Chapter 1

Direct Current (dc)

Electronics

INTRODUCTION

The advancement of science and technology has brought about important changes in the field of electronics. At one time, the field of electronics was very limited. Recent developments in solid-state electronics and microminiaturization have brought a number of significant changes. A person working in the field of electronics must be knowledgeable about many types of systems and numerous control devices in order to be successful today. Electronics is a very fascinating science that we use in many different ways. It would be difficult to list all the ways that we use electronics each day. Everyone today should have an understanding of electronics. This chapter deals with a basic topic in the study of electronics-direct current, or DC. Topics such as basic electrical systems; energy and power, the structure of matter, electrical charges, static electricity, electrical current, voltage, resistance, and measuring instruments are discussed. When studying this unit as well as others, you should refer to definitions of important terms in Appendix A. Preview these terms to gain a better understanding of what is discussed as the need arises.

OBJECTIVES

Upon completion of this chapter, you will be able to:

1. Explain the parts of an electronic system.

2. Explain the composition of matter.

3. Explain the laws of electrical charges.

4. Define the terms insulator, conductor, and semiconductor.

5. Explain electric current flow.

6. Diagram a simple electronic circuit.

7. Identify schematic electronic symbols.

8. Convert electrical quantities from metric units to English units and English units to metric units.

9. Use scientific notation to express numbers.

10. Define voltage, current, and resistance.

11. Describe basic types of batteries.

12. Explain how to connect batteries in series, parallel, and configurations.

13. Explain purposes of different configurations of battery connections,

14. Explain factors determining resistance.

15. Identify different types of resistors.

16. Identify resistor value by color code and size.

17. Explain the operation of potentiometers (variable resistors).

18. Construct basic electronic circuits.

19. Explain how to connect an ammeter in a circuit and measure current.

20. Explain how the voltmeter, ammeter, and ohmmeter are connected to a circuit.

21. Explain how to measure current, voltage, and resistance of basic dc circuits.

22. Solve basic math problems using a calculator.

23. Define Ohm's law and the power equation.

24. Solve problems finding current, voltage, and resistance.

25. Calculate power using the proper power formulas.

26. Define voltage drop in a circuit.

27. Solve circuit problems with resistors in different configurations.

28. Define the various terms relative to magnetism.

29. Explain the operation of various magnetic devices.

30. State Faraday's law for electromagnetic induction.

31. List three factors that affect the strength of electromagnets.

32. Explain how a capacitor operates.

33. Define the terms inductance and inductors.

34. Define the terms capacitance and capacitors.

35. List the factors determining capacitance.

36. Describe the construction of various types of capacitors.

37. Calculate total capacitance of capacitors in various series, parallel, and combination configurations.

38. Calculate total inductance of inductors in various series, parallel, and combination configurations.

39. List factors affecting inductance.

40. Identify different types of inductors.

41. Explain the concept of mutual inductance.

THE SYSTEM CONCEPT

For a number of years, people have worked with jigsaw puzzles as a source of recreation. A jigsaw puzzle contains a number of discrete parts that must be placed together properly to produce a picture. Each part then plays a specific role in the finished product. When a puzzle is first started, it is difficult to imagine the finished product without seeing a representative picture.

Studying a complex field such as electronics by discrete parts poses a problem that is somewhat similar to the jigsaw puzzle. In this case, it is difficult to determine the role that a discrete part plays in the operation of a complex system. A picture of the system divided into its essential parts therefore becomes an extremely important aid in understanding its operation.

The system concept will serve as the "big picture" in the study of electronics. In this approach, a system will first be divided into a number of essential blocks. The role played by each block then becomes more meaningful in the operation of the overall system. After the location of each block has been established, discrete component operation related to each block then becomes more relevant. Through this approach, the way in which some of the "pieces" of electronic systems fit together should be more apparent.

BASIC SYSTEM FUNCTIONS

The word system is commonly defined as an organization of parts that are connected together to form a complete unit. There are a wide variety of electronic systems used today. Each electronic system has a number of unique features, or characteristics, that distinguish it from other systems. More importantly, however, there is a common set of parts found in each system. These parts play the same basic role in all systems. The terms energy source, transmission path, control, load, and indicator are used to describe the various system parts. A block diagram of these basic parts of the system is shown in Figure 1-1.

