Click and expand the tabs below to get started. what you'll need
experimental procedure
Check out this video from Science Off Center and the St. Louis Science Center what's happening
When you run a plastic comb through your hair it pulls lots of very tiny particles called electrons away from your hair. We say these electrons have an electric charge, and when electrons (or other charged particles) move we call this electricity. If lots of extra charged electrons are stuck on the comb and not moving, however, we sometimes call this static electricity. This static electricity creates a force around the comb which can push or pull other charged materials, kind of like a magnet, and it turns out that water molecules are easily pulled by this force. That's why the stream of water from your faucet bends when you hold the charged comb nearby. The closer water is to the comb the stronger the force and the more it moves, but if the comb gets wet it can discharge, allowing those extra electrons to escape, thus there is no force left to pull on the water stream.
more detailed explanation
Everything is made up of matter, matter is made of atoms (which can combine to form molecules), and atoms are made up of protons, neutrons and most importantly for this experiment, electrons. The protons and neutrons are stuck together in the nucleus of the atom, while the much tinier electrons fly around outside the nucleus. It's kind of like the way planets orbit around the sun (the electrons are like the planets and the nucleus is like the sun), but electrons don't follow orderly paths like planets do, it's more like a cloud of electrons (called orbitals) surrounding the nucleus. What keeps electrons from flying away from the nucleus is a force called the electric field force. We say that electrons and protons have an electric charge, or just charge. Electrons have a negative charge (-), protons have a positive charge (+), and charged objects create an electric field that can attract or repel other charged objects (neutrons don't have any charge- they're neutral). Oppositely charged particles attract each other and the closer they are to each other, the stronger the field and the force of attraction. Particles with the same charge (such as two electrons or two protons) repel each other (the closer they are the stronger the field and repulsive force). [BTW- If this reminds you of the way magnets can attract or repel each other depending on their north or south poles and how close they are to each other, you're right. Electric and magnetic forces (and fields) are actually related to each other, so this is really called the electromagnetic force (and electromagnetic field). This is one of the four fundamental forces in nature (see the link below to learn more about this and the other fundamental forces that hold atoms together).] Different types of atoms (called elements) and the molecules they combine to form may be stronger or weaker when it comes to attracting and holding onto their electrons. In our case, the molecules that make up the plastic comb hold onto their electrons much more strongly than the molecules which make up your hairs hold onto their electrons. When you comb your hair, some of the comb's molecules get very close to some of the hair's molecules- so close, in fact, that the atoms in the comb actually start attracting or pulling on the oppositely charged electrons belonging to the atoms in the hair. Likewise, the hair's atoms also start pulling on the comb's electrons. It's kind of a tug-of-war battle, but with electrons (using the electric field force) instead of rope. Normally most molecules have the same number of negative and positive charges, (so we say they are neutral), but when you take the comb away from your hair some of the hair's electrons are actually pulled away, sticking to the comb's atoms instead (remember that the comb's atoms are stronger at attracting electrons). Thus the comb now has extra electrons- and extra electric charges- and your hairs have too few electrons, so we say the comb is now negatively charged and your hair is now positively charged. That's why your hairs are attracted or stand up if you bring the comb close to your head- because the opposite charges attract each other. The more you comb your hair, the more charges build up, and the stronger the electric field (and attractive force) will be. Charging an object this way is called contact electrification, or the tribo-electric effect (see the reference link below for more detail). This is also commonly called static electricity, but many scientists don't like this name, because electricity is defined as moving electric charges, thus static (non moving) electricity doesn't really make sense. Water molecules are made up of two hydrogen atoms and one oxygen atom attached to form a triangle shape, kind of like Mickey Mouse's head- oxygen is the face and the two