; that is, centripetal acceleration!
Physical Science Concepts
• Centripetal acceleration
• Velocity
• Vectors
Spin hanger
in the direction
of curved end.
Materials (for each demonstration)
Hanger, metal
Penny
Balance penny here
File
File end flat
Safety Precautions
Wear protective eyewear. Use caution and have all observers stand away as the demonstrator rotates the hanger and
penny—the penny may fly off.
Preparation
1. Bend a metal hanger so that it forms a diamond shape, as shown in the diagram.
2. File the end of the looped part so it is flat and so that a penny may be easily balanced without falling off. The looped part
may need to be bent slightly.
3. Practice the demonstration before class.
Procedure
1. Using one index finger pointed straight out, hold the hanger at the top of the diamond, so that the looped part hangs
downward.
PHYSICAL SCIENCE-FAX. . .makes science teaching easier.
013101
2. Using the other hand, balance the penny (heads-up) on the flat, filed end of the hanger (see Tips section).
3. Once the penny is balanced, gently and carefully rotate the hanger in a full circular motion at a constant speed. Start rotating slowly and without jerking the hanger or the penny will drop.
4. Observe the penny as it follows the loop of the hanger—it remains in place on the hanger. Discuss this observation with
your class.
Tips
• Balancing the penny on the flat, filed end of the hanger is not easy and may take some practice. The easiest way to balance
the penny is to hold the penny flat on your middle and index fingers. With those two fingers slightly spread, lower the
penny onto the flat, filed end of the hanger. It has also been found that raising the hanger to meet the penny works well.
• Allow students to practice and then have them demonstrate this activity in front of the class. This can be a motivational,
attention-getting activity at the beginning of the motion unit, or an activity to break up the period when students are doing
math calculations on velocity and acceleration.
Discussion
Velocity is the rate of motion in a specified direction and acceleration is a change in an object’s velocity. Both velocity and
acceleration are vector quantities in that they are based on a magnitude AND a specified direction (i.e., the car is traveling north at
55 mph). Speed, on the other hand, is an object’s rate of motion. Speed is a scalar quantity and is based only on a magnitude
(i.e., the car is traveling at 55 mph). Whenever there is a force, there is acceleration according to Newton’s second law of motion
(F = ma). A force is required to change an object’s speed, and a force is required to change an object’s direction. Since velocity is
a quantity of speed and direction, a change in an object’s speed OR a change in its direction, or both cause the object to accelerate. The force can change the speed of the object without affecting its direction (linear acceleration), or the force can change the
direction of the object with or without affecting its speed (centripetal acceleration).
Acceleration toward the center of a curved or circular path is called centripetal acceleration (ac) The word centripetal means
“toward the center.” If an object is spinning in a circle at a constant speed, the object is accelerating. This is because there is a
continuous change in direction (and velocity includes both speed and direction).
As an object moves in a circle, the change in its direction of motion is always toward the
center of the circle, while the direction of its motion is always perpendicular to the radius of the
circle. The force acting on the object in a direction toward the center of the curve is termed the
centripetal force. This force is based on the perpendicular speed of the object and its distance
from the center as measured by v2/r, where v is the object’s velocity and r is the circle’s radius.
Another common term associated with circular motion is centrifugal acceleration.
Centrifugal acceleration is a fictitious outward force (a pseudoforce) that balances centripetal
acceleration. It is a term commonly used to describe the “force” of inertia that wants to keep an
object traveling in a straight line.
Materials for Centripetal Acceleration are available from Flinn Scientific, Inc.
Catalog No.
Description
AP8975
File, Triangular, 10 ⁄2
AP8826
File, Triangular, Handle
1
–2–
© 2000 Flinn Scientific, Inc. All Rights Reserved.
Price/Each
Consult Your
Current Flinn
Catalog/Reference
Manual.
v
r
ac
“Your Safer Source for Science Supplies”
P.O. Box 219 • Batavia, Illinois 60510 • 1-800-452-1261 • flinn@flinnsci.com • Visit our website at: www.flinnsci.com
© 2002 Flinn Scientific, Inc. All Rights Reserved.
The Spinning Can
Lenz’s Law Demonstration
Publication No. 10240
Introduction
Use Lenz’s Law to “magically” spin an aluminum can without touching the can.
Science Concepts
• Lenz’s Law
• Induced current
Materials
Aluminum can (with entire top removed)
Small bar magnet
Container (to float aluminum can)
Thread
Spill tray
Tap water
Safety Precautions
This demonstration does not present any unusual safety hazards. Care should be taken in removing the can covers.
Procedure
1. Locate an aluminum can (as heavy gauge as possible) with a diameter about 1 greater than the length of your bar magnet.
An aluminum density catch bucket works well. (If the top of the can needs to be removed, do it carefully.)
2. Tie a thread tightly around the center of the bar magnet so that it can be spun in a circular fashion as shown in Figure 1.
Figure 1. Practice spinning and balancing the magnet.
PHYSICAL SCIENCE-FAX. . .makes science teaching easier.
013101
Teacher’s Notes continued
3. Locate a container (finger bowl, regular bowl, etc.) that is about 1
greater in diameter than the aluminum can.
