Static Electricity: Stationary Charges
Electricity is the name given to any effect resulting from the existence of stationary or moving electric charges. 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 (phenomena associated with a magnet). Rubbing two materials together, such as your feet against a carpet, causes two kinds of electric charges in the atoms that make up the materials to separate. A buildup of stationary charges is called a static charge. The effect of static charges is called static electricity. 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 a carpet, is an example of static discharge as a result of the loss of static charges.
In this project, you will discover how to polarize a material by electrostatic induction (separation of charges due to the presence of a charged object). You will learn how to charge a material by friction and conduction. You will also determine how the distance between charged materials affects the electric force between them.
Purpose To polarize a material by electrostatic induction.
- 2 Cheerios
- two 12-inch (30-cm) pieces of string
- transparent tape
- metric ruler
- 9-inch (22.5-cm) round balloon
- wool scarf
- yardstick (meterstick)
- Prepare two pendulums by tying Cheerios to the ends of the strings.
- Using tape, secure the free ends of the strings to the edge of a table. The strings should be far enough apart, about 1 cm, so that the Cheerios bobs hang freely and are near but not touching. Note the separation between the strings and bobs.
- Inflate the balloon to the size of a large grapefruit, and tie a knot in its neck to close it.
- Charge the balloon by rubbing it with the wool scarf five or more times.
- Hold the charged balloon under the hanging bobs so that the balloon is near but not touching them.
- Slowly move the charged balloon to the left, then move it to the right. Observe the motion of the bobs.
- Remove the balloon and place it at least 1 yard (1 m) from the bobs. Observe the position of the bobs in relation to each other for two or more minutes.
At first the strings with bobs attached hang parallel to each other. As the balloon moves beneath the bobs, the bobs both move in the direction of the balloon's motion. Immediately after the balloon is removed, the strings hang at an angle to each other, with the bobs more separated than before. After a time, the strings return to the parallel position.
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 the particles to attract (to pull together) or to repel (to push apart) one another or particles in other materials. The force between two objects due to their changes is called an electric force. The property of space around a charged object that causes forces on other charged objects is called an electric field. The source of positive and negative charges is atoms (the building blocks of matter), which contain a nucleus (the center part), with protons (positively charged particles inside the nucleus) and electrons (negatively charged particles outside the nucleus).
Physical contact between uncharged materials, such as rubbing objects together, is one method, called the friction method, of electrically charging them. Before the balloon and the wool were rubbed together, the balloon and the wool, like all materials, are neutral (having an equal number of positive and negative charges, thus having no electric charge). This is because they each had an equal number of protons (positive charges) and electrons (negative charges). When you rubbed the balloon with the wool, electrons were transferred from the wool to the balloon due to differences in attraction of the materials for electrons. An object with more of one kind of charge than another is said to be charged. The addition of electrons to the balloon gives the balloon a negative charge, and the loss of electrons by the scarf gives it a positive charge. The buildup of stationary electric charges is called static charges. The effect of static charges is called static electricity.
Two charged objects that have different kinds of charges attract each other, while two charged objects that have the same kinds of charge repel each other. So when the negatively charged balloon is brought near the neutral hanging Cheerios bobs, the positive and the negative charges in some of the molecules in the cereal separate slightly, because the negative charges are repelled by the negatively charged balloon. There is no increase in the net charge; there is only a redistribution of the charges. This redistribution gives the surface of the Cheerios a slight positive charge. This process of polarizing (separating positive and negative charges) a neutral material due to the proximity (nearness) of a charged object is called electrostatic induction (see Figure 15.2).
The positively charged side of the polarized bobs are attracted to the negatively charged balloon and move in the direction the balloon moves. When the charged balloon is removed, however, the now positively charged surfaces of the bobs repel each other, and they separate. But the induced positive charge is temporary, so when the charged balloon is no longer nearby, in a short time the separated charges return to their previous positions and the surfaces of the bobs become neutral and no longer repel one another. Thus the bobs hang straight down.