Describe two examples of electric forces in everyday life.
Static Electricity and Coulomb’s Law Objectives • Describe the historical development of the concepts of electrostatics. • Identify examples of electric forces in everyday life. • Identify the two types of electric charge and how they interact. • Describe the historical development of the concepts of electromagnetic force. • Describe and calculate how the magnitude of the electric force between two objects depends on their charges and the distance between them. Physics terms • Coulomb’s law • coulomb Equations The force between point charges is proportional to the product of the charges, divided by the square of the distance between them. Static electricity Benjamin Franklin was America’s first famous scientist. He and many others were studying the properties of static electricity even in the 1700’s. •The study of electric charges at rest is called electrostatics. •Static electricity results from a tiny excess of positive or negative electric charge on an object. An example Rub a balloon against a wool sweater. This transfers some negative charges from the sweater to the balloon. The sweater now has a net positive charge and the balloon has a net negative charge. They will attract and stick together. What makes the zap? Shuffling your feet on the carpet can also transfer charges. Touching a metal conductor, like a doorknob, causes the excess charge you’ve collected to be attracted to its opposite in the metal. A tiny electric current flows … and that’s what makes the zap! Electric charge There are two kinds of electric charge: positive (+) and negative (–). In electrically neutral objects, positive (+) charges are balanced by an equal number of negative (–) charges. Most objects are electrically neutral … otherwise there would be lightning everywhere all of the time! Opposites attract As with magnetic poles, unlike charges attract: Like electric charges repel: Neutral particles feel no electric force at all from positive or negative charges. Electric force Electric forces of attraction and repulsion can be enormously strong. These are the forces hold atoms and molecules together. Electric forces are so strong that, outside of atoms, positive and negative charges are rarely separated for long! The Van de Graaff generator A Van de Graaff generator is a device that can separate electric charge. Its sphere can reach thousands or even millions of volts, but it can only create a big spark – not a large electric current – if you touch it. Charge distribution Like charges repel, so charges will push each other apart as far as possible. If the charges are on a Van de Graaff generator, repulsive forces will push them to the outside edge of the sphere. Electrostatic induction It is also possible to charge an object without touching it. Here, a negatively-charged rod repels negative charges and attracts positive charges. The sphere is now “charged”. This process of separating charge is called electrostatic induction. Demo: the electroscope An electroscope uses induction to detect electric charge. Summary: charging an object There are three ways that objects typically become charged. • conduction: A charged object contacts an uncharged object and transfers charges to it. • friction: Different materials have different affinities for charge. Rubbing these materials together causes charges to transfer between the materials. • electrostatic induction: A charged object is brought close to a neutral object, resulting in charge separation in the neutral object. Assessment 1. Name one of the founding fathers of the United States who also contributed to our understanding of static electricity. Assessment 2. Describe two examples of electric forces in everyday life. Assessment 3. Which statement about electric forces is true? A. A negative charge attracts a negative charge. B. A positive charge repels a negative charge. C. A negative charge repels a negative charge. D. A positive charge attracts a positive charge. Where does charge come from? All ordinary matter is made of atoms, and all atoms contain charged particles. • The atomic nucleus contains positively-charged protons. • Outside of the nucleus are tiny fast-moving, negatively charged particles called electrons. Where does charge come from? The charge of an electron and proton are exactly equal and opposite. A complete atom has equal numbers of protons and electrons. Therefore, a complete atom has no net electric charge. The unit of electric charge The unit of electric charge is the coulomb (C) in honor of CharlesAugustin de Coulomb (1736-1806). Coulomb was a French physicist who made the first accurate measurements of electric force between charges. How big is a Coulomb? A coulomb is a very large amount of charge. One coulomb equals the charge of 6 × 1018 electrons or protons. Ordinary static electricity results from net charges of less than a millionth of a coulomb. Think: How can you increase the repulsive force between two positive charges? What does this force depend on? q1 Fe q2 r Fe It depends on the magnitude of each charge, and the distance between them. Can you guess the formula for Fe? Coulomb’s law Coulomb’s law quantifies the electric force: where: Fe = electrostatic force (N) ke = Coulomb constant = 9.0×109 Nm2/C2 q1 = electric charge of object 1 (C) q2 = electric charge of object 2 (C) r = distance between the two objects (m) Coulomb’s law Stating Coulomb’s law in words: The force between two charges equals the product of the charges divided by the square of the distance between them, all multiplied by a constant, ke. Engaging with the concepts Two silver balls with charges of +7.0 μC and +1.2 mC, are located 80 cm apart. Electric force What is the force between them? 7 e-6 7.0e-6 1.2 e-3 1.2e-3 0.80 8.99e+9 0.80 In what direction does it act? Hint: make sure you use the right units! Engaging with the concepts Two small metal spheres are separated by 25 mm. One has a charge of +2.0 μC, the other −2.0 μC. What is the force between them? Electric force 7 e-6 2.0e-6 1.2 e-3 -2.0e-6 0.80 8.99e+9 0.025 Engaging with the concepts Two small metal spheres are separated by 25 mm. One has a charge of +2.0 μC, the other −2.0 μC. What is the force between them if the distance doubles? What is the force between them if the charges ALSO double? Electric force 7 e-6 2.0e-6 1.2 e-3 2.0e-6 0.80 8.99e+9 50e-3 Fe increases with charge The electric force increases with increasing charge: Doubling one charge doubles the force: Doubling both charges quadruples the force: Fe obeys an inverse square law The electric force varies inversely with the square of the distance between charges. DOUBLING the distance reduces the force by a factor of ¼. HALVING the distance increases the force by a factor of 4. Test your knowledge Two +1.0 μC charges are separated by a distance of 1.0 cm. What is the magnitude and direction of their mutual electric force? Asked: Given: r Relationships: Solution: The electric force, Fe The electric force is enormously strong: One drop of water contains about 2,000 C of positive and negative charges. If you could separate these charges by one meter . . . The electric force, Fe The electric force is enormously strong: One drop of water contains about 2,000 C of positive and negative charges. If you could separate these charges by one meter, they would attract each other with a force of 36,000,000,000,000,000 N. Most applications in electrostatics involve small amounts of charge, often expressed as micro-coulombs (μC = 10-6 Coulombs). Comparing forces: Fe and Fg The electric forces between elementary particles are much much greater than their gravitational attraction. The force of gravity is so much smaller in these situations that we say it is negligible—meaning you can ignore it. Assessment 1. For whom was the unit of charge, the coulomb, named and in what way(s) did this person contribute to our understanding of the electric force? Assessment 2. Two charged particles are located 1.0 m apart. Describe the direction for the electric force between them if the two charges are: a. +1 μC and +1 μC b. +1 μC and -1 μC c. -1 μC and -1 μC Assessment 3. Two charged particles are located 1.0 m apart. a) Calculate the magnitude of the electric force between them if the two charges are +1.0 μC and +1.0 μC. a) What would be the magnitude of the force if the charges were 50 centimeters apart?