hydrogens are his ears. All of the electrons in these atoms move around in such a way that the oxygen corner of the triangle is more negatively charged and the hydrogen corners are more positively charged. This kind of molecule is called dipolar. Since water molecules in a liquid are free to move around and rotate, when you hold your negatively charged comb near a stream of water, the positive (hydrogen) sides of each water molecule are attracted to the comb's electric field and rotate so that they point towards the comb. This means that the side of the water stream closest to the comb is more positively charged and the opposite side of the stream is more negatively charged. Finally, the electric field of the negatively charged comb attracts (or pulls) on the positively charged side of the water stream, while it also repels (or pushes away) the opposite (negatively charged) side of the stream, but since the positive side is closer, the attractive force there is much stronger and the stream of water is pulled towards the comb. If you move the comb to the other side of the stream the water molecules simply rotate again to point the opposite direction, so that the side of the stream closest to the comb is once again more positive, and thus the stream still bends toward the comb. By the way- if you hold a magnet near the stream of water nothing happens. It is neither attracted or repelled by the magnet. But that doesn't mean that water (or anything made mostly of water, like frogs or even people) can't be influenced by a magnetic field- you just need a really powerful magnetic field. In fact, if your magnet is strong enough you can even levitate a frog. Did you notice that if you get the comb wet, it no longer bends the water? The extra electrons that were stuck on the comb are able to move into the water and escape, thus the comb is no longer charged (and no electric field present). Also, if you wet the comb first, then run it through your hair, the water prevents it from grabbing electrons from your hair and charging up, so this doesn't bend a stream of water either. Even a microscopically thin layer of water on the comb, which can happen on a humid day, will prevent it from charging up (if you're having trouble making the experiment work, this may be the problem). variations and related activities
You can also bend the water stream with a rubber (Latex) balloon or plastic rod (PVC plumbing pipe works very well). Just charge them up by rubbing with your hair, a cotton towel or T-shirt or a piece of fur. Try lots of different materials. This related activity (and video) shows you how to charge a balloon or PVC pipe and make some other objects move. Check out the references below for more information about the tribo-electric series and charging different materials.
Try making the faucet drip instead of a continuous stream of water. Are the droplets attracted to the comb? And while you're in the bathroom anyway, take a small piece of toilet paper and dangle it vertically near your comb. It should also be attracted to the comb, because the comb's electric field induces a small electric dipole moment (like a charge) in the paper's molecules that is attracted to the comb. In fact, if you tear the toilet paper into really tiny pieces you can even use the charged comb to lift them right off a table or countertop, and even make them seem to dance! Here is a different way to do this experiment: references and links for more information
Other science website do this experiment:
More on atoms and contact electrification (static electricity): The tribo-electric effect (contact electrification) and tribo-electric series: The fundamental forces within an atom: Induced dipole moments or charges: Have a question or comment? Let us know at the bottom of the page.
Return to Try Science at Home
1 Comment
This curious toy was first invented by René Descartes, an important scientist and mathematician in the early 1600's, and while his design has evolved into many different variations they all continue to amaze us while teaching some important scientific principles in a fun way. Click and expand the tabs below to get started. what you'll need
experimental procedure
Pipette "Submarine" diver:
Balloon "Fish" diver:
Ketchup packet diver:
what's happening
The pipette diver floats in the water at first because there is air trapped inside which makes it buoyant- just like the air inside a balloon or ping pong ball makes it float in water. When you squeeze the sides of the bottle you increase the pressure inside and that pushes more water into the pipette through the open hole at the bottom (you should be able to see it happen). This extra water makes it too heavy to keep floating (or more dense than the water in the bottle) and it sinks. When you stop squeezing the pressure drops and the extra water (and weight) flows back out of the pipette and it floats again.
