The Language of Physics

Transcription

1 SECTION 1 Plan and Prepare Preview Vocabulary Latin Word Origins The word schematic comes from the Latin word schema, meaning figure. This word is used in technology and science for a diagram or blueprint, especially of an electric circuit. Teach TEACH FROM VISUALS FIGURE 1.1 Students should be encouraged to create alternative representations of the circuit shown in the photo. Students should discuss what their symbols stand for, how convenient their symbols are for others to use, and in what way each symbol reflects relevant information. Ask Identify information about the group of elements that is not relevant to its function and is unnecessary in a schematic. Answer: The colors and sizes of the items shown and whether the wires are coiled, bent, or straight are irrelevant to the function of the group of elements. TEACH FROM VISUALS FIGURE 1.1 Students should recognize that the straight-line symbols connecting the battery symbol with the bulb symbol in (b) represent not only the wire but also all parts of the conducting connection between the bulb and battery. Ask Identify the parts of the photo symbolized by the black straight lines in the diagrams. Answer: The black lines symbolize the conducting path provided by the wires, clips, and socket. FIGURE 1.1 SECTION 1 Objectives Interpret and construct circuit diagrams. Identify circuits as open or closed. Deduce the potential difference across the circuit load, given the potential difference across the battery s terminals. schematic diagram a representation of a circuit that uses lines to represent wires and different symbols to represent components A Battery and Light Bulb (a) When this battery is connected to a light bulb, the potential difference across the battery generates a current that illuminates the bulb. (b) The connections between the light bulb and battery can be represented in a schematic diagram. 628 Chapter 18 Differentiated Instruction Schematic Diagrams and Circuits Key Terms schematic diagram Students may confuse schematic diagrams with other diagrams, such as geometric or architectural diagrams. Point out that schematic diagrams are diagrams in which the elements and components of a system, such as an electric circuit or an electric motor, are illustrated by previously defined symbols and icons rather than by their real pictures. Tell students that in an electric schematic diagram, Schematic Diagrams electric circuit Take a few minutes to examine the battery and light bulb in Figure 1.1(a); then draw a diagram of each element in the photograph and its connection. How easily could your diagram be interpreted by someone else? Could the elements in your diagram be used to depict a string of decorative lights, such as those draped over the trees of the San Antonio Riverwalk? A diagram that depicts the construction of an electrical apparatus is called a schematic diagram. The schematic diagram shown in Figure 1.1(b) uses symbols to represent the bulb, battery, and wire from Figure 1.1(a). Note that these same symbols can be used to describe these elements in any electrical apparatus. This way, schematic diagrams can be read by anyone familiar with the standard set of symbols. Reading schematic diagrams allows us to determine how the parts in an electrical device are arranged. In this chapter, you will see how the arrangement of resistors in an electrical device can affect the current in and potential difference across the other elements in the device. The ability to interpret schematic diagrams for complicated electrical equipment is an essential skill for solving problems involving electricity. As shown in Figure 1.2 on the next page, each element used in a piece of electrical equipment is represented by a symbol in schematic diagrams that reflects the element s construction or function. For example, the schematic-diagram symbol that represents an open switch resembles the open knife switch that is shown in the corresponding photograph. Note that Figure 1.2 also includes other forms of schematic-diagram symbols; these alternative symbols will not be used in this book. Untitled /26/2011 7:13:21 (a) for example, in this book, a capacitor is shown by two horizontal Ts positioned head to head and a resistor is shown by a squiggly wire. Point out or draw the schematic illustrations of a capacitor and a resistor. (b) 628 Chapter 18

