7.2: Static Electricity and Charge- Conservation of Charge
- Page ID
- 472592
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- Define electric charge, and describe how the two types of charge interact.
- Describe three common situations that generate static electricity.
- State the law of conservation of charge.
![photograph of a piece of amber.](https://chem.libretexts.org/@api/deki/files/470015/Figure_19_01_01a.jpg?revision=1&size=bestfit&width=300&height=225)
What makes plastic wrap cling? Static electricity. Not only are applications of static electricity common these days, its existence has been known since ancient times. The first record of its effects dates to ancient Greeks who noted more than 500 years B.C. that polishing amber temporarily enabled it to attract bits of straw (see Figure \(\PageIndex{1}\)). The very word electric derives from the Greek word for amber (electron).
Many of the characteristics of static electricity can be explored by rubbing things together. Rubbing creates the spark you get from walking across a wool carpet, for example. Static cling generated in a clothes dryer and the attraction of straw to recently polished amber also result from rubbing. Similarly, lightning results from air movements under certain weather conditions. You can also rub a balloon on your hair, and the static electricity created can then make the balloon cling to a wall. We also have to be cautious of static electricity, especially in dry climates. When we pump gasoline, we are warned to discharge ourselves (after sliding across the seat) on a metal surface before grabbing the gas nozzle. Attendants in hospital operating rooms must wear booties with aluminum foil on the bottoms to avoid creating sparks which may ignite the oxygen being used.
Some of the most basic characteristics of static electricity include:
- The effects of static electricity are explained by a physical quantity not previously introduced, called electric charge.
- There are only two types of charge, one called positive and the other called negative.
- Like charges repel, whereas unlike charges attract.
- The force between charges decreases with distance.
How do we know there are two types of electric charge? When various materials are rubbed together in controlled ways, certain combinations of materials always produce one type of charge on one material and the opposite type on the other. By convention, we call one type of charge “positive”, and the other type “negative.” For example, when glass is rubbed with silk, the glass becomes positively charged and the silk negatively charged. Since the glass and silk have opposite charges, they attract one another like clothes that have rubbed together in a dryer. Two glass rods rubbed with silk in this manner will repel one another, since each rod has positive charge on it. Similarly, two silk cloths so rubbed will repel, since both cloths have negative charge. Figure \(\PageIndex{2}\) shows how these simple materials can be used to explore the nature of the force between charges.
![three drawings illustrating the movement of charge between objects, as indicated in the caption.](https://chem.libretexts.org/@api/deki/files/470010/Figure_19_01_02a.jpg?revision=1&size=bestfit&width=520&height=146)
More sophisticated questions arise. Where do these charges come from? Can you create or destroy charge? Is there a smallest unit of charge? Exactly how does the force depend on the amount of charge and the distance between charges? Such questions obviously occurred to Benjamin Franklin and other early researchers, and they interest us even today.
Franklin wrote in his letters and books that he could see the effects of electric charge but did not understand what caused the phenomenon. Today we have the advantage of knowing that normal matter is made of atoms, and that atoms contain positive and negative charges, usually in equal amounts. We will learn more about atoms later in this text, but for now we will state that this negative charges come from something we call electrons. It's possible to have particles which carry either positive or negative charges, but for most of our observations of charge in the regular world, the charges we observe are due to variable amounts of electrons. A negatively charged object occurs because it has extra electrons. A positively charged object occurs because it does not have enough electrons. And an uncharged object (which is most objects) has the correct number of electrons. In each of the examples we looked at, the charge on an object occurred because electrons were transferred from one object to another, as we shall see next.
Separation of Charge in Atoms
Charges in matter can be separated—for example, by rubbing materials together. Some forms of matter have a greater affinity for electrons than others and will become negatively charged by close contact in rubbing, leaving the other material positively charged. (See Figure \(\PageIndex{6}\).) Positive charge can similarly be induced by rubbing. Methods other than rubbing can also separate charges. Batteries, for example, use combinations of substances that interact in such a way as to separate charges, as we will see later in this chapter.
![3 drawings of amber and cloth, before, during and after rubbing together, showing the charge transfer.](https://chem.libretexts.org/@api/deki/files/470016/Figure_19_01_06a.jpg?revision=1&size=bestfit&width=350&height=138)
No charge is actually created or destroyed when charges are separated as we have been discussing. Rather, existing charges are moved about. In fact, in all situations the total amount of charge is always constant. This universally obeyed law of nature is called the law of conservation of charge.
LAW OF CONSERVATION OF CHARGE
Total charge is constant in any process.
The law of conservation of charge is absolute—it has never been observed to be violated. Charge, then, is a special physical quantity, joining a very short list of other quantities in nature that are always conserved. Other conserved quantities include energy, momentum, and angular momentum.
MAKING CONNECTIONS: CONSERVATION LAWS
Only a limited number of physical quantities are universally conserved. Charge is one—energy, momentum, and angular momentum are others. Because they are conserved, these physical quantities are used to explain more phenomena and form more connections than other, less basic quantities. We find that conserved quantities give us great insight into the rules followed by nature and hints to the organization of nature. Discoveries of conservation laws have led to further discoveries, such as the weak nuclear force and the quark substructure of protons and other particles.
Section Summary
- There are only two types of charge, which we call positive and negative.
- Like charges repel, unlike charges attract, and the force between charges decreases with the square of the distance.
- Most of our observations of charge involve electrons, which carry a negative charge.
- The SI unit for charge is the coulomb (C), with protons and electrons having charges of opposite sign but equal magnitude; the magnitude of this basic charge \(\left|q_{e}\right|\) is
\[\left|q_{e}\right|=1.60 \times 10^{-19} \mathrm{C}. \nonumber\]
- Whenever charge is created or destroyed, equal amounts of positive and negative are involved.
- Most often, existing charges are separated from neutral objects to obtain some net charge.
- Both positive and negative charges exist in neutral objects and can be separated by rubbing one object with another. For macroscopic objects, negatively charged means an excess of electrons and positively charged means a depletion of electrons.
- The law of conservation of charge ensures that whenever a charge is created, an equal charge of the opposite sign is created at the same time.
Glossary
- electric charge
- a physical property of an object that causes it to be attracted toward or repelled from another charged object; each charged object generates and is influenced by a force called an electromagnetic force
- law of conservation of charge
- states that whenever a charge is created, an equal amount of charge with the opposite sign is created simultaneously
- electron
- a particle inside of an atom that carries the smallest unit of negative charge