Each block of a basic system has a specific role to play in the overall operation of the system. This role becomes extremely important when a detailed analysis of the system is to take place. Hundreds and even thousands of discrete components are sometimes needed to achieve a specific block function. Regardless of the complexity of the system, each block must still achieve its function in order for the system to be operational. Being familiar with these functions and being able to locate them within a complete system is a big step toward understanding the operation of the system.

The energy source of a system converts energy of one form into something more useful. Heat, light, sound, chemical, nuclear, and mechanical energy are considered as primary sources of energy. A primary energy source usually goes through an energy change before it can be used in an operating system.

The transmission path of a system is somewhat simplified when compared with other system functions. This part of the system simply provides a path for the transfer of energy. It starts with the energy source and continues through the system to the load device. In some cases, this path may be a single electrical conductor, light beam, or other medium between the source and the load. In other systems, there may be a supply line between the source and the load and a return line from the load to the source.

There may also be a number of alternate or auxiliary paths within a complete system. These paths may be series connected to a number of small load devices or parallel connected to many independent devices.

The control section of a system is by far the most complex part of the entire system. In its simplest form, control is achieved when a system is turned on or off (see Figure 1-2). Control of this type can take place anywhere between the source and the load device. The term full control is commonly used to describe this operation. In addition to this type of control, a system may also employ some type of partial control. Partial control usually causes some type of an operational change in the system other than an on or off condition. Changes in electric current or light intensity are examples of alterations achieved by partial control.

The load of a system refers to a specific part or number of parts designed to produce some form of work (see Figure 1-3). Work, in this case, occurs when energy goes through a transformation or change. Heat, light, chemical action, sound, and mechanical motion are some of the common forms of work produced by a load device. As a general rule, a very large portion of all energy produced by the source is consumed by the load device during its operation. The load is typically the most prevalent part of the entire system because of its obvious work function.

The indicator of a system is primarily designed to display certain operating conditions at various points throughout the system. In some systems the indicator is an optional part, whereas in others it is an essential part in the operation of the system. In the latter case, system operations and adjustments are usually critical and are dependent upon specific indicator readings. The term operational indicator is used to describe this application are also needed to determine different operating values. In this role, the indicator is only temporarily attached to the system to make measurements. Test lights, meters, oscilloscopes, chart recorders, and digital display instruments are some of the common indicators used in this capacity.

A SIMPLE ELECTRONIC SYSTEM EXAMPLE

A flashlight is a device designed to serve as a light source in an emergency or as a portable light source. In a strict sense, flashlights can be classified as portable electronic systems. They contain the four essential parts needed to make this classification. Figure 1-4 is a cutaway drawing of a flashlight, with each component part shown associated with its appropriate system block.

The battery of a flashlight serves as the primary energy source of the system. Chemical energy of the battery must be changed into electrical energy before the system becomes operational. The flashlight is a synthesized system because it utilizes two distinct forms of energy in its operation. The energy source of a flashlight is an expendable item. It must be replaced periodically when it loses its ability to produce electrical energy. The transmission path of a flashlight is commonly achieved via a metal case or through a conductor strip. Copper, brass, and plated steel are frequently used to achieve this function.

The control of electrical energy in a flashlight is achieved by a slide switch or a push-button switch. This type of control simply interrupts the transmission path between the source and the load device. Flashlights are primarily designed to have full control capabilities. This type of control is achieved manually by the person operating the system. The load of a flashlight is a small incandescent lamp.

When electrical energy from the source is forced to pass through the filament of the lamp, the lamp produces a bright glow. Electrical energy is first changed into heat and then into light energy. A certain amount of work is achieved by the lamp when this energy change takes place. The energy transformation process of a flashlight is irreversible. It starts at the battery when chemical energy is changed into electrical energy. Electrical energy is then changed into heat and eventually into light energy by the load device. This flow of energy is in a single direction.

