Electricity is the name given to an effect resulting from the presence of stationary or moving electric charges and the effects they cause. The word electricity was coined by William Gilbert (1544–1603), an English physicist and physician known primarily for his original experiments on the nature of electricity and magnetism (the force around a magnet). Rubbing two materials together, such as your feet against a carpet, causes the separation of two kinds of electric charges in the atoms that make up the materials. A buildup of stationary charges are called static charges. If enough charges separate, a spark, called static discharge, is produced when the charges recombine. American scientist and statesman Benjamin Franklin (1706–1790) named the two kinds of charges positive and negative. He also experimentally demonstrated that lightning, like the small spark created when you touch a metal doorknob after rubbing your feet on the carpet, is an example of static discharge, which is a loss of static charges. Static electricity is specifically the effect produced by static charges.
In this project, you will learn about two methods by which materials are charged: friction and conduction. You will discover the effects of electrostatic induction. You will also determine how materials and distance between charged materials affect the electric force.
Purpose: To charge materials electrically by friction and induction.
- 9-inch (22.5-cm) round balloon
- 9-ounce (270-ml) paper cup
- tap water
- wool cloth (scarf or sweater)
- Inflate the balloon to a size that is easily held in one hand. Tie a knot in the neck of the balloon.
- Use the pencil point to punch a small hole in the side of the cup near its bottom.
- Ask a helper to hold his or her finger over the hole in the cup, fill the cup with water, then set the cup on the edge of a sink with the hole pointing toward the sink.
- Rub the balloon on the cloth five or more times.
- Ask your helper to remove his or her finger from over the hole and observe the direction of the water coming from the hole in the cup.
- Hold the balloon near but not touching the stream and observe any change in the direction of the stream of water.
The stream of water bends toward the balloon.
Electricity is any effect resulting from the presence of stationary or moving electric charges. A charge (electric charge) is the property of particles within atoms that causes a force (a push or pull on an object) between the particles. The force between particles due to their charges is called an electric force. The two forms of charges are called positive and negative. When two like charges (positive and positive or negative and negative) are near each other, they repel (push apart) each other. But when two unlike charges (positive and negative) are near each other, they attract (pull together) each other. The property of space around a charged object that exerts an electric force on other charged objects is called an electric field. The source of positive and negative charges are the protons and electrons in atoms, which contain a nucleus. Protons are positively charged particles inside the nucleus (central part), and electrons are negatively charged particles outside the nucleus.
Physical contact, such as rubbing, between uncharged material is one method, called the friction method, of electrically charging them. Friction is the name of forces that oppose the motion of two surfaces in contact with each other, such as the rubbing of the balloon and cloth together. Before the balloon and the cloth are rubbed together, they are neutral (having an equal number of positive and negative charges, thus having no electric charge). This is because the atoms they are made of have an equal number of protons (positive charges) and electrons (negative charges). Atoms can become charged by either losing or gaining electrons. This happens because electrons, unlike protons, are free to move. If an atom loses an electron, the atom then has more positive charges (protons) than negative charges (electrons), and is therefore positively charged. If an atom gains an electron, it has more negative charges (electrons) than positive charges (protons), and is consequently negatively charged.
When two objects are rubbed together, one of them tends to lose electrons more than the other. The loss of electrons in one object results in its becoming positively charged, and the gain of electrons by the other object results in that object becoming negatively charged. In this experiment, when the rubbing stops and the balloon and cloth are separated, the electric charges stop moving. The balloon has a negative charge and the cloth a positive charge. An object with more of one kind of charge than another is said to be charged. These electric charges remain stationary, thus are called static charges (a buildup of stationary electric charges). Static electricity is the effect of static charges.
While water molecules are neutral, they are polarized, meaning their positive and negative charges are separated so that they have a positive and negative end. The presence of the negatively charged balloon causes the negative end of the water molecules to be repelled. Because water is a liquid, its molecules have more freedom of motion, so the water molecules rotate until the positive ends of the molecules are facing the balloon. Thus the side of the stream of water facing the balloon becomes more positively charged. Since unlike charges are attracted to one another, the positively charged water stream is attracted to the negatively charged balloon. The process of polarizing a neutral material by separating its positive and negative charges due to the proximity (nearness) of a charged object is called electrostatic induction. The rotation of the polarized water molecules by the charged balloon is an example of electrostatic induction.