The balloon "fish" also floats at first because their is a little air trapped inside, but this time when you squeeze the bottle there is no way to push any extra water inside (it's tied shut). But the balloon is made of rubber which is very flexible, so the pressure from the water outside squeezes the balloon and makes it a little smaller. The balloon still has the same weight, but now that everything inside is squished closer together (mostly the air molecules trapped inside) the balloon is more dense than the water and sinks. Stop squeezing and the balloon expands, becomes less dense and floats again. More detailed explanation
If this window doesn't open fully try closing and reopening the "What's Happening" window above while this window is open. The Earth's mass pulls other objects with less mass towards it's center (i.e. down). This is just the force of gravity that makes a ball fall to the ground, but the ball doesn't fall as long as you are holding it because your hand pushes up with a force that exactly balances the force of gravity pulling down. Of course if your hand pushes up with even more force you can make the ball move up. This is actually what's happening to our diver, we just need to understand what is causing these forces that make it move up and down. Obviously gravity is the force that makes it move down, but what force makes it move up? The surprising answer is actually gravity! But how can gravity make it move up and down? Okay, gravity doesn't directly push the diver up, but it is responsible for the force that does, which is called the buoyant force or buoyancy. As we said, the Earth's gravity pulls everything with mass down, including the water molecules in your bottle, and the weight (which is the amount of force of gravity) of all these water molecules trying to squeeze to the bottom of the bottle creates water pressure inside that pushes back and stops them. The deeper you go in the water the greater the pressure at that depth. Think of it like the cheerleaders making a pyramid. The cheerleaders on the bottom are being squished by the weight of the others above them, and must push back to keep them up. The cheerleaders in the second row have fewer above them, don't get squished as much and don't need to push back as much to hold up those above them. And of course the lone cheerleader on top isn't squished or pushing back at all. This water pressure doesn't only push on other water molecules, it pushes on everything inside the bottle as well as the bottle itself (and in every direction, not just up). So now we have a force that pulls the the diver down (its weight, which is equal to its mass multiplied by the acceleration due to gravity, i.e. W= M x g) and another force that pushes it up at the same time (its buoyancy, caused by the water pressure, which is also ultimately due to gravity). Whichever force is stronger determines the direction the diver will move. The weight of the diver is easy to measure, but how do we calculate its buoyant force? For the diver to move it must push any water molecules in its way aside or displace its volume (the amount of space it occupies). It turns out that the force needed to do this, or the buoyant force of the diver, is just the weight of the volume of water it needs to displace. This is called Archimedes Principle. To make things easier we can calculate the density, which is mass divided by volume (d=M/V), for both the diver and the water. If the density of the diver is greater than the density of water, then the weight of the diver is greater than its buoyant force in water and it sinks; if the diver is less dense than water the buoyant force is greater than its weight and it floats. The density of water doesn't change in this experiment (because water is incompressible- more on this later), so to move the diver up and down we must change its density instead. [BTW- water pressure pushes in all directions, so why does the diver go up if it's less dense than the water around it? Couldn't the water pressure just as easily push it down or sideways? Remember that pressure increases with depth, so at any depth the pressure on one side of the diver is always balanced by an opposite pressure push on the other side of the diver, i.e. they cancel out. But the water pressure pushing down on the top of the diver is always just a little less than the pressure pushing up on the bottom (because it's deeper), thus there is always just a little more force pushing up on the bottom than down or sideways and this is enough to insure that the diver (or any buoyant object) always floats upwards.] For the pipette diver we change its density by adding or removing mass while keeping its volume the same. When you first placed the pipette in the bottle of water it floated, so its density was less than the density of water. The pipette had only air inside and thus very little mass because there were very few air molecules (molecules in a gas are very far apart). When you squeeze the bottle it tries to squeeze the water inside, but the water molecules are already as close together as they can possibly get, i.e. we can't compress them any more, they're incompressible (which is effectively the case for all liquids). Gasses, however, are compressible since their molecules are very far apart and can easily be squeezed closer together. Thus the force of your squeezing the bottle increases the water pressure inside which pushes more water inside the pipette through the open hole in the bottom. The added water molecules increase the total mass of the pipette while the total volume inside stays the same (the air just takes up less volume now to make room for the added water volume), so the pipette density increases. Once enough water goes in to makes the pipette more dense than the water, it sinks. If you stop squeezing everything reverses- water flows out of the pipette, the air expands to fill more of the volume inside decreasing the overall density and the pipette floats once again. If you squeeze the bottle just hard enough you can force the right amount of water into the pipette to makes its density exactly equal to the water density and thus stop and hold the pipette motionless at any depth you choose. Give it a