Physics
Electric Current
Electric current is the flow of electric charge through a conductor. It is measured in amperes (A) and is the rate of flow of electric charge past a point in a circuit. The flow of current is driven by a potential difference, or voltage, and is fundamental to the operation of electrical devices and systems.
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11 Key excerpts on "Electric Current"
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
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. This flow of charge is known as an Electric Current. The Electric Current I is defined as the amount of charge per unit time that crosses the imaginary surface, as in Figure 20.3, in much the same sense that a river current is the amount of water per unit time that is flowing past a certain point. If the rate is constant, the current is I = Δq _ Δt (20.1) If the rate of flow is not constant, then Equation 20.1 gives the average current. Since the units for charge and time are the coulomb (C) and the second (s), the SI unit for current is a coulomb per second (C/s). One coulomb per second is referred to as an ampere (A), after the French mathematician André-Marie Ampère (1775–1836). If the charges move around a circuit in the same direction at all times, the current is said to be direct current (dc), which is the kind produced by bat- teries. In contrast, the current is said to be alternating current (ac) when the charges move first one way and then the opposite way, changing direction from moment to moment. Many energy sources produce alternating current—for example, generators at power companies and microphones. Example 1 deals with direct current. 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.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
789 C H A P T E R 2 6 26.1 Electric Current Learning Objectives After reading this module, you should be able to . . . 26.1.1 Apply the definition of current as the rate at which charge moves through a point, including solv- ing for the amount of charge that passes the point in a given time interval. 26.1.2 Identify that current is normally due to the motion of conduction electrons that are driven by electric fields (such as those set up in a wire by a battery). 26.1.3 Identify a junction in a circuit and apply the fact that (due to conservation of charge) the total current into a junction must equal the total current out of the junction. 26.1.4 Explain how current arrows are drawn in a schematic diagram of a circuit, and identify that the arrows are not vectors. Key Ideas ● An Electric Current i in a conductor is defined by i = dq ___ dt , where dq is the amount of positive charge that passes in time dt. ● By convention, the direction of Electric Current is taken as the direction in which positive charge carriers would move even though (normally) only conduction electrons can move. What Is Physics? In the last five chapters we discussed electrostatics—the physics of stationary charges. In this and the next chapter, we discuss the physics of Electric Currents— that is, charges in motion. Examples of Electric Currents abound and involve many professions. Mete- orologists are concerned with lightning and with the less dramatic slow flow of charge through the atmosphere. Biologists, physiologists, and engineers work- ing in medical technology are concerned with the nerve currents that control muscles and especially with how those currents can be reestablished after spi- nal cord injuries. Electrical engineers are concerned with countless electrical systems, such as power systems, lightning protection systems, information stor- age systems, and music systems.
eBook - PDF- Edward Purcell(Author)
- 2011(Publication Date)
- Cambridge University Press(Publisher)
4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 Electric Current and Current Density Steady Currents and Charge Conservation Electrical Conductivity and Ohm 's Law The Physics of Electrical Conduction Conduction in Metals Semiconductors Circuits and Circuit Elements Energy Dissipation in Current Flow Electromotive Force and the Voltaic Cell Networks with Voltage Sources Variable Currents in Capacitors and Resistors Problems Electric CurrentS 124 126 128 133 142 144 148 153 154 157 159 161 124 FIGURE 4.1 (a) A swarm of charged particles all moving with the same velocity u. The frame has area a. The particles which will pass through the frame in the next ;:;'1 sec are those now contained in the oblique prism (b). The prism has base area a and altitude u t:.t cos e, hence its volume is au t:.t cos 0 or a . u t:.t. (a) (b) --- 1'\'"--- 1\ I \ I \ I I I I I I I \ \ I 0 \ \I CHAPTER FOUR Electric Current AND CURRENT DENSITY 4.1 An Electric Current is charge in motion. The carriers of the charge can be physical particles like electrons or protons, which may or may not be attached to larger objects, atoms or molecules. Here we are not concerned with the nature of the charge carriers but only with the net transport of electric charge their motion causes. The Electric Current in a wire is the amount of charge passing a fixed mark on the wire in unit time. In CGS units current will be expressed in esu/sec. The SI unit is coulombs/sec, or amperes (amps). A current of 1 ampere is the same as a current of 2.998 X 10 9 esu/sec, which is equivalent to 6.24 X 10 18 elementary electronic charges per second. It is the net charge transport that counts, with due regard to sign. Negative charge moving east is equivalent to positive charge moving west.