When light is eventually produced, it consumes a large portion of the electrical energy coming from the source. When this energy is exhausted, the system becomes inoperative. The battery cells of a flashlight require periodic replacement in order to maintain a satisfactory operating condition. Flashlights do not ordinarily employ a specific indicator as part of the system. Operation is indicated when the lamp produces light. In a strict sense, we could say that the load of this system also serves as an indicator. In some electrical and electronic systems the indicator is an optional system part.

A DIGITAL ELECTRONIC SYSTEM EXAMPLE

Another example of an electronic system is a digital system. Some of the most significant developments that have taken place in electronics are used for automation.

Automatic fabrication methods, packaging, printing equipment, machining operations, and drafting equipment are all outgrowths of electronics in industry. In industrial systems that utilize digital control, instructions are supplied by magnetic tape, magnetic disk, or various physical changes such as pressure, temperature, or electricity. This information is then changed into digital signals and applied to the digital processor of the system. This information is decoded and directed to specific machines or machine parts, which then perform the necessary operations automatically.

A large portion of the automatic machinery being used by industry today receives instructions through digital signals. A digital, or numerical, system is therefore a very important part of the electronics field. In a strict sense, numerical signals are used primarily to perform the control function of a digital system. The computer numerical control (CNC) is also used to describe a specific type of digital system. A majority of the numerical systems in operation today are powered by electricity. This source of energy is primarily used to energize the load device, which in turn performs the work function of the system. The control function of the system must then be designed to respond to digital information. Inputs develop this information in the first operational step. Logic circuits, which provide full or partial control of the load device, are then activated. A numerically controlled milling machine is a type of digital system. CNC machine system is shown in Figure 1-5. Operating instructions are translated into electrical signals and processed by digital logic gates housed in the machine. As an end result, these signals are used to control various physical machine operations automatically. The load of a CNC system is typically electrical actuating motors or fluid-power cylinders designed to move the physical parts of a machine. When appropriate signals from the control unit are applied to the load, they move the two table axes or the cutting tool to a specific location. Machining operations can be performed according to the information programmed. Cutting speed, position location, clamping operations, and material flow can be controlled through this system. The operation is easily controlled by programming information into a system of this type.

ENERGY, WORK, AND POWER

An understanding of the terms energy, work, and power is necessary in the study of electronics. The first term, energy, means the capacity to do work. For example, the capacity to light a light bulb, to heat a home, or to move something requires energy. Energy exists in many forms, such as electrical, mechanical, chemical, and heat. If energy exists because of the movement of some item, such as a ball rolling down a hill, it is called kinetic energy. If energy exists because of the position of something, such as a ball that is at the top of the hill but not yet rolling, it is called potential energy. Energy has become one of the most important factors in our society. A second important term is work. Work is the transferring or transforming of energy. Work is done when a force is exerted to move something over a distance against opposition, such as when a chair is moved from one side of a room to the other. An electrical motor used to drive a machine performs work. Work is performed when motion is accomplished against the action of a force that tends to oppose the motion. Work is also done each time energy changes from one form into another. A third important term is power. Power is the rate at which work is done. It considers not only the work that is performed but the amount of time in which the work is done. For instance, electrical power is the rate at which work is done as electrical current flows through a wire. Mechanical power is the rate at which work is done as an object is moved against opposition over a certain distance. Power is either the rate of production or the rate of use of energy. The watt is the unit of measurement of power.