try! Watch the pipette closely as you squeeze the bottle and you should be able to see the water go in and out. This is exactly how a real submarine dives, surfaces or controls its depth in the ocean. Submarines contain ballast tanks that they can fill with water to increase their mass (and thus density) when they want to dive. To surface they blow air into the tanks, forcing the water out, which decreases their density and they float upwards. With just the right balance between air and water in the tanks they can control the density to remain at any depth they choose. The balloon "fish" also dives and sinks due to changes to its density, but instead of adding or removing mass we change its volume instead. Initially the balloon has just enough mass (mostly from the BB's inside) and volume (mostly due to the air inside) to make its overall density a little less than that of water, so it floats. When you squeeze the bottle and increase the water pressure inside it squeezes the balloon which then squeezes the air inside and reduces the volume of the balloon, i.e. it actually gets a little smaller. Since the balloon is closed nothing can go in or out (unlike the pipette diver), so the mass must stay the same even though the volume is smaller, and thus the density increases. If you squeeze the bottle and make the balloon small enough, the density will be greater than that of the water around it and it sinks. Stop squeezing and the balloon expands once again, its density decreases, and it floats back to the top of the bottle. Just as with the pipette diver, if you squeeze just hard enough you should be able to make the balloon stop and hover at any depth you like. We call this a balloon "fish" not only because it kind of looks like a little fish, but because this is very similar to the way a real fish controls how deep it swims in the water. Fish have a bladder inside their body that is filled with air (kind of like our lungs). They don't use it to breathe, but rather to control their density. Fish have muscles that can squeeze this bladder to make it a little smaller (just like you squeezed the bottle to make the balloon smaller) which in turn makes their whole body a little smaller and therefore denser when they want to sink to go deeper. To float up they just relax the muscles around the air bladder to make their body a little bigger and less dense. To hold any depth they squeeze their muscles just enough to keep the bladder the right size to match the density of the water. Can you make your "fish" perform all these movements? A ketchup packet usually contains a little air bubble inside so that it works just like the balloon in this experiment. The ketchup provides the mass (instead of BB's) and the flexible plastic or foil of the package allows it to change size as you squeeze the bottle. Finally, gravity also creates buoyancy in air and other gasses, which explains how helium and hot air balloons can float due to differences in density. In fact, this is true for all fluids, which can be composed of liquids, gasses or even solids (like sand) as well as mixtures of each. variations and related activities
More air inside the balloon increases the buoyant force, while adding more BB's makes it heavier, so by varying the size and weight of the balloons we can make it easier or harder for our diver to work. Try 2 or 3 different combinations in the same bottle. Another way to make it harder to operate is to leave a small amount of air in the bottle before screwing the top on. Now when you squeeze the bottle the increased pressure just compresses this air pocket rather than affecting the diver. Temperature also affects the density of the water (cold water is more dense than hot water), so a balloon which floats in cold water may sink if the water bottle warms up too much, perhaps by sitting in hot sunlight [to a lesser extent the warm water also warms the air in the balloon, which causes it to expand and increase its volume, thus increasing its buoyancy, so there is a trade off to be considered before determining if the balloon will now sink or continue floating. Generally speaking, the decreased density of the water with warming usually dominates, and the balloon sinks]. On the other hand, if you leave some air in the bottle, then warming the water also warms this air pocket, increasing its pressure, and effectively adding more "squeeze" force to make the balloon sink. A type of water thermometer named after Galileo uses this principle (see link below). You can also make the balloon sink by removing the bottle cap and blowing very hard into the bottle with your lips tight around the neck (as though you're trying to inflate it). This works best if your diver is very easy to move (i.e. its density is already very close to that of water). "Fizz-Saver" pumps can be used the same way. Buoyancy can turn raisins into divers, and you don't even need to squeeze the bottle. Buoyancy also makes Lava Lamps work. See the reference links below for more information. There are many other diver designs, including some that spin as they go up and down and one that works in reverse (i.e. it floats when you squeeze the bottle and sinks when you stop squeezing). You can even create a game to retrieve sunken treasure (and maybe think of a way to use buoyancy to retrieve real sunken treasures). Try other objects, such as restaurant salad dressing or honey packets and even little pieces or orange or lemon peels. Can you create a design of your own? You can also decorate your divers to make them look more like fish or squids- get creative! Check out the many links below for ideas. Can you use what you've learned in this experiment to design an instrument that measures the density of liquids? references and links to more information
History of René Descartes and Cartesian Divers:
More about density, buoyancy and Archimedes' Principle:
Other Cartesian Diver designs:
Make a reverse Cartesian Diver:
Make a game to retrieve sunken treasures: How submarines and fish use buoyancy to control their depth:
Make our cool Lava Lamp and raisins dance with buoyancy:
Galileo's thermometer: Make a hydrometer to measure liquid density: Have a question or comment? Let us know at the bottom of the page.