2 FIGURE 1.2 SCHEMATIC DIAGRAM SYMBOLS TEACH FROM VISUALS Component Wire or conductor Resistor or circuit load Bulb or lamp Plug Battery Symbol used in this book Other forms of this symbol Multiple cells Explanation Wires that connect elements are conductors. Because wires offer negligible resistance, they are represented by straight lines Resistors are shown having multiple bends, illustrating resistance to the movement of charges. The multiple bends of the filament indicate that the light bulb behaves as a resistor. The symbol for the filament of the bulb is often enclosed in a circle to emphasize the enclosure of a resistor in a bulb. The plug symbol looks like a container for two prongs. The emf between the two prongs of a plug is symbolized by lines of unequal length. Differences in line length indicate a potential difference between positive and negative terminals of the battery. The longer line represents the positive terminal of the battery. FIGURE 1.2 Be sure students recognize that the different symbols represent devices with different functions. Ask Challenge students to identify which devices have the following functions: storing energy, transforming energy, and conducting current. Answer: Batteries and capacitors store energy; Resistors, bulbs, and batteries transform energy; Wires, resistors, bulbs, plugs, closed switches, and batteries conduct current. The Language of Physics Although Figure 1.2 contains several schematic-diagram symbols, several stylistic variations exist. For example, some other symbols for light bulbs are shown below. Switch Capacitor Open Closed Open (j) Closed (k) (l) The small circles indicate the two places where the switch makes contact with the wires. Most switches work by breaking only one of the contacts, not both. The two parallel plates of a capacitor are symbolized by two parallel lines of equal length. One curved line indicates that the capacitor can be used with only direct current sources with the polarity as shown. Because light bulbs behave as resistors for small changes in voltage, the symbols for resistors are often used for light bulbs. Circuits and Circuit Elements 629 titled Create schematic diagrams that students can mark up. Create your own or make copies of schematic diagrams shown in this chapter. As practice, have students place one of the following labels on each of the symbols shown in a diagram: W: wire or connection R: resistor or circuit load Bu: bulb or lamp P: plug Ba: battery S: switch C: capacitor 5/26/2011 7:13:22 AM Circuits and Circuit Elements 629

3 Teach continued Answers Conceptual Challenge 1. Because there is no potential difference between the bird s feet, there is no current in the bird s body. 2. At first there is no potential difference between the parachutist s hands, so there is no current in the parachutist s body. If the parachutist s feet touch the ground and the parachutist continues to hold onto the wire, however, there will be current in the parachutist s body because of the potential difference between the wire in the parachutist s hands and the ground. FIGURE 1.3 A Complete Circuit When all electrical components are connected, charges can move freely in a circuit. The movement of charges in a circuit can be halted by opening the switch. electric circuit a set of electrical components connected such that they provide one or more complete paths for the movement of charges Electric Circuits Think about how you get the bulb in Figure 1.3 to light up. Will the bulb stay lit if the switch is opened? Is there any way to light the bulb without connecting the wires to the battery? The filament of the light bulb acts as a resistor. When a wire connects the terminals of the battery to the light bulb, as shown in Figure 1.3, charges built up on one terminal of the battery have a path to follow to reach the opposite charges on the other terminal. Because there are charges moving through the wire, a current exists. This current causes the filament to heat up and glow. Together, the bulb, battery, switch, and wire form an electric circuit. An electric circuit is a path through which charges can flow. A schematic diagram for a circuit is sometimes called a circuit diagram. Any element or group of elements in a circuit that dissipates energy is called a load. A simple circuit consists of a source of potential difference and electrical energy, such as a battery, and a load, such as a bulb or group of bulbs. Because the connecting wire and switch have negligible resistance, we will not consider these elements as part of the load. In Figure 1.3, the path from one battery terminal to the other is complete, a potential difference exists, and electrons move from one terminal to the other. In other words, there is a closed-loop path for electrons to follow. This is called a closed circuit. The switch in the circuit in Figure 1.3 must be closed in order for a steady current to exist. Without a complete path, there is no charge flow and therefore no current. This situation is an open circuit. If the switch in Figure 1.3 were open, as shown in Figure 1.2, the circuit would be open, the current would be zero, and the bulb would not light up. Conceptual Challenge Bird on a Wire Why is it possible for a bird to be perched on a high-voltage wire without being electrocuted? (Hint: Consider the potential difference between the bird s two feet.) Parachutist on a Wire Suppose a parachutist lands on a high-voltage wire and grabs the wire in preparation to be rescued. Will the parachutist be electrocuted? If the wire breaks, why should the parachutist let go of the wire as it falls to the ground? (Hint: First consider the potential difference between the parachutist s two hands holding the wire. Then consider the potential difference between the wire and the ground.) (b) blickwinkel/alamy 630 Chapter 18 Differentiated Instruction Inclusion Students with kinesthetic learning styles may benefit from using a fluid model for electric current in a circuit. In this model, charges moving due to potential difference are analogous to water moving to a level of lower gravitational potential energy. Wires are analogous to horizontal pipes, and resistors are analogous to water wheels, which transform the energy to another form. Batteries and generators act like pumps in that they lift water up, increasing its potential energy. If possible, have kinesthetic learners build a fluid model of a basic circuit. Untitled /26/2011 7:13: Chapter 18