eBook - PDF- Bhag Singh Guru, Hüseyin R. Hiziroglu(Authors)
- 2009(Publication Date)
- Cambridge University Press(Publisher)
The motion of charges is said to constitute a current. Because the charges are in motion only for a short duration, the current is commonly called a transient current. We define the current as the rate at which the charge is transported past a given point in a conducting medium. That is, i = dq dt (4.1) where dq is the amount of charge that flows past some given point in time dt. We have used the lowercase letter i for the current to indicate that it is generally a function of time. The SI unit of current is the ampere 137 138 4 Steady Electric Currents (A), named in honor of the French physicist, Andr´ e Marie Amp` ere. A current of one ampere corresponds to the transportation of one coulomb of charge in one second. In this chapter, we will limit our discussion to steady currents ; that is, only those currents that are constant in time. We will use the uppercase letter I to represent a steady current. A steady current is also called a direct current. 4.2 Nature of current and current density ................................. In this section, we introduce two types of currents: conduction current and convection current. We will also discuss the current per unit area, or current density, for both convection and conduction currents. 4.2.1 Conduction current In metals, like copper, silver, or gold, the free charge carriers are pre-dominantly electrons. More precisely, it is the valence electron of an atom which is free to contribute to the conduction process. An electron which may be considered as not being attached to any particular atom is called a free electron. A free electron has the capability of moving through a whole crystal lattice. However, the heavy, positively charged ions are relatively fixed at their regular positions in the crystal lattice and do not contribute to the current in the metal. Thus, the current in a metal conductor, called conduction current , is simply a flow of electrons.
eBook - PDF- Pierluigi Zotto, Sergio Lo Russo, Paolo Sartori(Authors)
- 2022(Publication Date)
- Società Editrice Esculapio(Publisher)
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. An infini- tesimal charge passes through such a section during an infinitesimal time, thus current intensity is defined as i = dq dt . The direction of the positive charge, which displaces from the positive pole to the negative pole of the applied generator, is arbi- trarily chosen as the direction of the flowing Electric Current, independently of the sign of the actual charge carriers. In a conductor in electrostatic equilibrium, thermal agitation makes free charges to move randomly, so the number of free charges en- + + + + + + + + V + V – A i – – – – – – – V + V – A i – Electric Current Chapter 7 100 tering or exiting per unit time a transverse section of the conductor match on average, im- plying that the current intensity is null. The current intensity unit of measurement is the ampère, symbol A, with [A] = [C][s –1 ]: 1 ampère is the intensity corresponding to a flow through a given surface of 1 e = 1 1.602176634 × 10 −19 ~ 6.25 × 10 18 elementary charges per second. 7.4 Current Density Consider an infinitesimal section dA of a conductor cut an an angle θ with respect to its cross section. The amount of charge which crosses this surface in an infinitesimal time is the one contained in a volume dV = ddA cos θ = vdtdA cos θ where v is the average speed of the charges during their motion towards the positive or the negative pole, called drift velocity. Such an amount of charge is dq = nq 0 dV = nq 0 vdtdA cos θ , where n = dN dV is the number of free charges per unit volume in the conductor, and q0 is the magnitude of a single free charge. The current intensity is i = dq dt = nq 0 vdA cos θ = nq 0 vdAi u n .
