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Chapter15electrical Charges and Forces Chapter 15 Electrical Charges and Forces Benjamin Franklin's famous kite experiment has been referred to many times, even though it is not known when, or if he even actually did the experiment at all. The major question of the day was "Is lightning an electrical phenomenon?" Back in the mid 1700's, the longest spark that could be generated was about 1 inch long, so it was not obvious that powerful lightning bolts could be similar to the small sparks seen in laboratory experiments. Popular legend explains that Franklin attached a metal key to the end of a kite string and flew the kite in a raging lightning storm. Because he knew the experiment would be dangerous, he most likely attached a second silk string to the key that insulated him from the electrical charges, and held onto that string. He did not Key Questions do his experiments during the peak of the storm, but 3 What is the actual source of chose to fly the kite as the storm was beginning to form. current in a household electrical He noticed, according to an account written several system? years later, that he did receive a shock when he touched 3 After you rub a balloon on your his knuckle to the metal key. He determined that hair, why can you then stick the lightning exhibited the same properties as small static balloon to a wall but not to a electricity sparks, and he was correct! metal doorknob? In this chapter, you will study how atoms are ultimately 3 How does a defibrillator work, responsible for electrical charges. You learned in an and what does it have to do earlier chapter that all atoms contain positive and with electricity? negative charges, and now you will see that these charges are the very same ones that allow us to benefit from generating, conducting, and using electricity. 339 15.1 Electric Charge and Current Vocabulary In chapters 13 and 14 we looked at how electricity is used and measured. Amps, volts, and ohms coulomb, electrically neutral, describe most of what you need to know to use electricity safely. However, we did really discuss charged, static electricity, what electricity is on the atomic level. What is current? What sort of thing can flow through solid Coulomb’s law, electroscope, metal? Why can a little electric motor do the work of two horses? charging by contact, charging by friction, polarized, charging by Positive and negative charge induction The true cause Virtually all the matter around you has electric charge because atoms are Objectives of electric made of electrons and protons (and neutrons). Electrons have negative charge 3 Distinguish between a positive current and protons have positive charge. The electrons and protons are usually stuck and negative net charge. together in atoms and are unable to separate from each other. However, in 3 Explain the meaning of Coulomb’s electrical conductors (like copper) a few electrons are not stuck to their law. atoms, but are free to move around. These mobile electrons are the real cause 3 Describe different ways of of electric current! charging an electroscope. Like charges Whether two charges attract or repel depends on whether they have the same repel and unlike or opposite sign. A positive and a negative charge will attract each other. Two charges attract positive charges will repel each other. Two negative charges will also repel each other. The force between charges is shown in Figure 15.1. Charge is The unit of charge is the coulomb (C). The name was chosen in honor of measured in Charles Augustin de Coulomb (1736-1806), the French physicist who coulombs performed the first accurate measurements of the force between charges. One coulomb is a huge amount of charge. A single proton has a charge of 1.602 × 10-19 coulomb (Figure 15.1). An electron has the exact same charge, only negative. The charge of an electron -1.602 × 10-19 coulomb. Two types of Electric charge, like mass, is a fundamental property of matter. An important charge difference between mass and charge is that there are two types of charge, which we call positive and negative. We know there are two kinds because electric charges can attract or repel each other. As far as we know, there is Figure 15.1: The direction of the forces on only one type of mass. All masses attract each other through gravity. We have charges depends on whether they have the never found masses that repel each other. same or opposite charges. 340 15.1 ELECTRIC CHARGE AND CURRENT CHAPTER 15: ELECTRICAL CHARGES AND FORCES Static electricity Neutral objects Matter contains trillions and trillions of charged electrons and protons because matter is made of atoms. Neutral atoms have the same number of electrons and protons. Therefore, the charge of an atom is exactly zero. Similarly, there is perfect cancellation between positive and negative in matter leaving a net charge of precisely zero. An object with a net charge of zero is described as being electrically neutral. Your pencil, your textbook, even your body are electrically neutral, at least most of the time. Charged objects An object is charged when its net charge is not zero. If you have ever felt a shock when you have touched a doorknob or removed clothes from a dryer, you have experienced a charged object. An object with more negative than positive charge has a net negative charge overall (Figure 15.2). If it has more positive than negative charge, the object has a positive net charge. The net charge is also sometimes called excess charge because a charged object has an Figure 15.2: An object is neutral if it has excess of either positive or negative charges. an equal number of positive and negative charges. Static electricity A tiny imbalance in either positive or negative charge on an object is the cause and charge of static electricity. If two neutral objects are rubbed together, the friction often pulls some electrons off one object and puts them temporarily on the other. This is what happens to clothes in the dryer and to your socks when you walk on a carpet. The static electricity you feel when taking clothes from a dryer or scuffing your socks on a carpet typically results from an excess charge of less than one-millionth of a coulomb, the unit of charge. What causes You get a shock because excess of charge of one sign strongly attracts charge shocks of the other sign and repels charge of the same sign. When you walk across a carpet on a dry day, your body picks up excess negative charge. If you touch a neutral door knob some of your excess negative charge moves to the door knob. Because the door knob is a conductor, the charge flows quickly. The moving charge makes a brief, intense electric current between you and the door knob. The shock you feel is the electric current as some of your excess Figure 15.3: The shock you get from negative charge transfers to the door knob (Figure 15.3). touching a door knob on a dry day comes from moving charge. UNIT 5 ELECTRICITY 341 Coulomb’s law The force Electric forces are so strong it is hard to imagine. A millimeter cube of carbon between charges the size of a pencil point contains about 77 coulombs of positive charge and is very strong the same amount of negative charge. If you could separate all the positive and negative charges by a distance of one meter, the attractive force between them would be 50 thousand billion newtons. This is the weight of three thousand, million cars. From the charge in a pencil point (Figure 15.4)! The huge forces between charges are the reason objects are electrically neutral. More charge The force between two charges depends on the charge and the distance. The means more force is directly proportional to the charge of each object. The greater the force charge, the stronger the force. Doubling the charge of one object doubles the force. Doubling the charge of both objects quadruples the force. Less distance The force is inversely proportional to the square of the distance between the Figure 15.4: If you could separate the means more charges (Figure 15.5). Electric forces get stronger as charges move closer and positive and negative charge in a pencil point force weaker as they move farther apart. Doubling the distance makes the force 1/4 by one meter, the force between the charges 2 1 2 as strong (1 ÷ 2 ). The force is /9 as strong (1 ÷ 3 ) at triple the distance. would be 50 thousand, billion newtons! Coulomb’s law Coulomb’s law explains the relationship between the amount of each charge (q1 and q2), the distance between their centers (r), and the electrical force (FE). The constant k relates the distance and charges to the force. Coulomb’s law is very similar to Newton’s law of gravitation (chapter 6). Action-reaction The force between two charges acts along a line joining their centers. As Figure 15.5: How the electric force pairs required by Newton’s third law of motion, the forces on each charge make an changes with charge and distance in action-reaction pair. They are equal in strength and opposite in direction. Coulomb’s law. 342 15.1 ELECTRIC CHARGE AND CURRENT CHAPTER 15: ELECTRICAL CHARGES AND FORCES Electrostatics What is Electrostatics is the part of physics that deals with the forces created by electrostatics? unmoving charges.
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