Benjamin Franklin’s Static Motor



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Benjamin Franklin’s

Static Motor

This version developed by: ScienceScene 2013©



This unique adaptation of the world’s first electric motor uses static electric charges to power the motor. The ability to produce a static charge varies with temperature and humidity. A humidity of less than 40% has been found to be satisfactory to successfully run the motor.

A digital copy of this document can be obtained by going to http://www.sciencescene.com.
Select Workshop Materials then select Physical Science Activities. Follow directions to download document.

Materials

Drywall screw, Phillips head, 2.5 inch long

Square block of wood approx. 1” x 1”

Plastic 4 oz. Disposable Portion Cup

Wool fabric or cat’s fur 6" x 6"

Styrofoam Block 3" x 5" or ½ Styrofoam Plate

“Great Value” 8 7/8” Foam Plates - Wal-Mart



Rubber band 3 ½ ” x ¼”

Pushpin or thumbtack

Reflective Mylar® Cut into rectangles 3/8 “ by 1 ¾”

Ruler


Scissors




1. 2. 3. 4. 5



Directions

1. Screw the drywall screw into the center of the block of wood.

2. Carefully position the tip of the Pushpin or thumbtack over the exact center of the bottom of the cup. Slowly push the pushpin or thumbtack straight down. Twisting will help the tip penetrate the cup.
Please Note: the cup, in the kit, already has a hole placed in it. Use as a model for future motors.

3. Place the rubber band around the outside of the cup as shown.

4. Insert a Mylar® strip (vane) under the rubber band and then bend it at the rubber band to create a crease. Evenly insert the remaining vanes, for a total of four, under the rubber band.

5. Assemble the motor by placing the point of the pushpin into the cross on the head of the screw.

6. Rub the Styrofoam with the fabric for 20 to 30 seconds then place the Styrofoam on one side of the motor and the fabric on the other. Vary the position of the fabric and Styrofoam until the motor rotates continuously. Repeat the rubbing as needed. Additional adjusting of the fabric and Styrofoam will help the motor spin faster as the vanes come close to new areas that have not yet lost their charge imbalance.

Extensions - Turn this activity into a engineering challenge by creating a motor and invite students to create a “better “motor.


  1. Explore motors with different number vanes to determine the optimal number of veins. The drawing, Vein Placement, indicates the placement for rotors containing 1–6 veins. Place cup on drawing and mark placement of veins.

  2. Explore the size of the materials that are rubbed together to generate a charge imbalance to determine the optimal size.

  3. Explore other materials that can be rubbed together to generate a charge imbalance. See section “Electrostatic Series”.

  4. Create different shaped veins to create new versions of the rotor.

Background

There are two kinds of electricity: (1) static electricity, and (2) current electricity. Essentially both are really the same. Static electricity consists of electrons or ions that are not moving. Current electricity consists of ions in motion, which produces important magnetic affects.

Static electricity makes your hair crackle when combed, or it may give you a shock if you touch a person or a metal doorknob after walking across a carpet on a cold dry day. You can produce static electricity by rubbing a comb with a piece of wool. The atoms in the wool lose some of their electrons, and the atoms in the comb gain them, so the comb becomes negatively-charged, and the wool becomes positively-charged.

Ordinarily, an atom is neutral. This means that it has an equal number of electrons and protons. If an atom gains some electrons, it is said to be negatively charged. If an atom loses some electrons, it becomes positively charged. Atoms that either gain or lose electrons become electrically charged, and are called ions. Atoms with like charges repel each other, while atoms with unlike charges attract each other.

a. friction objects can be charged by rubbing them together. As the two objects touch, one gives up loosely held outermost electrons to the other. Thus, the material that receives electrons becomes negatively charged and the material that loses electrons becomes positively charged.
b. contact touching two items together and then separating them can move electrons from one item to the other. The item that gains electrons will have a net negative (-) charge. The item that loses electrons will have a net positive (+) charge. Note that electrons are moved, not created, in this process! Charges will "stay put" unless the item is a conductor (metal, etc.) that lets electrons move about easily.

c. induction a material may be charged by induction when a charged body it is brought near a neutral material. This causes the electrical charges to be redistributed in the material, resulting in one material having an excess of either positive (+) or negative (−) charges. Certain non-conducting materials can also be given a static electric charge by electrostatic induction. In these cases, it is caused by polarization of their molecules.

Electrostatic Series

With the knowledge that some atoms either lose or gain electrons to become charged, we have been able to make lists of materials that will receive or lose electrons. Such a list is called an “electrostatic series.” An ‘electrostatic series” lists common materials in the order of their tendency to receive or lose electrons. The following electrostatic series is arranged with materials that tend to give up electrons and become positively charged at the bottom of the list. Those that have a tendency to gain electrons and become negatively charged are found at the top of the list. Any material on the list gives up electrons to a material above it. The further apart the materials are on the list, the more easily electrons are transferred from one to the other.

The Motor

Reflective Mylar® has a thin coating of aluminum, which is a conductor. The electron movement and electron transfer happen so quickly that the strips spin without stopping. Only a relatively small number of electrons are moved to or from a strip each time. The charged object(s), having a much larger net charge, can supply or take on extra electrons for many rotations of the strips.

When a Mylar® strip moves near enough to a negatively charged object, some electrons will move toward the Mylar®. If the object is positively charged the electrons move from the Mylar® toward the positively charged object.

Either way, the Mylar® will end up with the same charge (+ or -) as the nearby charged object. A Mylar® strip, which can rotate, will be repelled from the charged object, since like (same) charges repel each other. The strip is now strongly attracted to the oppositely charged second object. When a charged strip moves near enough to the second object, some electrons are again transferred. The Mylar® strip now acquires the same type of charge as the second object. The strip is now repelled from this second object, but will be attracted to the first charged object. The Mylar® strip continues rotating due to momentum. When a Mylar® strip rotates near the first charged object, the process repeats. The attractive force changes to a repelling force and the Mylar® strip rotates away, repeating the cycle.

Resources

Basic Background Information on Static Electricity

Benjamin Franklin and his electric motor

Benjamin Franklin's Electric Motor



Raft Resources for Teaching - Static Merry-Go-Round (for purchase)

Benjamin Franklin’s Static Motor


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