4. Place the bowl in the center of a spill tray.
5. Fill the bowl to its very top with tap water until it can not hold any
more water. A “bulging” bowl of water is ideal.
Thread
6. Carefully float the aluminum can open-side-up on top of the water.
The final setup should look like Figure 2.
Bar Magnet
Heavy Gauge
Aluminum Can
H2O
7. With the magnet hanging outside the can, start the magnet spinning
on the hanging thread. Spin and balance the magnet so it is spinning
in a level, even fashion. Then slowly lower the spinning magnet
down inside the lip of the can, being careful to not touch the side of
the can. (This might take a little practice and a steady hand.)
Bowl
Spill Tray
8. Watch what happens to the floating can. What direction does it spin compared to the magnet? What happens when the
thread is completely unwound and the magnet spins in the opposite direction?
Discussion
In 1834 Henrich Lenz put forward his Law of Induction: An induced current in a closed conducting loop will appear in such
a direction that it opposes the motion that produced it.
This “induced opposition” (often called Eddy currents) has been demonstrated in many ways since Lenz first formulated the
principle. A classic demonstration has been to drop a strong magnet through a conducting tube to show how the created Eddy currents “oppose” the pull of gravity. The induced opposition causes the magnet to fall very slowly through the tube. It is a great
demonstration to show the relationship between Lenz’s Law and the Law of Conservation of Energy.
The spinning aluminum can in this demonstration illustrates the same principles. The can will spin in this demonstration in
response to the induced current created between the spinning magnet and the aluminum can. The can is actually being repelled in
response to the induced current from the spinning magnet. The “phase shift” is very dramatic when the induced field direction is
reversed and the magnet starts spinning in the opposite direction. The can “magically” reverses its spinning direction too.
Tips
• Practice spinning and balancing the magnet prior to the demonstration as shown in Figure 1.
• The aluminum can should be as thick walled as possible. A soda can does not work well and has sharp edges when the
entire top is removed. An aluminum catch bucket works well.
• This demonstration can be very dramatic if performed on an air table instead of on floating water.
• The right combination of sizes of parts is what makes this demonstration successful. The can needs to float on a high
meniscus of water and therefore the bowl should be only slightly bigger in diameter than the can. If floating properly, the
can should not touch the sides of the bowl. (A tea cup of the right size works well.)
Disposal
Pour water down the drain and save all of the other materials.
Reference
Dindorf, W. Lenz’s Law in the Kitchen. The Physics Teacher. Vol. 37, May, 1999.
Lenz’s Law products are available from Flinn Scientific, Inc.
Catalog No.
AP4698
AP9261
AP4626
Description
Eddy Current Experiment Kit
Bar Magnet, Cobalt Steel
Catch Bucket, Aluminum
–4–
© 2001 Flinn Scientific, Inc. All Rights Reserved.
Price/Each
Consult Your
Current Flinn
Catalog/Reference
Manual.
“Your Safer Source for Science Supplies”
P.O. Box 219 • Batavia, Illinois 60510 • 1-800-452-1261 • flinn@flinnsci.com • Visit our website at: www.flinnsci.com
© 2003 Flinn Scientific, Inc. All Rights Reserved.
Van de Graaf Generator Safety
Catalog Nos. AP4699, AP6476
Publication No. 10552
Introduction
Van de Graaf generators build up and maintain a high voltage static electric charge—some of them up to 500,000 volts.
Safely operating a Van de Graaf generator is essential if students are to have the opportunity to see the many exciting and memorable static electricity demonstrations performed with a Van de Graaf generator.
Science Concepts
• Van de Graaf generator safety
• Static electricity
Materials
Van de Graaf generator
Discharge electrode
Meter stick, wood (no metal), or 1-m wooden dowel (optional)
How a Van de Graaf Generator Works
Most Van de Graaf generators build up a positive electric charge on their domes by separating
negative electric charge from positive electric charge. This is accomplished by a rotating insulated
belt. When two different materials are rubbed together, one object takes away electrons from the
other object. The electron-removing object becomes negatively charged, leaving the second object
with a positive charge. The tendency of an object to become positively or negatively charged is a
property of the materials being rubbed together.
In a Van de Graaf generator, the lower pulley connected to an electric motor rotates a rubber
belt that is also looped over the upper metal pulley. The lower pulley and belt rub together causing
electric charge transfer and separation. The lower pulley becomes negatively charged and the inside
of the rubber belt becomes positively charged. Because the rubber belt is an insulator and not a very
good conductor of electricity, the positive charge does not evenly distribute over the belt. Instead,
the inside of the belt remains positively charged, while the outside of the belt becomes negatively
charged as a result of electric induction. Electric induction occurs because the negatively charged
particles (electrons) on the belt experience a repulsive force from the negatively charged pulley
(like charges repel). The electrons on the belt tend to move as far from the pulley as possible—to
the outside of the belt. At the base of the Van de Graaf generator, a grounded metallic comb is
SAFETY-FAX. . .makes science teaching safer.
Figure 1.
