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Soda Bottle Electrostatic Motor

You can build this simple electrostatic motor from 2-liter soda bottles and aluminum foil. Its construction does not require access to a machine shop. It draws a fraction of a microamp during operation, and can run at unexpectedly high speeds (1000 RPM!).

One of these motors is featured in the Electricity exhibit at the Museum of Science in Boston, powered by a hand-cranked Van de Graaff machine.

NOTE: To power your soda-bottle motor, you will need a "static electric" power supply (one that puts out high voltage--a minimum of 5000 volts DC--at low current). Batteries do not supply this voltage, but there are two safe, low-current electrostatic energy sources that you can use: 1) A balloon rubbed with artificial fur or other material, or 2) a simple electrophorus. Both of these power sources require low humidity in order to function properly.

• three 2-liter pop bottles, one with a METAL cap (Note: If you can't find a metal cap, you may need a hard object such as a 1/4" dia. glass test tube to glue into one of the plastic caps.)
• roll of aluminum foil or adhesive aluminum foil tape (available at some hardware stores)
• rubber cement
• silicone caulk
• 13"L x 1/8" dia. metal welding rod (works best), or a metal coat hanger
• two 8" pieces of solid copper wire or metal coat hanger wire
• wood plank or piece of metal or plastic for the base
• duct tape
• wire for grounding your motor
• balloon(s) for charging
• charging material such as wool or artificial fur to rub the balloon(s)

                      rotor
                     bottle
                      |^|         
            |||     /     \     |~|
  stator  /     \  (       )  /     \   stator
  bottle (       ) |       | (       )  bottle
   (+)   |       | |       | |       |   (-)
         | |||---------    |-|       |
         |       | |       | |       |
         |       | (_______) |       |
     ____(_______)_____|_____(_______)___
    |____________________________________| base 
                  SIDE VIEW


                                   brush
                           ______  support
   stator    ___       ___/     _|\|_ 
   bottle  /     \   /     \   /     \
          |  (|)  | |  (|)  | |  (|)  |
           \_   _/   \     /   \ ___ /  stator
       brush |\|______/~~~rotor         bottle
       support            bottle

TOP VIEW

Construction

• Cut the rod or coat hanger so it's about 1" longer than the middle bottle. Note: As stated above, 1/8" welding rod works best for as the center rod. A coat hanger wire will work, but it's very wobbly.
• Sharpen the rod using a file. Note: Instead of filing a sharp point, you can attach a piece of a sharpened pencil to the top of the rod. The sharp graphite point makes a good bearing, and is a better option if you must use a plastic bottle cap and do not have a test tube as described in "The Bearing" below.
• With an adult's help and wearing safety goggles, drill a hole in the center of the base using a drill bit slightly smaller than the sharpened rod.
• Carefully force the rod into the base, unsharpened end first.

• Find the exact center of the bottom of the middle bottle, and with an adult's help and wearing safety goggles, drill a hole there that's slightly larger than the rod diameter. Note: It is important to center the bottom hole well so the bottle surface won't move so much as it turns, making for easier adjustments later on.
• Slide the bottle onto the sharpened rod. Note: Check the spinning operation of the bottle. The hole should be large enough that the bottle spins very freely on the rod; however, if the hole is too big, the bottle will rattle around and make the brushes drag on its surface.

The Bearing (inside the cap of the Rotor Bottle):

The bearing gives the point on the rod (or the pencil point) something to ride in to stay centered.

NOTE: I have found through experimentation that Method #1 works best for making the bearing on the inside of the cap whether it is metal or plastic, and makes a plastic cap a more workable option. The sharpened rod spins nicely against the glass of the test tube.

• Method #1: With an adult's help, cut the bottom 1/2" off the 1/4" dia. test tube by nicking it with a file and snapping it by hand while wearing gloves.
• Being careful to avoid any sharp edges, glue the piece of test tube into the exact center of the bottle cap.
Note: Avoid dropping the center bottle suddenly down onto the metal rod, or the sharp point will shatter the glass bearing.


