Electric Charges: Balloon "Pets", Electric Field Detectors & Pop Can Racers
[Shopping List: balloon, including clear latex; soda bottles with caps; Styrofoam; string or yarn; aluminum pop cans; funnels; wool, cotton, polyester cloth, animal fur; fluorescent tubes; metal spoons; plastic pipe or tubes; helium if available]
Electric Field Detectors
- Mash Styrofoam into tiny pieces (perhaps in Ziploc bags)
- Using a funnel if necessary, carefully put a hundred or so Styrofoam bits into a balloon (preferably transparent) or a plastic soda bottle
- Carefully blow up balloon or cap soda bottle.
- Rub balloon or bottle with cloth (or on shirt or hair)
- Observe what happens to the Styrofoam bits as you bring fingers or various objects near the balloon (especially metal objects such as spoons)
Balloon Pets
- Blow up a balloon, tie off and attach a string or yarn
- Rub balloon with cloth (or on shirt or hair)
- Dangle balloon from string and observe what happens as you bring it near your body or other objects, including other balloons
Pop Can Racers
- Place an aluminum soda pop can on its side on a smooth table top
- Rub a balloon with cloth or fur or on shirt or hair
- Bring the balloon near the pop can and observe what happens
What's happening: All materials are made of atoms, which in turn are made of protons, neutrons and electrons. The electrons, which have a negative electric charge, are the smallest and most mobile of these sub-atomic particles and can easily move from one atom to another, even from an atom on one material to an atom on a different material. When two different materials, such as the latex balloon and the cotton or wool cloth, touch each other, the atoms of each material begin to tug at the electrons on the other, and the material which holds onto electrons the strongest will steal many of them from the weaker material. This gives the stronger material a lot of excess electrons, and thus a lot of excess negative electric charge, while the other material is left with an opposite or excess positive charge. [We can find tables showing which materials steal electrons from other materials, but it's not always clear which way the electrons will go in all cases.] The same thing happens when the Styrofoam bits touch the balloon; electrons can move from one to the other and leaving them with different electric charges. These charged materials create an invisible electric field of force in the space around them, which can attract or repel other charge objects (or electrons). Similarly charged objects (positive and positive or negative and negative) repel each other, while oppositely charged objects attract each other. Since the Styrofoam bits are very small, they can easily move in response to electric fields.
Materials can also be classified as conductors if their electrons can move around easily through the material, or insulators if their electrons cannot move easily. Latex and Styrofoam are insulators, while metals such as the aluminum can are conductors. Thus when we charge the balloon by rubbing it, the charges stay put for a long time since their electrons can't easily move around. Metals are very different however. When we bring an aluminum can or a metal spoon near a charged object such as a balloon, electrons in the metal can move around easily. If the balloon has a negative charge, electrons in the pop can (which also have negative charge) move as far away from the balloon as they can, leaving a positive charge on the side of the can nearest the balloon, and since opposite charges attract each other, the can will move towards the balloon. The same thing happens when a metal spoon is held close to the balloon; if the balloon is hanging from a string and free to move, it will be attracted to the spoon. Your fingers are also conductors (though not as good as the aluminum in the pop can), so the balloon will also move towards your fingers or body. When a balloon filled with Styrofoam bits is charged (let's say it's negative), the bits will touch the balloon and acquire the opposite charge (positive) and stick to the wall of the balloon. Now when a metal spoon (or your fingers) is held close to the balloon, the side of the spoon facing the balloon becomes positively charged, and this positive charge (or the electric field arising from the positive charge) is strong enough to repel the Styrofoam bits (because they're are also positively charged), thus they jump way very quickly. Electric fields around conductors are also much stronger near sharp points and edges, thus as you turn the spoon (or point your fingertips), the Styrofoam bits will experience stronger forces and move more quickly.
Variations: Try different objects. Try hanging the pop can from a string and bringing the balloon close without touching- they should attract each other. Then let the balloon touch the pop can, and they should immediately repel each other. This is because when they touch, electrons can move from one to the other which now gives the pop can an excess charge of the same sign as the balloon. After a few seconds this excess charges drains away (because the electrons can move through the string and into your body), and the can will again move towards the balloon. Try bringing your finger close to a charged balloon until you feel or hear (or even see) a spark. This happens when the electric field between your finger and the balloon becomes so strong that the air molecules nearby become charged and then jump onto your finger. The same thing happens between charged clouds and the ground during a storm to create lightning. Rub a balloon with a cloth in a dark room to see the sparks. Hold a charged balloon near a fluorescent light bulb (straight tubes work best) to charge the gas molecules in the tube which then emit photons and light up. A plastic pipe or tube can also be charged by rubbing with various fabrics, or even your hand.
You can make a floating version of the balloon buddy by filling it with helium and attaching a little masking tape to the bottom (like the tail on a kite) to control its density. You want it to be neutrally buoyant- neither rising or falling- just floating in mid air. This is a good opportunity to talk about buoyant forces and density. Floating balloon buddies can perform many cool tricks. Use your hand, a plastic tube, or another balloon to guide your buddy through the air- up, down and sideways. You can also use a marker to draw a face, then carefully rub only one side of the balloon. Because the latex is an insulator, the electrons will stay where they are, resulting in unequal electric charge on the two sides of the balloon. Now you can use another charged object not only to move the balloon through the air as before, but to make it rotate as well. Thus two balloons may face or even kiss each other, or turn away if they don't like each other.
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