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
Surface Figure 20.3 The Electric Current is the amount of charge per unit time that passes through an imaginary surface that is perpendicular to the motion of the charges. EXAMPLE 1 | A Pocket Calculator The battery pack of a pocket calculator has a voltage* of 3.0 V and delivers a current of 0.17 mA. In one hour of operation, (a) how much charge flows in the circuit and (b) how much energy does the battery deliver to the calculator circuit? Reasoning Since current is defined as charge per unit time, the charge that flows in one hour is the product of the current and the time (3600 s). The charge that leaves the 3.0-V battery pack has 3.0 joules of energy per coulomb of charge. Thus, the total energy delivered to the calculator circuit is the charge (in coulombs) times the energy per unit charge (in volts or joules/coulomb). Solution (a) The charge that flows in one hour can be determined from Equation 20.1: Dq 5 I( Dt) 5 (0.17 3 10 23 A)(3600 s) 5 0.61 C (b) The energy delivered to the calculator circuit is Energy 5 Charge 3 Energy Charge 5 (0.61 C)(3.0 V) 5 1.8 J u Battery voltage *The potential difference between two points, such as the terminals of a battery, is commonly called the voltage between the points. Conventional current, I Electron flow Device + – Figure 20.4 In a circuit, electrons actually flow through the metal wires. However, it is customary to use a conventional current I to describe the flow of charges. 484 Chapter 20 | Electric Circuits conventional current originating from the positive terminal and moving clockwise around the circuit. A conventional current of hypothetical positive charges is consistent with our earlier use of a positive test charge for defining electric fields and potentials. The direction of conventional current is always from a point of higher potential toward a point of lower potential—that is, from the positive toward the negative terminal. In this text, the symbol I stands for conventional current.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
Surface Figure 20.3 The Electric Current is the amount of charge per unit time that passes through an imaginary surface that is perpendicular to the motion of the charges. EXAMPLE 1 | A Pocket Calculator The battery pack of a pocket calculator has a voltage* of 3.0 V and delivers a current of 0.17 mA. In one hour of operation, (a) how much charge flows in the circuit and (b) how much energy does the battery deliver to the calculator circuit? Reasoning Since current is defined as charge per unit time, the charge that flows in one hour is the product of the current and the time (3600 s). The charge that leaves the 3.0-V battery pack has 3.0 joules of energy per coulomb of charge. Thus, the total energy delivered to the calculator circuit is the charge (in coulombs) times the energy per unit charge (in volts or joules/coulomb). Solution (a) The charge that flows in one hour can be determined from Equation 20.1: Dq 5 I( Dt) 5 (0.17 3 10 23 A)(3600 s) 5 0.61 C (b) The energy delivered to the calculator circuit is Energy 5 Charge 3 Energy Charge 5 (0.61 C)(3.0 V) 5 1.8 J u Battery voltage *The potential difference between two points, such as the terminals of a battery, is commonly called the voltage between the points. Conventional current, I Electron flow Device + – Figure 20.4 In a circuit, electrons actually flow through the metal wires. However, it is customary to use a conventional current I to describe the flow of charges. 20.2 | Ohm’s Law 543 conventional current originating from the positive terminal and moving clockwise around the circuit. A conventional current of hypothetical positive charges is consistent with our earlier use of a positive test charge for defining electric fields and potentials. The direction of conventional current is always from a point of higher potential toward a point of lower potential—that is, from the positive toward the negative terminal. In this text, the symbol I stands for conventional current.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
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. This flow of charge is known as an Electric Current. The Electric Current I is defined as the amount of charge per unit time that crosses the imaginary surface, as in Figure 20.3, in much the same sense that a river current is the amount of water per unit time that is flowing past a certain point. If the rate is constant, the current is I = ∆q ∆t (20.1) If the rate of flow is not constant, then Equation 20.1 gives the average current. Since the units for charge and time are the coulomb (C) and the second (s), the SI unit for current is a coulomb per second (C/s). One coulomb per second is referred to as an ampere (A), after the French math- ematician André-Marie Ampère (1775–1836). If the charges move around a circuit in the same direction at all times, the current is said to be direct current (dc), which is the kind produced by batteries. In contrast, the current is said to be alternating current (ac) when the charges move first one way and then the opposite way, changing direction from moment to moment. Many energy sources produce alternating current—for example, generators at power companies and microphones. Example 1 deals with direct current. 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.
- Jerry D. Wilson, Anthony J. Buffa, Bo Lou(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
He advanced an “electric fluid” theory of electric- ity. When objects became electrostatically charged, he postulated that the fluid moved from a place of excess (“positive”) to a place of deficit (“nega- tive”). The idea or convention that electricity “flowed” from positive to negative was adopted and is applied even today. + – Battery Resistor Capacitor Wire Open switch Closed switch + – Putting it together A complete circuit Two unconnected wires Two connected wires or ▲ FIGURE 17.5 Some circuit symbols The symbols shows are the standard and common ones used in circuit diagraming. ΔV ΔV ΔV AB A B S 1 S 2 ▲ FIGURE 17.6 What happens to the voltage? See text for discussion. 447 Electric Current and Resistance Quantitatively, Electric Current (I ) is defined as time rate of flow of net charge. Assuming dc conditions and thus a steady charge flow, if a net charge Δq passes a point in a wire in a time interval Δt, then the Electric Current at that point is defined as I q t = Δ Δ (Electric Current) (17.1) SI unit of current: coulomb per second (C/s), or ampere (A) The coulomb per second is named the ampere (A), in honor of the French physicist André Ampère (1775–1836), an early investigator of electrical and magnetic phenomena. In everyday usage, the ampere is commonly shortened to amp. Thus, a cur- rent of 10 A is read as “ten amps.” Smaller currents are expressed in milliamperes (mA, or 10 −3 A), microamperes (μA, or 10 −6 A), or nanoamperes (nA, or 10 −9 A). These are also commonly short- ened to milliamps, microamps, and nanoamps, respectively. In a typical household circuit, it is not unusual for wires to carry several amps of current. To understand the relationship between charge and current, consider Example 17.2. EXAMPLE 17.2: COUNTING ELECTRONS – CURRENT AND CHARGE Suppose a steady current of 0.50 A in a wire lasts for 2.0 min. How much charge moves past a point in this wire during this time? How many electrons does this represent? THINKING IT THROUGH.
