Physics
Electromotive Force
Electromotive force (EMF) is the energy per unit charge that is supplied by a source, such as a battery or generator, to move a charge around a circuit. It is not a force in the traditional sense, but rather a potential difference that drives the flow of electric current. EMF is measured in volts and is the driving force behind the movement of charge in an electrical circuit.
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eBook - PDFElectrotechnics N4 Student's Book
TVET FIRST
- SA Chuturgoon(Author)
- 2021(Publication Date)
- Troupant(Publisher)
So, let us highlight the differences between the two in order to avoid confusion. Electromotive Force (emf) is the voltage (electrical potential) measured across the terminals of an electrical energy source of an open circuit, that is, when no current is flowing through the circuit. Important SI unit: a unit of measurement defined by the International System of Units (a metric system used in science and industry) dissipate: cause energy to be lost through its conversion to heat Electromotive ‘force’ is a misnomer . It is not a force measured in newtons but an electrical potential instead. Interesting open circuit: an incomplete electrical connection through which current cannot flow misnomer: a word or concept suggesting a meaning that is known to be wrong generator: a machine that converts one form of energy into another, especially mechanical energy into electrical energy solar energy: radiant energy emitted by the sun radiant: sending out light; shining or glowing brightly friction: the resistance that one surface encounters when moving over another 4 Module 1 TVET FIRST • The switch is open. • Current does not flow, so the light bulb does not glow. • The emf is measured across the energy source (the battery in this case). • The switch is closed. • Current is now flowing, so the light bulb glows. • The potential difference is measured across the energy source (the battery in this case). switch (open) light bulb (not glowing) battery emf + – Figure 1.2: Emf in an electric circuit Potential difference (PD) is the voltage (electrical potential) measured across the terminals of an electrical energy source of a closed circuit, that is, when current is flowing through the circuit. switch (closed) light bulb (glowing) battery PD + – I Figure 1.3: Potential difference in an electric circuit Figure 1.4 shows what actually happens in any electric circuit.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
As we have seen in Section 19.2, electric potential is energy per unit charge, which is not force. Without electric circuits this exuberant display of colorful lights in Sydney, Australia, would not be possible. Virtually every aspect of life in modern industrialized society utilizes or depends upon electric circuits in some way. Chapter | 20 LEARNING OBJECTIVES After reading this module, you should be able to... 20.1 | Define Electromotive Force and current. 20.2 | Solve problems using Ohm’s law for a simple series circuit. 20.3 | Relate resistance and resistivity. 20.4 | Solve problems involving electric power. 20.5 | Solve ac circuit problems for current and power. 20.6 | Analyze resistor circuits with series connections. 20.7 | Analyze resistor circuits with parallel connections. 20.8 | Analyze resistor circuits with both series and parallel connections. 20.9 | Solve circuit problems that include internal resistances of batteries. 20.10 | Solve complex circuit problems by applying Kirchhoff’s rules. 20.11 | Describe how ammeters and voltmeters operate. 20.12 | Analyze capacitor circuits. 20.13 | Analyze RC circuits. 20.14 | Explain why electrical grounding is important. Battery pack MENU iPod Music Videos Photos Podcasts Extras Settings Shuffle Songs Sleep MENU iPod Music Videos Photos Podcasts Extras Settings Shuffle Songs Sleep Moving charge Conducting wire To MP3 player mechanism + – Figure 20.1 In an electric circuit, energy is transferred from a source (the battery pack) to a device (the MP3 player) by charges that move through a conducting wire. 482 20.1 | Electromotive Force and Current 483 battery the emf is 1.5 V. In reality, the potential difference between the terminals of a battery is somewhat less than the maximum value indicated by the emf, for reasons that Section 20.9 discusses.