STRUCTURE OF MATTER

In the study of electronics, it is necessary to understand why electrical energy exists. By looking first at how certain natural materials are made, it will then be easier to see why electrical energy exists. Here are a few basic scientific terms that are often used in the study of chemistry. They are also very important in the study of electronics. First, matter is anything that occupies space and has weight. Matter can be either a solid, a liquid, or a gaseous material. Solid matter includes such things as metal and wood; liquid matter is exemplified by water and gasoline; and gaseous matter includes such things as oxygen and hydrogen. Solids can be converted into liquids, and liquids can be made into gases. For example, water can be solid in the form of ice. Water can also be a gas in the form of steam. Matter changes state when the particles of which they are made are heated. As they are heated, the particles move and strike one another, causing them to move farther apart. Ice is converted into a liquid by adding heat. If heated to a high temperature, water becomes a gas (steam). All forms of matter exist in their most familiar states because of the amount of heat they contain. Some materials require more heat than others to become a liquid or a gas. However, all materials can be made to change from a solid to a liquid or from a liquid to a gas if enough heat is added. Also, these materials can change into liquids or solids if heat is taken from them. The next important term in the study of the structure of matter is the element. An element is considered to be the basic material that makes up all matter. Materials such as hydrogen, aluminum, copper, iron, and iodine are a few of the over 100 elements known to exist. A table of elements is shown in Figure 1-6 (opposite). Some elements exist in nature and some are manufactured. Everything around us is made up of elements. There are many more materials in our world than there are elements. Other materials are made by combining elements. A combination of two or more elements is called a compound. For example, water is a compound made from the elements hydrogen and oxygen. Salt is made from sodium and chlorine. Another important term is molecule. A molecule I believed to be the smallest particle that a compound cabe reduced to before being broken down into its basic elements. For example, one molecule of water has two hydrogen atoms and one oxygen atom. In an even deeper look into the structure of matter, there are particles called atoms. Within these atoms are the forces that cause electrical energy to exist. An atom is considered to be the smallest particle to which an element can be reduced and still have the properties of that element. If an atom were broken down any further, the element would no longer exist. The smallest particles that are found in all atoms are called electrons, protons, and neutrons. Elements differ from one another on the basis of the numbers of these particles found in their atoms.

The relationship of matter, elements, compounds, molecules, atoms, electrons, protons, and neutrons is shown in Figure 1-7. The simplest atom, hydrogen, is shown in Figure

1-8. The hydrogen atom has a center part called a nucleus, which has one proton. A proton is a particle that has a positive (+) charge. The hydrogen atom has one electron, which orbits around the nucleus of the atom. The electron has a negative (-) charge. Most atoms also have neutrons in the nucleus. A neutron has neither a positive nor a negative charge and is considered neutral. A carbon atom is shown in Figure 1-9. A carbon atom has 6 protons (+), 6 neutrons (N), and 6 electrons (-). The protons and the neutrons are in the nucleus, and the electrons orbit around the nucleus. The carbon atom has two orbits or circular paths. In the first orbit, there are 2 electrons. The other 4 electrons are in the second orbit.

Look at the table of elements in Figure 1-6. Notice there are a different number of protons in the nucleus of each atom. This causes each element to be different. For example, hydrogen has 1 proton, carbon has 6, oxygen has 8, and lead has 82. The number of protons that each atom has is called its atomic number. The nucleus of an atom contains protons ( +) and neutrons (N). Since neutrons have no charge and protons have positive charges, the nucleus of an atom has a positive charge. Protons are believed to be about one third the diameter of electrons. The mass, or weight, of a proton is over 1800 times that of an electron, whose mass is about 9

10-28 g. Electrons move easily in their orbits around the nucleus of an atom. It is the movement of electrons that causes electrical energy to exist. Early models of atoms showed electrons orbiting around the nucleus, in analogy with planets around the sun. This model is inconsistent with much modern experimental evidence. Atomic orbitals are very different from the orbits of satellites. Atoms consist of a dense, positively charged nucleus surrounded by a cloud or series of clouds of electrons that occupy energy levels, which are commonly called shells. The occupied shell of highest energy is known as the valence shell, and the electrons in it are known as valence electrons. Electrons behave as both particles and waves, so descriptions of them always refer to their probability of being in a certain region around the nucleus. Representations of orbitals are boundary surfaces enclosing the probable areas in which the electrons are found. All s orbitals are spherical, p orbitals are egg-shaped, d orbitals are dumbbell shaped, and f orbitals are double-dumbbell- shaped. Covalent bonding thus involves the overlapping of valence shell orbitals of different atoms. The electron charge then becomes concentrated in this region, thus

attracting the two positively charged nuclei toward the negative charge between them. In ionic bonding, the ions are discrete units, and they group themselves in crystal structures, surrounding themselves with the ions of opposite charge.