Click and expand the tabs below to get started. what you'll need
You will also want plenty of space to move around as you levitate objects. experimental procedure
what's happening
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: Have a question or comment? Let us know at the bottom of the page.
Click and expand the tabs below to get started. experimental procedure
what's happening
This experiment is all about what's happening on the surface of the water. It's tempting to say that the little bread clip boat is floating in the water, but it's actually held up by water molecules at the surface pulling on other water molecules around them (cohesion) to create a strong force called surface tension. It's almost as if they form an invisible film that covers the entire surface, and it's strong enough to keep your boat from sinking! You can prove that your boat can't float on it's own by pushing it into the water- it will sink to the bottom of the pan. When you add a drop of dish soap to the water soap molecules begin to spread out like a film or puddle, taking the place of water molecules at the surface. This is because soap is type of chemical called a surfactant, and its molecules have a much lower or weaker surface tension, i.e. they can't pull other molecules around them as strongly as the water molecules do. Imagine a tug-of-war competition between two teams- the soap team and the water team. The water team is much stronger, so they win, pulling the soap molecules back with them and stretching the soap film out across the surface. This process of a liquid with a higher surface tension pulling a liquid with a lower surface tension is called the Marangoni effect (the reference link below has a nice video explaining what is happening). When you add a drop of dish soap to the water inside the hole of your boat it lowers the surface tension inside and the water molecules behind your boat pull soap out of the hole through the little gap (like a rocket nozzle). The force of these soap molecules moving backwards produces an equal and opposite force on the boat, moving it forward. This is an example of Newton's Third Law of Motion (see the reference links below to learn more about Sir Isaac Newton's famous Laws of Motion). Because the surface tension of water is so much stronger than that of soap, the water quickly stretches out and expands the soap film until it covers the entire surface of your pan. Once this happens there is no more force to propel your boat and it stops. Even a tiny drop of soap contains more than enough molecules to cover the entire surface (the film can actually keep stretching until it is only a single molecule thick!), so if you want to race your boat again you must remove all of the soap from the surface by dumping out the water and rinsing everything. Variations and related activities
There are more fun experiments that demonstrate the cohesive force or surface tension of water. First, place a glass or plastic cup (it should have a smooth rim) in your empty pan (just to catch any water that spills), then fill the cup with water completely to the top (just before it spills over). Using a spoon (or a pipette if you have one) carefully add more water a few drops at a time until the surface of the water bulges over the top of the glass without spilling. You can really see just how strong the surface tension is as it keeps the water from spilling over the side (if it does spill over the side you probably added the extra water too quickly; try again). Now that you are impressed by the surface tension of pure water, add a single drop of dish soap to the water in the cup (or touch it with your soapy Q-tip or toothpick). The water should instantly spill over the side as the soap lowers the surface tension, making it too weak to keep bulging above the top of the cup. Another fun experiment is to see what else you can "float" on the water surface. [Note- we'll say "float" here, but what we're really doing is suspending objects on the surface of water due to surface tension. An object only truly floats in a fluid when its density is less than that of the fluid, and the objects we'll be using are more dense than water, so they would sink if not for the surface tension.] Again place your glass or plastic cup in the empty pan and fill it to the top with clean water. Try to place a paper clip on the surface of the water without it sinking. This can be a bit tricky, but keep trying. Some tips that might help: use smaller paper clips, bigger ones may just be too heavy for surface tension to hold; first balance the paper clip like a teeter-totter on the edge of the cup, then gently nudge it onto the water surface; another trick that might help is to first float a small piece of paper towel or napkin on the surface, lay the paper clip on the paper, then gently sink the paper using another paper clip or toothpick. See if you can "float" small buttons or thumb tacks