IN10552
103003
Teacher’s Notes continued
located near the rotating belt and pulley. The grounded metallic comb allows the built-up negative charge on the outside of the
belt to “bleed” off, thereby making the entire belt (inside and outside) more positively charged.
At the top of the Van de Graaf generator, another metallic comb assembly is connected to the metal dome and is located near
the rotating belt and upper metal pulley. The upper metal pulley retains the positive charge as the positively charged belt moves
over the pulley. The outside of the belt passes the metal comb connected to the dome and negative charges on the dome drain onto
the more positively charged belt (opposite charges attract). The negative charge remains on the belt until it is allowed to drain off
at the grounded base comb assembly.
The positive charge that accumulates on the upper metal pulley quickly distributes itself evenly over the metal dome because
like electric charges repel each other, and because the metal dome is an excellent conductor of electricity. The Van de Graaf generator dome can continue to gain positive charge because the positive charge that accumulates on the pulley on the inside the
dome does not “feel” the positive charge on the outside of the dome. This is due to the fact an electrically charged shell does not
produce a net electric field (and therefore a net electric force) inside the shell. So, the positive charges that are deposited on the
metal pulley continually move to the outside of the dome due to their own repulsive forces until a large electric potential is created on the dome. The amount that can be collected is based on the diameter of the dome and the dielectric breakdown of the air
surrounding the Van de Graaf Generator. The larger the dome (more surface area), the more charge that can be collected.
However, when the dielectric breakdown of air is reached (30,000 volts/cm in dry air), electric discharge occurs into the air which
removes some of the static charge on the dome. Dielectric breakdown prevents the dome from collecting an “infinite” amount of
charge.
Static Electricity Hazards
Van de Graaf generators typically produce a very small amount of current (microamperes). Therefore, an accidental shock
from a Van de Graaf generator may be startling and it may be painful, but it will not cause serious harm to most individuals, even
at a high voltage. The discharge may stimulate the nerves and cause muscles to contract briefly, but it will not produce thermal
burns or damage to cells. The strength of the current is difficult to determine since it is based on the total resistance of the shocked
individual and the voltage discharge by the Van de Graaf generator (potentially up to 500,000 volts). Table 1 gives the approximate physiological effects on a “normal” male for different levels of direct current (DC) applied for one second. The static discharge from a Van de Graaf generator lasts only a fraction of a second. However, electric currents running through the body, even
a small amount, can still lead to possible problems and injuries.
Current (milliamperes)
Physiological Effects
1–5
Threshold of sensation
>5
Mild Shock
>30
Painful shock
>50
Muscle paralysis
>75
Severe shock
>250
Possibly fatal
Although the shock from a Van de Graaf generator is not permanently damaging, methods to prevent electric shock should
always be taken. Purposely discharging an individual quickly so that they can “feel” a shock should never be done. Everyone’s
body responds differently to electric shocks. What might be slightly painful to one person might be seriously dangerous to
another.
When operating a Van de Graaf generator, always have a grounded discharge electrode that can be used to discharge the Van
de Graaf generator before the generator is touched. Most Van de Graaf generators have a ground terminal on the base where a discharge electrode can be connected. The discharge electrode protects the operator from being shocked before getting near the generator or turning it off. However, discharging an individual requires a special technique. When an individual is charged by a Van
de Graaf generator for a hair-raising demonstration it is very important to discharge the individual in a slow, controlled manner
before he removes his hand from the Van de Graaf generator dome. Do NOT use a grounded discharge electrode to ground the
individual. Instead, discharge the charged individual by first turning off the Van de Graaf generator by flipping the ON/OFF
switch with a wooden meter stick (no metal ends) or a 1-m long dowel rod. Don’t touch the Van de Graaf generator with your
–6–
© 2003 Flinn Scientific, Inc. All Rights Reserved.
IN10552
Teacher’s Notes continued
bare hand because this will cause a quick discharge. When the Van de Graaf Generator is turned off, touch the charged individual
with the wooden meter stick until his hair falls. Continue to touch the individual for another 15–30 seconds so that he is completely discharged before allowing him to step off the insulated platform. Caution: Only perform hair-raising type demonstrations
with a Van de Graaf generator that has an ON/OFF switch so the Van de Graaf generator can be turned off with an insulated rod,
and does not have to be touched with a bare (grounded) hand.
Additional Safety Precautions
Before operating a Van de Graaf generator for experiments or demonstrations, it is important to become knowledgeable and
familiar with the principles of the experiment and the expected results. Ask an experienced Van de Graaf generator operator if
you have questions.
Do not use a Van de Graaf generator near water, grounded water faucets, or other grounded objects such as doors or walls.
Also, do not operate near electrical equipment such as computers, televisions, magnetic recordable devices (VCR tapes or floppy
disks). Make sure no flammable gases are present.
Materials for Van de Graaf Generator Safety are available from Flinn Scientific, Inc.
Catalog No.
AP6476
AP4699
AP5634
Description
Van de Graaf Generator, 400-kV
Van de Graaf Generator
Discharge Electrode
Price/Each
Consult Your
Current Flinn
Catalog/Reference
Manual.
–7–
© 2003 Flinn Scientific, Inc. All Rights Reserved.
IN10552