The electron arrangement in the atoms of elements can be described from atomic mathematical theory. The first energy level, or shell, contains up to 2 electrons. The next shell contains up to 8 electrons. The third contains up to 18 electrons. Eighteen is the largest quantity any shell can contain. New shells are started as soon as shells nearer the nucleus have been filled with the maximum number of electrons. An atom with an incomplete outer shell is very active. When two unlike atoms with incomplete outer shells come together, they try to share their outer electrons. When their combined outer electrons are enough to make up one complete shell, stable atoms are formed. For example, oxygen has 8 electrons, 2 in the first shell and 6 in its outer shell. There is room for 8 electrons in the outer shell. Hydrogen has one electron in its outer shell. When two hydrogen atoms come near, oxygen combines with the hydrogen atoms by sharing the electrons of the two hydrogen atoms. Water is formed in a manner similar to the sketch of Figure 1-10. All the electrons are then bound tightly together, and a very stable water molecule is formed. The electrons in the incomplete outer shell of an atom are known as valence electrons. They are the only electrons that will combine with the electrons of other atoms to form compounds. They are also the only electrons that cause electric current to flow. For this reason it is necessary to understand the structure of matter.

ELECTROSTATIC CHARGES

In the preceding section, the positive and negative charges of protons and electrons were studied. Protons and electrons are parts of atoms that make up all things in our world. The positive charge of a proton is similar to the negative charge of an electron. However, a positive charge is the opposite of a negative charge. These charges are called electrostatic charges. Figure 1-11 shows how electrostatic charges affect each other. Each charged particle is surrounded by an electrostatic field.

The effect that electrostatic charges have on each other is very important. They either repel (move away) or attract (come together) each other. This action is as follows:

1. Positive charges repel each other (see Figure 1- 11(a)).

2. Negative charges repel each other (see Figure 1- 11(b)).

3. Positive and negative charges attract each other (see Figure 1-11c)).

Therefore, it is said that like charges repel and unlike charges attract.

The atoms of some materials can be made to gain or lose electrons. The material then becomes charged. Oneway to do this is to rub a glass rod with a piece of silk cloth. The glass rod loses electrons (-), so it now has a positive (+) charge. The silk cloth pulls electrons (-) away from the glass. Since the silk cloth gains new electrons, it now has a negative (-) charge. Another way to charge a material is to rub a rubber rod with fur.

It is also possible to charge other materials because some materials are charged when they are brought close to another charged object. If a charged rubber rod is touched against another material, the second material may become charged. Remember that materials are charged due to the movement of electrons and protons.

Also, remember that when an atom loses electrons (-), it becomes positive (+). These facts are very important in the study of electronics.

Charged materials affect each other due to lines of force. Try to visualize these, as shown in Figure 1-11 These imaginary lines cannot be seen. However, they exert a force in all directions around a charged material. Their force is similar to the force of gravity around the earth.

This force is called a gravitational field.

STATIC ELECTRICITY

Most people have observed the effect of static electricity. Whenever objects become charged, it is due to static electricity. A common example of static electricity is lightning. Lightning is caused by a difference in charge (+ and -) between the earth's surface and the clouds during a storm. The arc produced by lightning is the movement of charges between the earth and the clouds. Another common effect of static electricity is being "shocked" b a doorknob after walking across a carpeted floor. Static electricity also causes clothes taken from a dryer to cling together and hair to stick to a comb. Electrical charges are used to filter dust and soot in devices called electrostatic filters. Electrostatic precipitators are used in power plants to filter the exhaust gas that goes into the air. Static electricity is also used in the manufacture of sandpaper and in the spray painting of automobiles.

A device called an electroscope is used to detect a negative or positive charge.

ELECTRIC CURRENT

Static electricity is caused by stationary charges. However, electrical current is the motion of electrical charges from one point to another. Electric current is produced when electrons (-) are removed from their atoms. Some electrons in the outer orbits of the atoms or certain elements are easy to remove. A force or pressure applied to a material causes electrons to be removed. The movement of electrons from one atom to another is called electric current flow.