the same way (make sure you use the type of thumb tacks shaped like little umbrellas, and larger or plastic-coated ones usually work better). Once you have some objects suspended on the surface, add a drop of dish soap again and watch what happens. [Note that some buttons may still float even after you add the soap- if they are only slightly more dense than water the surface tension of the soap layer may be strong enough to hold them up.] You can indirectly observe a soap film spreading across the water. Add some clean water to your pan then sprinkle some pepper flakes all over the surface. Add a drop of dish soap near the middle of the pan and watch what happens. Since the pepper flakes are floating on the water they will move as it pulls away, allowing you to actually measure the size of the soap film. You can also add pepper flakes to the water before you launch your boat to better observe what is happening when you add the soap. Try various sizes and shapes of bread bag clips, and trim them to make different boat shapes. You can also cut boats from old playing cards or other paper that has a waterproof coating, thin Styrofoam sheets, plastic or other materials. Try various shapes for the "fuel" hole and "nozzle" channel. How does this affect the performance of your boat? Trying racing boats with your friends. Remember that you will need to dump the soapy water and refill the pan with clean water before each race. Could you design a boat that that will always move in a circle, or just spin without really going anywhere (see the science4fun reference link below)? Instead of adding the soap to the hole inside the bread clip while it is in the water try placing a small drop of soap directly on the boat first (somewhere around the fuel hole or the nozzle port), then placing the boat in the water. You can also try touching your soapy toothpick behind the boat once it's in the water rather than inside the fuel hole. Does it still move? Does it go as fast or as far as before? When you add the soap behind the boat the surface tension of the water will try to pull water and soap into the boat hole which- according to Newton's 3rd Law- should make the boat move backwards, but it still goes forward instead. How can that be? In this case something different is happening. As the soap film forms in back of the boat the water behind the soap pulls backwards, but the water in front of the boat is pulling forward, and this drags the boat along with it (sprinkle pepper in the water to observe this). This dragging effect also contributes some of the force to move the boat even when you do add the soap inside the hole. Finally, soap is not the only liquid with a lower surface tension than water. Try adding a small drop of rubbing alcohol (isopropyl alcohol) to the fuel hole in your boat instead of dish soap (you will need to use a pipette or small straw). Your boat might not move any faster than it did with soap- at first- but if you keep adding more drops of alcohol your boat will keep going, and going, and going! It doesn't "run out of gas" the way it did with soap. This is because rather than forming a film on the surface (as a surfactant like soap does), alcohol is miscible in water, i.e. alcohol mixes or dissolves completely into the water, allowing the surface tension to recover fairly quickly so that the next drop is just as effective as the last. Theoretically your boat could keep going until you have added almost as much alcohol as the water you started with. This reference link below explains what's happening and describes some other liquids that you can experiment with as alternative "fuels" to better propel your boat. references and links for more information
Other's versions of this activity:
Surface tension and cohesion force:
More surface tension and cohesion experiments:
Nice video explaining the Marangoni effect: An even better "fuel" to power your little boat and a great way to make this activity into a real experiment: Newton's Laws of Motion:
Return to Try Science at Home
what you'll need
You'll also want a large washable table or drop cloth if you're working inside, as this can be a pretty messy experiment. Towels and a bucket of clean water are a good idea. prepare your oobleck
Preparing a cornstarch and water Oobleck mixture. experimental procedure
What's happening