eBook - PDF- Pierluigi Zotto, Sergio Lo Russo, Paolo Sartori(Authors)
- 2022(Publication Date)
- Società Editrice Esculapio(Publisher)
E = V + − V − = E ds + − ∫ , where the integration path connects the positive and negative poles and it is fully inside the device, is called Electromotive Force. Field E appearing in the integral is homogenous to electrostatic fields, but it has different vector properties, as it will become clear while dis- cussing its performance in paragraph 7.12. An Electromotive Force E is, therefore, the difference in potential between the two ter- minals of a device measured when the circuit is open, that is, in absence of an electric cur- rent, a condition which occurs if the two poles are not connected by a wire. In this condi- tion, i.e. in absence of an electric current even if the poles are connected by a conductor, the Electromotive Force is the voltage across the terminals E = V + − V − = E ds + − ∫ , where the path integral is computed along the connecting wire which corresponds to the actual path that would be covered by moving charges in presence of a current. Field E is the electrostatic field in the wire, which appears because the wire is not in electrostatic equilibrium since different potentials are present at its extremities. In general, however, the integral along a closed circuit, for instance from the positive pole to it again, first along the wire and then across the device, is not null. Such a property is discussed later in paragraph 7.12. The working principle of any Electromotive Force generators is related to electrochemi- cal effects. Examples of Electromotive Force generators are batteries and accumulators (Volta effect), thermocouples (Seebeck effect) and Peltier cells (Peltier effect). 7.3 Current Intensity An electric current is a flow of charges which move in a conductor under the action of the electric field that appears when a voltage is kept constant at its extremities by an elec- tromotive force generator. The amount of charge which flows per unit time through a transverse section of the conduc- tor is called current intensity.
eBook - PDF- David J. Griffiths(Author)
- 2017(Publication Date)
- Cambridge University Press(Publisher)
Whatever the mechanism, its net effect is determined by the line integral of f around the circuit: E ≡ f · d l = f s · d l. (7.9) (Because E · d l = 0 for electrostatic fields, it doesn’t matter whether you use f or f s .) E is called the Electromotive Force, or emf, of the circuit. It’s a lousy term, since this is not a force at all—it’s the integral of a force per unit charge. Some people prefer the word electromotance, but emf is so established that I think we’d better stick with it. Within an ideal source of emf (a resistanceless battery, 4 for instance), the net force on the charges is zero (Eq. 7.1 with σ = ∞), so E = −f s . The potential difference between the terminals (a and b) is therefore V = − b a E · d l = b a f s · d l = f s · d l = E (7.10) (we can extend the integral to the entire loop because f s = 0 outside the source). The function of a battery, then, is to establish and maintain a voltage difference equal to the Electromotive Force (a 6 V battery, for example, holds the positive ter- minal 6 V above the negative terminal). The resulting electrostatic field drives cur- rent around the rest of the circuit (notice, however, that inside the battery f s drives current in the direction opposite to E). 5 Because it’s the line integral of f s , E can be interpreted as the work done per unit charge, by the source—indeed, in some books Electromotive Force is defined this way. However, as you’ll see in the next section, there is some subtlety in- volved in this interpretation, so I prefer Eq. 7.9. 4 Real batteries have a certain internal resistance, r , and the potential difference between their termi- nals is E − Ir , when a current I is flowing. For an illuminating discussion of how batteries work, see D.