• Method #2: If you do not wish to use a glass test tube, then with an adult's help, use a hard, sharp object such as a nail to make a dimple in the center of the bottle cap's inside (whether metal or plastic). (Note: Take care not to poke through the bottle cap with the sharp object!) If you used the pencil point mentioned above rather than sharpening the rod, a plastic bottle cap might work with simply this dimple.
• Next, precisely cut three broad strips of aluminum foil or adhesive foil tape just wide enough to leave a 1/2" vertical space between them when attached to the center bottle.
• Round the corners of the foil or foil tape, then test-fit the foil strips on the bottle and trim as needed.

• If using rubber cement and foil rather than adhesive foil tape, coat the entire center bottle with rubber cement, then coat one side of each pre-cut foil strip and allow the glue to dry for a couple of minutes. Note: It doesn't matter whether the shiny side of the foil faces out or in.
• Carefully lay the glue-covered foil or adhesive foil strips on the bottle, burnishing them down with a spoon as you go. As shown in the drawing, the end result should look like an aluminum coating on the bottle, with three broad foil sections separated by 1/2" gaps running vertically. Note: No part of each foil section should touch any other foil section. Bubbles in the foil are harmless, and can be punctured with a pin and flattened with a spoon.

• Secure large sheets of foil (with rubber cement if not using adhesive foil tape) around the entire center areas of both of the two outer 'stator' bottles, leaving a 2" foil-free space at the bottoms.
• Note: Keep the bottom clear of foil, and make sure that no foil on these bottles comes close to touching the base or close to any duct tape you might use to connect the stator bottles to the base.

• Attach one of the 8" pieces of solid copper wire or coat hanger to each stator bottle (on the front side of one bottle and on the back side of the other) by bending one end of the wire into an S-shape and embedding the S-shaped part in silicone caulk on the side of the bottle.
• Extend each wire sideways so that the ends are very near (but not touching) the rotor bottle surface.
• Bend the tip of each wire so it points toward the surface of the rotor bottle as shown in the drawing.
• After the caulk sets, bend the remaining short ends of each S-shape so they make solid contact with their stator bottle's foil. Note: Be sure that the silicone does not insulate the wire from the foil.

• Using duct tape (or a nut and bolt as described below), attach the two stator bottles to the base so they are spaced about 1/2" from the rotor bottle. Note: If you use tape to attach the stator bottles to the base, make sure it DOES NOT reach up to contact the foil. Duct tape, masking tape and wood are all slightly conductive, and when the humidity is high, they can provide an unwanted leakage path to ground, preventing motor operation.
• If you can figure out a way to position a bolt inside each bottle, you can use a nut and bolt through the bottom of each stator bottle to attach it to the base. This allows you to rotate the bottles a bit for easy adjustment of the spacing between the commutator wire tips and the center bottle.

Running Your Motor

Note: Electrical ground can be found in many places, for example, use a metal faucet, metal sink, or the metal screw on the cover plate of a light switch or electric outlet. In a pinch you can use a metal tabletop or a few feet of aluminum foil layed on the floor, or even touch the other bottle with your finger to provide a crude ground-path out through your feet.

• Connect one stator bottle foil to ground.
• Charge a balloon by rubbing it with the wool or artificial fur.
• Pass the surface of the charged balloon along the foil of the other, non-grounded stator bottle, and the motor will slowly turn. Hint: This works better if you place the balloon against the stator bottle at the top edge of the foil, then carefully ROTATE the balloon so its charged surface passes over the foil edge.
• Enlist the help of several friends to keep the motor turning by sequentially charging several balloons and passing each across the foil.

An electrophorus will run your Soda Bottle Motor very slowly. Note: If humidity is high it will not work properly. See "Is the Humidity Too High?" in the "Electrostatic Motor Debugging/Repair" section below.

After building your electrophorus:

• Connect the foil of one of the stator bottles to ground.
• Charge the electrophorus disk as described in the electrophorus instructions and touch it to the foil on the non-grounded stator bottle.
• Do this over and over fairly fast, and the motor will slowly turn.

Electrostatic Motor Debugging/Repair

The first thing to think of is whether you followed the instructions exactly when you built your motor. It can seem boring to simply copy someone else's idea exactly, and it's easy to insert many improvements as you go, but maybe one of your improvements actually made the motor not work. The solution? Make sure you followed the instructions exactly, then, after you have a working motor, go and add all your alterations and improvements one at a time and see what happens. This could even make for some interesting experiments!