There are three main states of matter- solids, liquids and gasses (there is also a fourth state called plasma, and even more states that you may learn about if you study science in college). These three states have very different physical properties, so it is usually very easy to tell them apart. We know that solids keep their shape and are often hard, liquids take the shape of their container but can pour and flow, and gasses can expand to fill all available space. Scientists have created another category (although it's not a true state of matter) called a fluid which includes any substance that will pour or flow if pushed. Liquids like water are obviously fluids, but a bucket of sand or rocks (which are solids) can also be fluids, as can mixtures of solids and liquids, and even gasses. But what if a material has properties that fit with more than one state of matter at the same time? We're not talking about cooling a liquid until it freezes and becomes solid, or boiling it until turns to gas- all substances act this way depending on their temperature. Oobleck can act like a liquid- it takes the shape of its container, it's easy to stir and you can pour it- but if you push it too hard or too quickly it instantly acts like a solid- it stops flowing, keeps its shape and can even break. So is Oobleck a solid or a liquid? The answer is both- and neither. Confusing, right? It turns out that not everything easily fits into just one state of matter category. Oobleck is a fluid because it can flow, but it's certainly not like water or other normal fluids, so we call this a Non-Newtonian fluid (named after the famous scientist Sir Isaac Newton). Scientists aren't exactly sure why Oobleck acts this way, but the full explanation below will help you understand what might be happening. There are many other common substances (like ketchup or toothpaste) that also have some strange properties if you study them closely. You can find out more about these in the related activities and links below. More detailed explanation
When a large amount of cornstarch is added to a smaller amount of water it does not dissolve to make a liquid solution. It can flow, however, so we refer to this mixture as a fluid. A fluid is not a phase or state of matter (such as a solid, liquid or gas) but rather any substance that will deform or flow when a shear stress (i.e. a sideways push or force) is applied. If you pour some water onto a plate to form a small puddle then lift one edge slightly, the water will slide or flow along the surface of plate due to gravity. The term fluid and liquid are often used as synonyms, but a fluid can actually be liquid, gas, mixtures of either or both, mixtures of solids in a liquid (called suspensions or colloids), or even mixtures of two or more solids. Examples include sand, rocks, honey and maple syrup. If it flows as a shear stress is applied- even very slowly- it can be considered to be a fluid. Many materials thought to be solid- such as glass or tar pitch- will actually flow very slowly, often taking years for any noticeable change to occur. Most common fluids flow faster as higher shear stress is applied- i.e. the harder you push (or the more you tip a cup) the faster it flows. Resistance to movement or flow is called viscosity- a fluid with low viscosity will flow easily even under very low shear stress (an example is water), while a highly viscous fluid flows very slowly (such as honey). If the flow rate or viscosity is simply proportional to the stress and stops flowing when there is no stress, the substance is called a Newtonian fluid (after Isaac Newton, who first studied them). Oobleck, however, is an example of a (very) non-Newtonian fluid. Its flow rate decreases dramatically (i.e. the viscosity increases dramatically) as the sheer stress increases. The harder and faster you push or pull on the Oobleck, the more viscous or solid-like it becomes. You can even run across the surface of a large pool filled with oobleck (check out the video links below), but if you stop you will sink. This specific type of non-Newtonian fluid is called a stress thickening or dilatant fluid. More advanced dilatant materials are now being used in protective gear for football and other impact sports. The pads flex and move easily under normal motion, but when subjected to a sudden blow (a hit or tackle) they instantly stiffen and absorb the force of the impact, protecting the athlete. Bullet-proof vests are also being developed with these materials. To understand what is happening inside the Oobleck, picture the cornstarch molecules suspended or floating around in the water. As long as little or no stress is applied, the cornstarch molecules are free to move easily (sort of lubricated by the water), sliding over and around each other. However, when a large or sudden force is applied the solid cornstarch molecules instantly clump or stick together (flocculate) and the entire matrix acts like a solid. There is so much more cornstarch than water in the mixture that there is just no room for the cornstarch molecules to quickly move. If the mixture is made with a much higher proportion of water, however, this behavior is not observed, even though the fluid may still be thicker and more viscous. This is what happens when you use cornstarch to make gravy or pudding. Oobleck Cornstarch Monsters (find out what's happening in the Related Activities below) variations and related activities