eBook - ePub- S. Bobby Rauf(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
The term “electromotive” force stems from the early recognition of electrical current as something that consisted, strictly, of the movement of “electrons.” Nowadays, however, with the more recent breakthroughs in the renewable and non-traditional electrical power generating methods and systems like microbial fuel cells and hydrocarbon fuel cells, electrical power is being harnessed, more and more, in the form of charged particles that may not be electrons. In batteries, such as those used in automobiles, as we will see in the batteries chapter, the flow of current driven by voltage potential difference consists not only of negatively charged electrons, e −, but also types of ions, including H + and HSO 4 − ions. 1 Two, relatively putative, analogies for voltage in the mechanical and civil engineering disciplines are pressure and elevation. In the mechanical realm – or more specifically in the fluid and hydraulic systems – high pressure or pressure differential pushes fluid from one point to another and performs mechanical work. Similarly, voltage – in the form of voltage difference between two points, as with the positive and negative terminals of an automobile battery – moves electrons or charged particles through loads such as motors, coils, resistive elements, wires, or conductors. As electrons or charged particles are pushed through loads like motors, coils, resistive elements, light filaments, etc., electrical energy is converted into mechanical energy, heat energy, or light energy. In equipment like rechargeable batteries, during the charging process, applied voltage can push ions from one electrode (or terminal) to another and thereby “charge” the battery
eBook - PDF- William Moebs, Samuel J. Ling, Jeff Sanny(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
The second section of this chapter covers the analysis of series and parallel circuits that consist of resistors. Later in this chapter, we introduce the basic equations and techniques to analyze any circuit, including those that are not reducible through simplifying parallel and series elements. But first, we need to understand how to power a circuit. Chapter 10 | Direct-Current Circuits 431 10.1 | Electromotive Force Learning Objectives By the end of the section, you will be able to: • Describe the Electromotive Force (emf) and the internal resistance of a battery • Explain the basic operation of a battery If you forget to turn off your car lights, they slowly dim as the battery runs down. Why don’t they suddenly blink off when the battery’s energy is gone? Their gradual dimming implies that the battery output voltage decreases as the battery is depleted. The reason for the decrease in output voltage for depleted batteries is that all voltage sources have two fundamental parts—a source of electrical energy and an internal resistance. In this section, we examine the energy source and the internal resistance. Introduction to Electromotive Force Voltage has many sources, a few of which are shown in Figure 10.2. All such devices create a potential difference and can supply current if connected to a circuit. A special type of potential difference is known as Electromotive Force (emf). The emf is not a force at all, but the term ‘Electromotive Force’ is used for historical reasons. It was coined by Alessandro Volta in the 1800s, when he invented the first battery, also known as the voltaic pile. Because the Electromotive Force is not a force, it is common to refer to these sources simply as sources of emf (pronounced as the letters “ee-em-eff”), instead of sources of Electromotive Force. 432 Chapter 10 | Direct-Current Circuits This OpenStax book is available for free at http://cnx.org/content/col12074/1.3
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
Within a battery, a chemical reaction occurs that transfers electrons from one terminal (leaving it positively charged) to another terminal (leaving it negatively charged). Figure 20.2 shows the two terminals of a car battery and a flashlight battery. The drawing also illustrates the symbol used to represent a battery in circuit drawings. Because of the positive and negative charges on the battery terminals, an electric potential difference exists between them. The maximum potential difference is called the Electromotive Force* (emf) of the battery, for which the symbol ℰ is used. In a typical car battery, the chemical reaction maintains the potential of the positive terminal at a maximum of 12 volts (12 joules/coulomb) higher than the potential of the negative terminal, so the emf is ℰ = 12 V. Thus, one coulomb of charge emerging from the battery and entering a circuit has at most 12 joules of energy. In a typical flashlight 551 *The word “force” appears in this context for historical reasons, even though it is incorrect. As we have seen in Section 19.2, electric potential is energy per unit charge, which is not force. 552 CHAPTER 20 Electric Circuits battery the emf is 1.5 V. In reality, the potential difference between the terminals of a battery is somewhat less than the maximum value indicated by the emf, for reasons that Section 20.9 discusses. In a circuit such as the one shown in Figure 20.1, the battery creates an electric field within and parallel to the wire, directed from the positive toward the negative terminal. The electric field exerts a force on the free electrons in the wire, and they respond by moving. Figure 20.3 shows charges moving inside a wire and crossing an imaginary surface that is perpendicular to their motion.