4 (br) Gustoimages/Photo Researchers, Inc.; (tr) GIPhotoStock/Photo Researchers, Inc. CFLs and LEDs T he most familiar of light bulbs, incandescent bulbs, may soon be a relic of the past. Thomas Edison first invented these bulbs in 1879 and they have been in use ever since. They work by heating a small metal filament that glows and produces light. Although incandescent bulbs give off very warm and pleasant light, they are extremely inefficient. Nearly 90% of the energy they use is converted into heat and only 10% is converted into light. New federal law requires that by 2014 all bulbs be at least 30% more efficient. Two new types of bulbs look to replace incandescent bulbs. The first type of light bulb is called compact fluorescent light (or CFL for short). CFLs work by running an electrical current through a tube that contains a mixture of gases. The atoms of gas absorb energy from the electricity and emit ultraviolet light. Humans, however, cannot see ultraviolet light. What happens next is that the ultraviolet light hits the surface of the tube that has been coated with a chemical that absorbs the ultraviolet light and emits visible light. The second type of light bulb is called light-emitting diode (or LED for short). LEDs work by moving electrons and protons in a solid piece of material called a semiconductor. As the electrons move through this material they lose energy and release light. The Short circuits can be hazardous. Without a load, such as a bulb or other resistor, the circuit contains little resistance to the movement of charges. This situation is called a short circuit. For example, a short circuit occurs when a wire is connected from one terminal of a battery to the other by a wire with little resistance. This commonly occurs when uninsulated wires connected to different terminals come into contact with each other. When short circuits occur in the wiring of your home, the increase in current can become unsafe. Most wires cannot withstand the increased current, and they begin to overheat. The wire s insulation may even melt or cause a fire. electrons here release no energy, so LEDs are more energy efficient than both incandescent and CFLs. In addition, because LEDs are made of solid material, they can be very small and are very durable so they last a long time. Although both CFLs and LEDS cost considerably more than incandescent bulbs, they use much less energy to produce the same amount of light. In addition, they have a much longer life span. When both of these factors are taken into account, replacing your incandescent bulbs with CFLs or LEDs may cost more up front, but they end up saving money over the life of the bulb. Why It Matters CFLs and LEDs Many electrical products, such as decorative lights, extension cords, and appliances, have a prominent tag labeled UL. This mark, from Underwriters Laboratories, indicates that the product has been tested by UL engineers for electrical, fire, and other hazards. Circuits and Circuit Elements 631 titled Because batteries are said to run down, many students believe that current is consumed by a circuit. To check for this misconception, ask students to draw arrows representing the current in a simple circuit. Some may believe that current is used up in the resistor. Their diagrams will show charges moving only from the battery to the bulb. Others may think that the current comes back to the battery but has decreased in magnitude. Arrows representing current in their diagrams may get smaller after the resistor. 5/26/2011 7:13:29 AM Point out that the number of charges entering a part of the circuit in some time interval equals the number of charges leaving it in the same time interval. Explain that the chemicals in the battery react to produce a potential difference. Eventually, most of these reacting chemicals are converted to other substances, and the battery no longer produces a potential difference. Circuits and Circuit Elements 631