CONDUCTORS

A material through which current flows is called a conductor. A conductor passes electric current very easily. Copper and aluminum wire are commonly used as conductors. Conductors are said to have low resistance to electrical current flow. Conductors usually have three or fewer electrons in the outer orbit of their atoms. Remember that the electrons of an atom orbit around the nucleus. Many metals are electrical conductors. Each metal has a different ability to conduct electric current. Materials with only one outer orbit or valence electron (gold, silver, copper) are the best conductors. For example, silver is a better conductor than copper, but it is too expensive to use in large amounts. Aluminum does not conduct electrical current as well as copper, but it is commonly used, since it is cheaper and lighter than other conductors. Copper is used more than any other conductor.

INSULATORS

There are some materials that do not allow electric current to flow easily. The electrons of materials that are insulators are difficult to release. In some insulators, their valence shells are filled with eight electrons. The valence shells of others are over half-filled with electrons. The atoms of materials that are insulators are said to be stable. Insulators have high resistance to the movement of electric current. Some examples of insulators are plastic and rubber.

SEMICONDUCTORS

Materials called semiconductors have become very important in electronics. Semiconductor materials are neither conductors nor insulators. Their classification also depends on the number of electrons their atoms have in their valence shells. Semiconductors have 4 electrons in their valence shells. Remember that conductors have outer orbits less than half-filled and insulators ordinarily have outer orbits more than half-filled. Figure 1-12 compares conductors, insulators, and semiconductors. Some common types of semiconductor materials are silicon, germanium, and selenium.

CURRENT FLOW

The usefulness of electricity is due to its electric current flow. Current flow is the movement of electrical charges along a conductor. Static electricity, or electricity at rest, has some practical uses due to electrical charges. Electric current flow allows us to use electrical energy to do many types of work. The movement of valence shell electrons of conductors produces electrical current. The outer electrons of the atoms of a conductor are called free electrons. Energy released by these electrons as they move allows work to be done. As more electrons move along a conductor, more energy is released. This is called an increased electric current flow. The movement of electrons along a conductor is shown in Figure 1-13.

To understand how current flow takes place, it is necessary to know about the atoms of conductors.

Conductors, such as copper, have atoms that are loosely held together. Copper is said to have atoms connected together by metallic bonding. A copper atom has one valence shell electron, which is loosely held to the atom. These atoms are so close together that their valence shells overlap each other. Electrons can easily move from one atom to another. In any conductor the outer electrons continually move in a random manner from atom to atom. The random movement of electrons does not result in current flow, since electrons must move in the same di12 rection to cause current flow. If electric charges are placed on each end of a conductor, the free electrons move in one direction. Figure 1-14 shows current flow through a conductor caused by negative and positive electrical charges.

Current flow takes place because there is a difference in the charges at each end of the conductor. Remember that like charges repel and unlike charges attract.

When an electrical charge is placed on each end of the conductor, the free electrons move. Free electrons have a negative charge, so they are repelled by the negative charge on the left of Figure 1-13. The free electrons are attracted to the positive charge on the right and move to the right from one atom to another. If the charges on each end of the conductor are increased, more free electrons will move. This increased movement causes more electric current flow. Current flow is the result of electrical energy caused as electrons change orbits. This impulse moves from one electron to another. When one electron moves out of its valence shell, it enters another atom's valence shell. An electron is then repelled from that atom. This action goes on in all parts of a conductor. Remember that electric current flow produces a transfer of energy.

Electronic Circuits

Current flow takes place in electronic circuits. A circuit is a path or conductor for electric current flow. Electric current flows only when it has a complete, or closed-circuit, path. There must be a source of electrical energy to cause current to flow along a closed path. Figure 1-14 shows a battery used as an energy source to cause current to flow through a light bulb. Notice that the path, or circuit, is complete. Light is given off by the light bulb due to the work done as electric current flows through a closed circuit. Electrical energy produced by the battery is changed to light energy in this circuit.