• Interesting fact: Just a microscopic coating of oil will prevent "frictional charging." (The anti-static "dryer sheets" used to prevent static charging in your clothes dryer actually contain oil.) So if you're rubbing fur on rubber, those surfaces need to be extremely clean. The slightest bit of oil can simulate the effects of 90% humid weather and halt all charging.
• If your "frictional" electrostatic machine doesn't work or stops working, yet the weather is dry, suspect that your balloon and/or what you are rubbing it with are too oily.
• Give both objects a good scrubbing in running water with some detergent (not soap!) to carry away the microscopic oil coating. Blot dry, then thoroughly dry the surfaces with a hair dryer.

• Does the center bottle turn VERY freely? It must, otherwise the feeble electrostatic forces won't be able to move it.
• To judge whether its friction is low enough, get the bottle spinning slowly by hand, turning about once per second, then let go. It should keep turning for one or two revolutions or more. If not, the friction is far too high.

To decrease the friction:

• Verify that the tips of the commutator wires do not drag against the center bottle as it rotates.
• If you used a metal bottle cap and sharpened metal rod, check out the metal point bearing and make sure it hasn't drilled itself into the metal bottle cap.
• If you used a plastic bottle cap without the test tube glued inside, then be sure to attach a sharpened pencil point to the metal rod so as not to drill into the soft plastic.
• Also put a little oil near the bottom of the metal rod where it rides against the hole in the rotor bottle's bottom.

• If the humidity is high (above 40% to 50%), store the whole setup in a cold, air-conditioned room for a couple of hours, then operate it in that room. Or, wait to try again on a day that isn't as humid.
• Hint: To test the humidity, rub a balloon on your arm hairs. If the charged balloon makes the arm-hairs rise, the humidity is low enough that it is not the cause of any problems with your motor. If no amount of rubbing with a balloon can lift your arm-hair, then the humidity is probably too high.

• Make sure that there are no surface connections between the three foil strips of the rotor bottle.
• Ensure that there is about 1/2" space between the three foil strips.
• Make sure there are no connections between the foil on the stator bottle and the base.
• If you used tape to attach the stator bottles to the base, make sure it is not making contact with the foil.

"Organized" Debugging

If after trying the above, your motor still won't work, then you need to look for the problem systematically. Use the "debugging sequence" below.

Reduce the Commutator Gap

• Verify that the tips of the commutator wires are VERY CLOSE to the surface of the rotor bottle, since tiny sparks will be jumping from the wires to the patches of foil.
• If you centered the bottom hole in the rotor bottle, the surface won't move so much as it turns, and you can easily move the wire tips closer in.
• If the gap between the foil and the wire tips is too big, then your motor will only work if you use a VERY hi-volt supply, like a VandeGraaff generator.
• If the gap is small, then even a feeble power supply (like the balloon/wool or an electrophorus) will make your motor spin.

• Sometimes there is an invisible leakage path between the foils on the rotor bottle.
• Ensure that there is 1/2" spacing between the foil strips on the rotor bottle. If not, then with an adult's help, use a razor knife to trim the foil strips so that there is 1/2" spacing between them.
• Next, use something to scrape the plastic between the foils to remove all traces of glue or other material from the plastic bottle.

• Give the rotor bottle a slow spin.
• Let it coast, then immediately use your power source to create the high voltage.
• You may hear little clicking noises as sparks jump between the commutator wire tips and the rotor foils.
• If you DON'T hear any clicking, then the sparks may be far too small to hear or see. Hint: Place an AM radio within a few feet of your device and tune it to a blank station. The radio will pick up the electromagnetic pulses of even the tiniest sparks.
• If you still don't hear any sparking on the AM radio, then the high voltage is being shorted out somehow.
• If you DO hear clicking with either method above, then almost everything is working, but there isn't enough force to turn the rotor. Possible reasons:
  
  - Your power source could be too weak
  - The friction in the rotor bearing could be too high. Hint: To decrease friction in the rotor bearing, use the glass test tube/pencil point bearing as described in the instructions.
  - The middle bottle could be much higher than the side bottles.
  - The weather could be too humid; try moving into an air-conditioned room.

