An electrically charged challenge from Science Buddies
Key concepts Electricity Electronics Insulators Conductors
Introduction
Have you ever wondered why rubbing a balloon or a blanket—or even a winter hat—on your head makes your hair stand up? The effect is due to static electricity, but how is the static electricity made, and why does it make your hair stand on end?
Static electricity is the buildup of electrical charge in an object. Sometimes static electricity can suddenly discharge, such as when a bolt of lightning flashes through the sky. Other times, static electricity can cause objects to cling to one another. Think of how socks fresh out of the dryer stick together. This happens when objects have opposite charges, positive and negative, which attract. (Objects with the same charges repel one another.) Could enough static electricity make a balloon stick to a wall? How much do you think you would have to rub it?
Background
When one object is rubbed against another, static electricity can be created. This is because the rubbing creates a negative charge that is carried by electrons. The electrons can build up to produce static electricity. For example, when you shuffle your feet across a carpet, you are creating many surface contacts between your feet and the carpet, allowing electrons to transfer to you, thereby building up a static charge on your skin. When you touch another person or an object, you can suddenly discharge the static as an electrical shock.
Similarly, when you rub a balloon on your head it causes opposite static charges to build up both on your hair and the balloon. Consequently, when you pull the balloon slowly away from your head, you can see these two opposite static charges attracting one another and making your hair stand up.
Materials • Balloon • An object made out of wool (such as a sweater, scarf, blanket or ball of yarn) • Stopwatch • A wall • A partner (optional)
Preparation
• Blow up the balloon and tie off the end. • Have your partner prepare to use the stopwatch.Procedure
• Hold the balloon in a way that your hand covers as little of its surface area as possible, such as by using only your thumb and pointer finger or by gripping the balloon by its neck where it is tied off. • Rub the balloon on the woolly object once, in one direction.• Hold the balloon up on the wall with the side that was rubbed against the wool facing the wall, then release it. Does the balloon stay stuck on the wall? If the balloon stays stuck, have your partner immediately start the stopwatch to time how long the balloon remains bound to the wall. If the balloon does not stick, move to the next step.
• Touch the balloon to a metal object. Why do you think this is important to do?
• Repeat the above process but each time increase the number of times you rub the balloon on the woolly object. Rub the balloon in the same direction each time. (Do not rub the balloon back and forth.) How many rubs does it take to make the balloon stick to the wall for a few seconds? What about multiple minutes?
• You can repeat this whole process two more times. Do your observations for each trial match with the previous trials?
• Extra: Does rubbing in one direction give a different result than rubbing back and forth? Try comparing the same number of rubs in one direction with those done back and forth. Does one stay on the wall longer than the other?
• Extra: Try comparing the effectiveness of different materials for producing a static charge. Does rubbing wool work better than rubbing silk? Design an experiment to test several different materials: silk, wool, nylon, polyester, plastic, metal, etcetera.
Observations and results
In general, did the balloon stick to the wall for a longer amount of time as you increased the number of times you rubbed the balloon on the woolly object?
Wool is a conductive material, which means it readily gives away its electrons. Consequently, when you rub a balloon on wool, this causes the electrons to move from the wool to the balloon's surface. The rubbed part of the balloon now has a negative charge. Objects made of rubber, such as the balloon, are electrical insulators, meaning that they resist electric charges flowing through them. This is why only part of the balloon may have a negative charge (where the wool rubbed it) and the rest may remain neutral.
When the balloon has been rubbed enough times to gain a sufficient negative charge, it will be attracted to the wall. Although the wall should normally have a neutral charge, the charges within it can rearrange so that a positively charged area attracts the negatively charged balloon. Because the wall is also an electrical insulator, the charge is not immediately discharged. However, because metal is an electrical conductor, when you rub the balloon against metal the extra electrons in the balloon quickly leave the balloon and move into the metal so the balloon is no longer attracted and does not adhere.
More to explore
"Static Electricity: Learn about Static Charge & Static Shock" from Science Made Simple
"The Shocking Truth Behind Static Electricity" from Live Science
"Static Electricity: Background Information for the Teacher" from The Museum of Science, Boston
"Rubbing Up against Static Electricity" from Science Buddies
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Describe the movement of electrons from one material to another.
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Determine the resulting charge of two materials rubbing together.
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Explain how static charge causes materials to attract or repel each other.
Materials
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see individual activities for materials.
Background
Everything we see is made up of tiny particles of matter called atoms. The atoms are made up of even smaller parts called protons, electrons and neutrons. An atom usually has the same number of protons and electrons, but sometimes electrons can be moved away from their atoms.
If you comb your hair, for example, electrons leave the atoms and molecules in your hair and travel to the plastic comb. The comb, covered in negatively charged electrons, becomes negatively charged as well, and your hair is left with a positive charge. This “separation of charge” is the reason for the collection of effects we call static electricity.
If two objects have different charges, they attract (or pull towards) each other. If two objects have the same charge, they repel (or push away) from each other. After you’ve combed your hair, every single hair has the same positive charge. Since things with the same charge repel each other, the hairs try to move away from each other by standing up and away from all the other hairs, resulting in you having a very funny-looking hairdo!
Another example: if you walk across a carpet, electrons move from the rug to you. Now you have extra electrons. If you have extra electrons piled on you, they will spill off when you touch an object like a doorknob, and give you a shock. Shocks come from gaining or losing electric charge in a hurry.
When a charged object is brought close to a neutral material, the electrons on the neutral material will either move toward the charged object (if it has a positive charge) or away from the charged object (if it has a negative charge). In other words, the neutral material “picks up” charge on its near and far side, relevant to the charged object. This phenomenon is called an induced charge. The result is that a normally neutral material will have a slight charge when near the charged object, and it is enough for the two to attract.
Electrostatic charges are not caused by friction, although many assume this to be the case.
Rubbing a balloon on your head or dragging your feet on the carpet will build up a charge, but so will ordinary walking or repeatedly touching your head with a balloon! It’s the mere contact between two different materials that causes charge to move from one object to another. Rubbing materials together can help move charge more quickly because more surface area is being contacted. Friction has nothing to do with the charge.
An important thing to consider when doing any of these activities is the weather: humidity in the air can make it difficult to build up charges, causing experiments to behave in unexpected ways!
The best “static” weather is clear, sunny, and cool.
Vocabulary
atom – the particle of matter made of protons, electron and neutrons
electron – A subatomic particle that has a negative electrical charge.
electroscope – A device that detects electrical charge.
induced charge – Separation of charges within an otherwise neutral object caused by the proximity of a charged object.
proton – A subatomic particle that has a positive electrical charge.
static electricity – Electrical effects caused by the charge imbalance between a negatively charged object and a positively charged object.
Triboelectric series – A list that ranks various materials according to their tendency to gain or lose electrons.
Other Resources
BC Hydro | Power Smart for Schools
How Stuff Works | How do Van de Graaff Generators Work
To purchase a fly stick or Van de Graaff generator: Arbor Scientific