One of the most interesting things you can do with Oobleck is to vibrate it (move back and forth) very quickly, typically by placing it in a speaker cone and driving it with a frequency generator and amplifier (as shown in the video above). Once we start the amplifier the cone vibrates back and forth between 50-90 Hz (cycles per second) which pushes on the Oobleck as the speaker moves forward, causing it to solidify instantly. As the cone stops and then reverses direction the Oobleck can relax and become liquid-like once again. These solid to liquid transitions occur so quickly that the Oobleck is able to build solid-like fingers and other shapes that seem to grow and come to life right out of the liquid. Some even break free to hop and dance around. Watch the behavior change in the video as we vary the frequency and amplitude of the vibration. There are many other interesting and very different types of non-Newtonian fluids. If the viscosity decreases as the stress increases, the fluid is called a psuedo-plastic or stress thinning fluid. Latex paints (as well as some nail polish and cosmetics) are designed to have psuedo-plastic behavior- you want the paint to flow easily off the brush when it's moving, but stop flowing once it's on your wall. Another example is toothpaste. Put a small amount of toothpaste on an electric toothbrush. As long as it is turned off, the toothpaste behaves like a solid and does not flow, but as soon as you vibrate the toothbrush the toothpaste will begin to flow and even dance around. Another example of a non-Newtonian fluid is the slime or silly putty, like we make with Elmer's Glue and borax in one of our other popular activities. While the actual behavior is rather complex and difficult to categorize, it will flow fairly easily (i.e. act like a liquid) under low stress, but break (like a solid) if too much stress is applied. Jell-O has somewhat similar behavior. It is also possible for the viscosity to change with the duration (rather than the magnitude) of the applied stress. If the viscosity continues to decrease over time as a constant stress is applied (even if it's relatively small in magnitude), the fluid is said to be thixotropic. Ketchup or tomato paste is an example of a thixotropic fluid [actually ketchup is even more complicated, exhibiting both thixotropic and pseudo-plastic properties]. Remember the Hines Ketchup commercial (to the soundtrack of Carly Simon's "Anticipation")? To help pour ketchup out of the bottle you should shake or vibrate it quickly, the viscosity soon drops and the ketchup flows more easily (see the video link below). Another odd but difficult to categorize non-Newtonian fluid is quicksand (fine sand particles suspended in water). When quicksand vibrates (such as during an earthquake or someone stuck squirms around) water flows around the sand particles and makes them more buoyant (a process called liquefaction), decreasing the apparent viscosity. When this happens a building or person on the surface can easily sink into the quicksand. When a person is trapped in quicksand, however, his movements can create local regions where the water flows away and the sand particles are compacted into a solid. Thus the quicksand actually has a very complicated behavior. To escape from quicksand, simply relax and move very gently and slowly. Because the sand/water fluid is much more dense than your body, you will literally float to the surface over time, rather than sinking to your death as is always portrayed in the movies. Yet another strange fluid is a mixture of cornstarch in vegetable oil (such as corn or canola oil). Prepare a much less viscous mixture (about 2 parts cornstarch to 1 part oil). It should pour easily from one cup to another with the consistency of pancake batter under normal conditions, but if a strong electric field is present the viscosity will increase dramatically. This is an example of an electro-rheological fluid. To demonstrate this, rub a balloon on your shirt or in your hair to create trapped electric charges on its surface (sometimes called static electricity). As a partner slowly pours the cornstarch-oil fluid from one cup into another, carefully bring your charged balloon near the pouring stream and the fluid will not only bend towards the balloon, but will actually stop flowing and "freeze" in mid-air. Remove the balloon and the stream will begin to flow again immediately. Materials with this property are being used in some automotive transmissions, clutches, brakes and shock absorbers. references and links to more information
More on Non-Newtonian fluids [Note that as you read more about Non-Newtonian fluids you may find a lot of contradictory information. This is because the behavior of these materials are often very complex and difficult to categorize]:
More activities and videos with Oobleck:
Running across a pool filled with Oobleck: More Oobleck Cornstarch Monsters:
Why is Ketchup so hard to get out of its bottle?:
Electro-rheological and magneto-rheological fluids:
|