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what you'll need
You will also want plenty of space to move around as you levitate objects.
All materials are made up of molecules that are composed of atoms (except for pure elements which are just atoms). Atoms are made of even smaller sub-atomic particles called protons, neutrons and electrons. Protons and neutrons form the nucleus of the atom, while the much, much smaller electrons move around the nucleus. You can sort of picture the nucleus like the sun and the electrons orbiting the nucleus like the planets orbiting the sun (this isn't exactly correct, but it's okay for our simple picture). We say that electrons have a negative electric charge while protons have an opposite or positive charge, and these oppositely charged particles are attracted to each other (like a magnet that sticks to your refrigerator), which is what keeps the electrons in orbit around the nucleus (the neutrons have no electric charge and we can just ignore them for now).
These electrons are so tiny and move around so easily that they can sometimes jump from one atom to another atom in the material, or even from an atom on one object to an atom on a different object. When two objects made of different materials, such as the latex balloon and the cotton or wool cloth, are rubbed together, the protons in each object not only attract their own electrons, but may begin to attract or pull on the electrons in the other object, sort of like a game of tug-of-war. If we now separate these two objects the one made of a material which can attract electrons the strongest may actually steal many electrons from the weaker material. This gives the "winning" object a lot of extra electrons, and thus a lot of excess negative electric charge, while the other object is left with the opposite or excess positive charge. This process is called contact electrification or triboelectricity, although it's more commonly referred to as static electricity. There are triboelectric series charts (see the reference links below) showing which materials can steal electrons from other materials, i.e. which will become negatively or positively charged upon contact with another material in the chart, but triboelectricity can be finicky, so it's not always clear which way the electrons will go in all cases, and sometimes the behavior just doesn't agree with the chart at all.
To explain how electrically charged objects (or individual protons and electrons) attract each other scientists say that they create an invisible electric field or force in the space around them. This electric field can not only attract an object or particle with an opposite electric charge (like a proton attracting an electron), but can also repel a particle if it has the same charge, i.e. two electrons will repel each other. Thus similarly charged objects (either positive and positive or negative and negative) repel each other, while oppositely charged objects attract each other, sort of like the attractive and repulsive forces of magnets (in fact, magnetism is just another form of exactly the same fundamental force observed here- but that's for another activity). This is what's happening when you press the polyethylene plastic hoop against the tabletop surface, or rub the balloon with the cotton cloth. You have probably also noticed this when you rubbed a balloon on your hair. Rubbing objects together just brings more of their surfaces close enough so that the atoms in one material can tug at the electrons of the other (it's not due to friction, so you don't need to rub hard). When you pull them apart you each will have opposite electric charge: the plastic hoop and the tabletop have opposite charge, i.e. one is positive and the other negative and they attract each other (that's why they cling or stick together); and the ballon or PVC and the cloth also have opposite charge, one positive the other negative. It can be very difficult to tell which one is positive and which is negative, but as long as the plastic hoop has the same charge as the balloon or PVC wand (whether it's actually positive or negative) they will repel each other (of course if they are oppositely charged they will instead attract and perhaps even stick to each other).
When you toss the plastic hoop in the air it starts to fall due to the pulling force of gravity, but your charged balloon or PVC wand produces a repelling or pushing force on the hoop. The closer you bring it the stronger that force will be, so that the hoop stops falling and rises up instead. If you hold the balloon or wand at just the right distance below the hoop you should be able to make it levitate almost motionless- bring it closer and the hoop rises, farther away and the hoop falls. Move your wand or balloon around and you can make the hoop dance and perform tricks!
Different materials are classified as conductors if their electrons can move around easily through the material, or insulators if their electrons cannot move easily. Latex rubber, plastic and Styrofoam are insulators, while metals such as copper or the aluminum in a soda can are conductors. When we charge insulating materials like the Latex balloon by rubbing it, the separated charges stay put for a long time since their electrons can't easily move around, and that's why it remains charged. Metals are very different however, which is why electric wires and cables are made with metal conductors so that electricity (moving electrons) can flow more easily. These electric wires are covered with rubber or plastic insulation so we don't get a shock when we touch them.
troubleshooting - what can go wrong
Contact electrification or triboelectricity can be very finicky, often due to various types of contamination or environmental factors which can influence the process. Thus you might have trouble charging your objects, or even get the opposite of what you expect to see. Some days it just doesn't want to work at all, especially on a very humid day. Moisture in the air can deposit a thin layer of water molecules on some objects which allows the static electric charges to move away. Oil from your hands can also contaminate objects in this way, so if your plastic hoop doesn't seem to work , try cutting off a fresh one.