- Saeed Benjamin Niku, Saeed Benjamin(Authors)
- 2022(Publication Date)
- Springer(Publisher)
155 CHAPTER 6 Electron1otive Force Motors, Transformers, AC and DC Currents 6.1 INTRODUCTION Each of the two generators of the Diablo Canyon nuclear power plant in San Luis Obispo County generates over 1,100 megawatts of power, enough for about 3 million people. The Ames Research Center national full-scale subsonic wind tunnel in Mountain View, California, is 40 x 80 ft and creates winds of up to 350 mph (560 km/h), large enough to test a real, full-scale Boeing 737. The fans and the motors running these fans are enormous. And yet, the generators used to recharge a hand-held flashlight are the size of a large olive and the motors used in small remote-control servomotors are about ¼ inch in diameter. What is important is that the largest and the smallest of motors and generators are actually very similar in the way they work and that they all follow Faraday's Law which we will study later. When you simply plug in an electric motor (whether as part of a device or stand-alone) it simply turns and provides a torque that allows the device to do its job. The same is true for a DC motor that is connected to a battery. You may also use a simple charger (or transformer) to recharge your batteries, whether in a cell phone, camera, computer, hybrid car, or toy. In fact, you may have heard that the high voltage (as large as 500,000 volts) of electric power is lowered to the household voltage (110 volts) with a transformer before it is delivered to your place of residence or work. All these examples are based on a phenomenon called electromotive farce or emf A similar phenomenon that works in the opposite way, called back-emf, is also an important issue that affects the way these systems work or are designed. In this chapter we will study these two phenomena, how they are used, and where they appear to affect our daily lives. But first let's learn the difference between voltage and a current, and their relationship.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
Currents of approximately 0.2 A are potentially fatal because they can make the heart fibrillate, or beat in an uncontrolled manner. Substantially larger currents stop the heart completely. However, since the heart often begins beating normally again after the current ceases, the larger currents can be less dangerous than the smaller currents that cause fibrillation. CONCEPT SUMMARY 20.1 Electromotive Force and Current There must be at least one source or generator of electrical energy in an electric circuit. The Electromotive Force (emf) of a generator, such as a battery, is the maximum potential difference (in volts) that exists between the terminals of the generator. The rate of flow of charge is called the electric current. If the rate is constant, the current I is given by Equation 20.1, where Dq is the magnitude of the charge crossing a surface in a time Dt, the surface being perpendicular to the motion of the charge. The SI unit for current is the coulomb per second (C/s), which is referred to as an ampere (A). When the charges flow only in one direction around a circuit, the current is called direct current (dc). When the direction of charge flow changes from moment to moment, the current is known as alternating current (ac). Conventional current is the hypothetical flow of positive charges that would have the same effect in a circuit as the movement of negative charges that actually does occur. 20.2 Ohm’s Law The definition of electrical resistance R is R 5 V/I, where V (in volts) is the voltage applied across a piece of material and I (in amperes) is the current through the material. Resistance is measured in volts per ampere, a unit called an ohm (V). If the ratio of the voltage to the current is constant for all values of voltage and current, the resistance is constant. In this event, the definition of resistance becomes Ohm’s law, Equation 20.2.
eBook - PDFEngineering Problem Solving
A Classical Perspective
- Milton C. Shaw(Author)
- 2001(Publication Date)
- William Andrew(Publisher)
This is called electroplating . If a copper part is made the anode, material will be removed (deplated). This is called electrolytic machining. By masking the cathode or anode, different shapes may be generated by differential plating or deplating. When material is differentially deposited to form a shape, this is called electrolytic forming. The flow of current in a conductor is analogous to the flow of fluid in a pipe where the current is equivalent to the rate of fluid flow, and the potential difference is equivalent to pressure drop along the pipe. Just as work is done when lifting a weight in mechanics, work ( dW ) is done when voltage ( V ) causes a displacement of a charge ( dq ). Eq. (10.2) dW = Vdq where work is in joules when V is in volts and dq is in coulombs. Power ( P ) is the rate of doing work, hence under steady state conditions, (constant V and I ): Eq. (10.3) P = VI where P is in watts (W ) when V is in volts and I is in amps. A watt is one joule per second. 4.0 SOURCES OF EMF The production of an electrical potential capable of causing a flow of current is generally accomplished by converting another form of energy into electrical energy. This may involve: Thermal energy (from fossil fuel, geothermal, or thermonuclear) into mechanical energy and then into electrical energy Potential or kinetic energy (hydro, wind, and tidal) into mechanical energy and then into electrical energy Solar energy into electrical energy Chemical energy into electrical energy (as in batteries) Electrical Engineering 231 The first of these is the most important, and involves generation of high-pressure steam or high velocity products of combustion to drive an engine of some sort coupled to an electrical generator. The second involves a turbine coupled to an electrical generator. The third involves use of a solar cell to convert energy in the form of thermal radiation directly into electrical energy.
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