eBook - PDF- G. G. Járos, B. J. Meyer(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
SECTION II ELECTRICITY This page intentionally left blank CHAPTER 4 The Origin of Electricity and the Flow of Electricity in Conductors Introduction Despite the fact that electricity is used daily in all spheres of life, and that its fundamental principles are relatively simple, many people think of it as something mysterious. The study of electricity deals mainly with the study of the behaviour of charged particles. The problem arises from the fact that man has no sense organ with which he can perceive charged particles. He can only sense the effects of their flow, as for example, when he puts his fingers into a wall plug, or turns on a light switch, or places the two poles of a battery on his tongue. In the first instance, his muscles will contract painfully, in the second, he will perceive light, while in the third instance he will experience an acid taste on his tongue. Until relatively recently we were unable to see viruses, but were nevertheless aware of their existence and of their effects on tissues, including the tissues of the human body. According to the symptoms and signs of diseases produced by viruses, certain properties of these viruses could be inferred. Today we have advanced in the technical field to such an extent that we are able to see viruses by means of very strong microscopes (incidentally microscopes that are based on the principles of electricity). What is more, the viruses appear pre-cisely as was predicted from their properties. What applied to viruses, holds for electricity today: we cannot see the structural units of electricity, even with the most sensitive microscope, but we can infer certain details about them from their behaviour. Atoms and molecules All substances existing in nature can be divided into elements and compounds. There are over a hundred known elements. Each of these is represented by a symbol, e.g., hydrogen by H, oxygen by O, iron by Fe, etc. Compounds are formed by the chemical reaction between two or more elements.
eBook - PDFThe Electronics Companion
Devices and Circuits for Physicists and Engineers, 2nd Edition
- Anthony C. Fischer-Cripps(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
of charge carriers per unit volume t A v n q t A v n q Q Q ) ( ) ( 2 2 2 1 1 1 A t v n q Q 1 1 1 The total charge passing any given point in coulombs per second is called A t v n q Q ) ( ) ( 2 2 2 The total charge passing any given point in coulombs per second is called Electric Current: ) ( ) ( ) ( ) ( 2 2 2 1 1 1 2 2 2 1 1 1 v n q v n q A I A v n q A v n q t Q Q Current density J = I/A amps m 2 or coulombs m 2 s 1 1 amp is the rate of flow of electric charge when one coulomb of electric charge passes a given point in an electric circuit in one second. In metallic conductors, the mobile charge carriers are negatively charged electrons; hence n 1 = 0 dt dq i Lower case quantities refer to instantaneous values. Upper case refers to steady-state or DC values In general, charged electrons; hence n 1 0. Note that the amp is a measure of quantity of charge per second and on its own provides no information about the net drift velocity of the charge carriers ( 0.1 mm s 1 ). refers to steady state or DC values. 1.6 Conventional Current 7 1. Electricity Electric Current involves the net flow of electrical charge carriers which in a metallic electrical charge carriers, which, in a metallic conductor, are negatively charged electrons. Often in circuit analysis, the physical nature of the actual flow of charge is not important - whether it be the flow of free electrons in a wire or the movement of positive ions in a solution. But, in the 1830s, no one had heard of the electron. At that time, Faraday noticed that when current flowed through a wire connected to a chemical cell, one electrode, the anode, lost weight and the other electrode, the cathode, gained weight. Hence it was concluded that charge carriers, whatever they were, flowed through the wire from the anode to the cathode.
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