In our experience rubbing a Latex balloon or PVC pipe with cotton cloth and rubbing a polyethylene bag on a wooden surface should produce the best results, but if these combinations don't seem to work for you, try something different. You can substitute wool, fur or polyester fabric for cotton, and you can try different wooden surfaces (painted, varnished, waxed, bare wood, etc.). Some kitchen countertops (like Formica) may also work very well, but plastic tables do not.
In any case, the more you practice the better you will get at levitation, so keep experimenting. And who knows- a little Wingardium Leviosa probably couldn't hurt either!
variations and related activities
You can also use the electric field force from a charged balloon or PVC wand to move other objects. Lay an empty aluminum soda can on its side, charge up your balloon or PVC wand then hold it very close to the can without touching it. The can should begin to roll towards your wand, so be sure to move it away as the can rolls to keep from touching. Place your wand on the other side of the can and you can make it stop, then begin rolling the other direction. Notice that you didn't need to charge the can for this to happen! That's because metals like aluminum are conductors, unlike the PVC plastic or Latex balloon. When we bring an aluminum can or a metal spoon near a charged object such as a balloon, the electrons in the metal can move around easily. If the balloon has a negative charge, electrons in the soda can (which also have negative charge) are repelled (remember similar charges repel each other) and move as far away from the balloon as possible (i.e. to the back side of the can), leaving fewer electrons and thus a positive charge on the side of the can nearest the balloon. 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 a balloon; if a 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 soda can), so the balloon should also move towards your fingers or body. That's also why the polyethylene hoop likes to stick to your body!
Have you ever noticed little bits of Styrofoam (like packing "peanuts") clinging to your hand or other objects? Do clothes cling to you or other clothes when you take them our of the dryer (i.e. "static cling")? Styrofoam peanuts (and clothes) can pick up an electric charge very easily by bumping into other peanuts (or clothes) or just about anything else, and thus will be attracted strongly to other objects- which you've probably noticed if you ever tried to pick them up! Try moving them around with your charged balloon or wand. The tiny beads inside a beanbag chair are also made of Styrofoam, so you if you have some you can put them inside a dry, empty plastic soda bottle and screw on the cap (if you don't have an old beanbag chair you can tear some packing peanuts or other pieces of Styrofoam into tiny bits instead). Now rub the outside of the bottle on your cotton shirt or hair and shake the bottle to transfer electric charge to the bits inside, then move a charged balloon or PVC wand near the bottle to see what happens! Also try moving a metal spoon, fork (or your aluminum soda can) near the plastic bottle filled with Styrofoam bits. Just as when you rolled an aluminum soda can above, electrons in the metal can move around due to the influence of the electric field from a charged object. Electric fields around conductors are 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.
If you rub a balloon on your cotton shirt or hair in a very dark room you may see sparks and hear crackling! The sparks are actually tiny lightning bolts, and the crackling tiny thunder claps! The electric field around your charged balloon is strong enough to move or even rip electrons from nearby air molecules, and the energy released as they crash into other air molecules heats the air to make the sparks and crackles. This is also what happens when air moves rapidly in clouds during a storm to make the really big lightning and thunder, and when you rub your feet on carpet and touch a doorknob or "shock" a friend. You might also see sparks at night when you rub against your blankets in bed.
We can even use the force of an electric field from a charged object to bend water! Adjust the faucet in your bathroom or kitchen sink so that a very thin continuous stream of water flows (rapid dripping also works). Charge up a PVC wand, balloon or even run a plastic comb through your hair then carefully hold it close to the stream of water. Make sure your charged object doesn't get wet, however, as this will discharge it (electrons can move through water- that's why you never drop a toaster or hair dryer in the bathtub!) If this happens dry the PVC or balloon completely, then charge it up and try again. The stream moves because water molecules are dipolar, i.e. they have negatively and positively charged ends.
There are even more electric charge activities and experiments in the links below.
references and links to more information
More about triboelectricity, contact electrification and the Triboelectric Series:
Measure electric charge with a DIY electroscope:
Fun with static electricity videos:
More static electricity activities:
Just for fun, levitating a frog in a very strong magnetic field:
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