Electric current cannot flow if a circuit is open. An open circuit does not provide a complete path for current flow. If the circuit of Figure 1-14 were to become open, no current would flow. The light bulb would not glow. Free electrons of the conductor would no longer move from one atom to another. An example of an open circuit is a "burned-out" light bulb. Actually, the filament (the part that produces light) has become open. The open filament of a light bulb stops current flow from the source of electrical energy. This causes the bulb to stop burning, or producing light. Another common circuit term is a short circuit. A short circuit, which can be very harmful, occurs when a conductor connects directly across the terminals of an electrical energy source. For safety purposes, a short circuit should never happen because short circuits cause too much current to flow from the source. If a wire is placed across a battery, a short circuit occurs. The battery would probably be destroyed and the wire could get hot or possibly melt due to the short circuit.

Direction of Current Flow

Electric current flow is the movement of electrons along a conductor. Electrons are negative charges.

Negative charges are attracted to positive charges and are repelled by other negative charges. Electrons move from the negative terminal of a battery to the positive terminal. This is called electron current flow. Electron current flow is in a direction of electron movement from negative to positive through a circuit. Another way to look at electric current flow is in terms of charges. A high charge can be considered positive and a low charge, negative. Using this method, an electrical charge is considered to move from a high charge to a low charge. This is called conventional current flow. Conventional current flow is the movement of charges from positive to negative.

Electron and conventional current flow should not be confusing. They are two different ways of looking at current flow. One deals with electron movement and the other deals with charge movement. In this book, electron current flow is used.

Amount of Current Flow (The Ampere)

The amount of electric current that flows through a circuit depends on the number of electrons that pass a point in a certain time. The coulomb (C) is a unit of measurement of electric current. It is estimated that 1 C is 6,280,000,000,000,000,000 electrons (6.28 1018 in scientific notation). Since electrons are very small, it takes many to make one unit of measurement. When 1 C passes a point on a conductor in one second, 1 ampere (A) of current flows in the circuit. The unit is named for A.M. Ampere, an 18-century scientist who studied electricity. Current is commonly measured in units called milliamperes (mA) and microamperes (A). These are smaller units of current. A milliampere is one thousandth (1/1000) of an ampere and a microampere is one millionth (1/1,000,000) of an ampere.

Current Flow Compared to Water Flow

An electrical circuit is a path in which an electric current flows. Current flow is similar to the flow of water through a pipe. Water flow can be used to show how current flows in an electrical circuit. When water flows in a pipe, something causes it to move. The pipe offers opposition, or resistance, to the flow of water. If the pipe is small, it is more difficult for the water to flow. In an electric circuit, current flows through wires (conductors),

The wires of an electric circuit are similar to the pipes through which water flows. If the wires are made of a material that has high resistance, it is difficult for current to flow. The result is the same as water flow through a pipe with a rough surface. If the wires are large, it is easier for current to flow in an electrical circuit. In the same way, it is easier for water to flow through a large pipe. Electric current and water flow are compared in Figure

1-15. Current flows from one place to another in an electrical circuit. Similarly, water that leaves a pump moves from one place to another. The rate of water flow through

a pipe is measured in gallons per minute. In an electronic circuit, the current is measured in amperes. The flow of electric current is measured by the number of coulombs that pass a point on a conductor each second.

ELECTRICAL FORCE (VOLTAGE)

Water pressure is needed to force water along a pipe. Similarly, electrical pressure is needed to force current along a conductor. Water pressure is usually measured in pounds per square inch (psi). Electrical pressure is measured in volts (V). If a motor is rated at 120 V, it requires

120 V of electrical pressure applied to the motor to force the proper amount of current through it. More pressure would increase the current flow and less pressure would not force enough current to flow. The motor would not operate properly with too much or too little voltage.

Water pressure produced by a pump causes water to flow through pipes. Pumps produce pressure that causes water to flow. The same is true of an electrical energy source.

A source such as a battery or generator produces current flow through a circuit. As voltage is increased, the amount of current in the circuit is also increased. Voltage is also called electromotive force (EMF).