5 Teach continued The Language of Physics The term emf originally stood for electromotive force. This term may be misleading because emf is not a force. Rather, it refers to a potential difference measured in volts. The voltage value on a battery label denotes its emf. In this text, internal resistance will be disregarded unless specifically noted. The value of the terminal voltage, ΔV, can be found from the emf (ε ), the total current (I ), and the internal resistance (r ) with the following equation: QuickLab ΔV = ε Ir Teacher s Notes To light the bulb, students should connect the bottom of the bulb to one terminal of the battery and the side of the bulb s base to the other terminal. The bulb can be lit with one wire by holding the base of the bulb to one of the battery s terminals and using the wire to connect the side of the bulb s base to the other terminal MATERIALS 1 miniature light bulb 1 D-cell battery wires rubber band or tape SAFETY Do not perform this lab with any batteries or electrical devices other than those listed here. Never work with electricity near water. Be sure the floor and all work surfaces are dry. SIMPLE CIRCUITS Connect the bulb to the battery using two wires, using a rubber band or tape to hold the wire to the battery. Once you have gotten the bulb to light, try different arrangements to see whether there is more than one way to get the bulb to light. Can you make the bulb light using just one wire? Diagram each arrangement that you try, and note whether it produces light. Explain exactly which parts of the bulb, battery, and wire must be connected for the light bulb to produce light. The source of potential difference and electrical energy is the circuit s emf. Will a bulb in a circuit light up if you remove the battery? Without a potential difference, there is no charge flow and no current. The battery is necessary because the battery is the source of potential difference and electrical energy for the circuit. So, the bulb must be connected to the battery to be lit. Any device that increases the potential energy of charges circulating in a circuit is a source of emf, or electromotive force. The emf is the energy per unit charge supplied by a source of electric current. Think of such a source as a charge pump that forces electrons to move in a certain direction. Batteries and generators are examples of emf sources. For conventional current, the terminal voltage is less than the emf. Look at the battery attached to the light bulb in the circuit shown in Figure 1.4. As shown in the inset, instead of behaving only like a source of emf, the battery behaves as if it contains both an emf source and a resistor. The battery s internal resistance to current is the result of moving charges colliding with atoms inside the battery while the charges are traveling from one terminal to the other. Thus, when charges move conventionally in a battery, the potential difference across the battery s terminals, the terminal voltage, is actually slightly less than the emf. Unless otherwise stated, any reference in this book to the potential difference across a battery should be thought of as the potential difference measured across the battery s terminals rather than as the emf of the battery. In other words, all examples and end-of-chapter problems will disregard the internal resistance of the battery. FIGURE 1.4 A Battery s Internal Resistance (a) A battery in a circuit behaves as if it contains both (b) an emf source and (c) an internal resistance. For simplicity s sake, in problem solving it will be assumed that this internal resistance is insignificant. (a) (b) (c) Small internal resistance 632 Chapter 18 Differentiated Instruction Comparing electromotive force with voltage drop from the electrons' perspective could provide students with a basis for understanding fluctuations in potential energy. Point out that raising the potential energy of electrons in a source yields electromotive force, while decreasing the potential energy of electrons in a load results in a voltage drop. Untitled /26/2011 7:13: Chapter 18

6 Potential difference across a load equals the terminal voltage. When charges move within a battery from one terminal to the other, the chemical energy of the battery is converted to the electrical potential energy of the charges. As charges move through the circuit, their electrical potential energy is converted to other forms of energy. For instance, when the load is a resistor, the electrical potential energy of the charges is converted to the internal energy of the resistor and dissipated as thermal energy and light energy. Because energy is conserved, the energy gained and the energy lost must be equal for one complete trip around the circuit (starting and ending at the same place). Thus, the electrical potential energy gained in the battery must equal the energy dissipated by the load. Because the potential difference is the meas urement of potential energy per amount of charge, the potential increase across the battery must equal the potential decrease across the load. SECTION 1 FORMATIVE ASSESSMENT Reviewing Main Ideas 1. Identify the types of elements in the schematic diagram illustrated in Figure 1.5 and the number of each type. 2. Using the symbols listed in Figure 1.2, draw a schematic diagram of a working circuit that contains two resistors, an emf source, and a closed switch. 3. In which of the circuits pictured below will there be no current? FIGURE 1.6 FIGURE 1.7 FIGURE 1.5 Key Models and Analogies From an energy-transformation perspective, think of batteries as electrical-energy-supply devices and of resistors and light bulbs as electricalenergy-consuming devices. The electric current conveys this energy from the battery to the resistor. Assess and Reteach Assess Use the Formative Assessment on this page to evaluate student mastery of the section. Reteach For students who need additional instruction, download the Section Study Guide. Response to Intervention To reassess students mastery, use the Section Quiz, available to print or to take directly online at HMDScience.com. FIGURE 1.8 FIGURE If the potential difference across the bulb in a certain flashlight is 3.0 V, what is the potential difference across the combination of batteries used to power it? Critical Thinking 5. In what forms is the electrical energy that is supplied to a string of decorative lights dissipated? Answers to Section Assessment Circuits and Circuit Elements 633 led one battery, one closed switch, two resistors, and three bulbs 2. Students' diagrams should include the circuit elements as they appear in Figure Figure 1.7 and Figure 1.9 will have no current in them V 5. It is converted to thermal energy and light energy. 5/26/2011 7:13:30 AM Circuits and Circuit Elements 633