• Generate another burst of hi-voltage, then bump your knuckle against the foil on the ungrounded stator bottle.
• If you don't feel a spark (or hear it on the AM radio), then there may be a short circuit in the motor.

• If you did get a spark from the ungrounded stator bottle in the step above, then use your power source again to make more hi-voltage, and bump the grounded stator bottle with your knuckle. You should not feel (or hear on the AM radio) a spark.
• If you DO get a spark, it means that your source of electrical ground is bad, or there is a bad connection between the "ground" bottle and the distant earth.

• The metal of the commutator wires must be directly connected to the foil of their respective stator bottles. Commutator wires are electrically a part of the stator foils, so when the power supply is operating, both the wires and the stator foils acquire the same polarity of charge imbalance.
• If the commutator wires don't make good contact with the foil on their corresponding stators, either the stator foils won't get charged, or no charge will flow down the wire to leap across to the rotor bottle.
• To ensure good contact, make sure there is no plastic on either end of the commutator wires, and that bare metal touches bare foil. Make sure that there is no caulk, tape, hot glue, or other material that you used to secure the wire to the foil, between the foil and the wire.

How it Works

• When high voltage is applied between the two stator bottle foils, one stator bottle acquires a negative charge imbalance, while the other one becomes positive.
• Also, a tiny spark jumps from the tip of each commutator brush to one of the foil sectors on the rotor bottle.
• The sector under the positive brush becomes positive, while the one under the negative brush becomes negative.
• The rotor's foil sectors are then repelled from the alike-charged stator bottle and attracted to the unlike charged stator bottle.
• This sideways electrostatic force causes the center bottle to rotate, which brings new foil sectors under the brushes.
• Tiny sparks then jump to the new sectors and charge them, which makes them attract/repel from the stator bottles, etc.
• The foil on the rotor bottle that's under the commutator is always charged the same as the commutator, so it's always being repelled/attracted sideways.

• The force is continuous, therefore the speed of the rotor bottle will keep rising higher and higher.
• In practice the rotor speed will not increase forever, but will stabilize because of air turbulence, bearing resistance and bearing chatter, etc.
• If the entire motor could be put inside a vacuum chamber, it would REALLY run fast, but then the sparks couldn't jump from the commutator wires, and you'd have to arrange some kind of sliding contact brushes instead. (Sparks cannot exist in vacuum, since a spark is made of air that has turned into plasma.)

Here's another way to visualize the motor:


• If you turn it sideways, you'll see that the motor is sort of like a waterwheel.
• On one side, the excess negative charge is pouring into the side of the rotor bottle and "falling" toward the positive bottle.
• On the other side, the positive excess is "falling" upward and dragging the rotor bottle surface up.
• The moving charge gets temporarily stuck in the foil surfaces of the rotor, and it drags the rotor along with it as it moves from one stator bottle to the other.
• The motor rotates because it blocks the flow of moving electric charge, just as a waterwheel rotates because it blocks the flow of moving water.

Yet another way to imagine it:


• Normal coil-and-magnet motors often are built like this: three electromagnet coils in the rotor, two permanent magnets as the stator, and a commutator. They operate by magnetic attraction/repulsion.
• In the pop-bottle motor, all the coils are replaced with capacitor plates. The rotor has three capacitor plates instead of three electromagnetic coils. The stator permanent magnets are replaced by (+) and (-) charged stator plates. All the electric currents have become voltages, and all the magnetic fields are replaced by electric fields.
• In electronics, the swapping of coils with capacitors and voltage with current is called the "electromagnetic dual."
• Capacitors are the "dual" of inductors.
• The pop-bottle motor is the "electromagnetic dual" of the common coil/magnet motor.
• Small slot-car PM (permanent magnet) motors represent one side of the electric/magnetic duality found in light, radio, electric energy, and all EM phenomena.
• The pop-bottle electrostatic motor is the other half of the duality.
• Light waves, radio waves, and electrical energy are where the voltage/current and electric/magnetic duality blends into a single thing called...electromagnetism.

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Soda Bottle Electrostatic Motor