Why is I the symbol for electric current? - Quora Quora Written Jun 18, 2011 It's from the French for intensite de courant or current intensity in Engish. Wikipedia elaborates: The conventional symbol for current is I, which may seem puzzling. It originates from the French phrase intensité de courant, or in Englishcurrent intensity.[3][4] This phrase is frequently used when discussing the value of an electric current, especially in older texts; modern practice often shortens this to simply current but current intensity is still used in many recent textbooks. The I symbol was used by André-Marie Ampère himself, after whom the unit of electric current is named, in formulating the eponymous Ampère's force law which he discovered in 1820.[5] The notation travelled from France to England where it became standard, although at least one journal did not change from using C to I until 1896.[6] Electric Current - DJI Wiki Electric Current Jump to: navigation , search An electric current is a flow of electric charge through an electrical conductor. Electric currents flow when there is voltage present across a conductor.

In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma. The SI unit for measuring an electric current is the ampere, which is the flow of electric charges through a surface at the rate of one coulomb per second. Electric current can be measured using an ammeter. Electric currents cause many effects, notably heating, but also induce magnetic fields, which are widely used for motors, inductors and generators. Contents 8 Electromagnetism Symbol The conventional symbol for current is I, which originates from the French phrase intensité de courant, or in English current intensity. This phrase is frequently used when discussing the value of an electric current, especially in older texts; modern practice often shortens this to simply current but current intensity is still used in many recent textbooks. The I symbol was used by André-Marie Ampère, after whom the unit of electric current is named, in formulating the eponymous Ampère's force law which he discovered in 1820.

The notation travelled from France to Britain, where it became standard, although at least one journal did not change from using C to I until 1896. Conventions The electrons, the charge carriers in an electrical circuit, flow in the opposite direction of the conventional electric current. The symbol for a battery in a circuit diagram. A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention for the direction of current which is independent of the type of charge carriers is needed. The direction of conventional current is defined arbitrarily to be the direction of the flow of positive charges. In metals, which make up the wires and other conductors in most electrical circuits, the positive charges are immobile, and the charge carriers are electrons. Because the electron carries negative charge, the electron motion in a metal conductor is in the direction opposite to that of conventional (or electric) current. Reference direction When analyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosen reference direction.

When the circuit is solved, the circuit element currents may have positive or negative values. A negative value means that the actual direction of current through that circuit element is opposite that of the chosen reference direction. In electronic circuits the reference current directions are usually chosen so that all currents flow toward ground. This often matches conventional current direction, because in many circuits the power supply voltage is positive with respect to ground. Ohms law Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance,= one arrives at the usual mathematical equation that describes this relationship: I = V/R where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.= AC and DC The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage. Direct current Direct Current (DC) is the unidirectional flow of electric charge.

Direct current is produced by sources such as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was galvanic current. Alternating current In Alternating Current (AC, also ac), the movement of electric charge periodically reverses direction. In direct current (DC, also dc), the flow of electric charge is only in one direction. AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal. Occurrences Natural examples include lightning and the solar wind, the source of the polar auroras (the aurora borealis and aurora australis).

The human caused occurrences of electric current includes the flow of conduction electrons in metal wires, such as the overhead power lines that deliver electrical energy across long distances and the smaller wires within electrical and electronic equipment. Eddy currents are electric currents that occur in conductors exposed to changing magnetic fields. Similarly, electric currents occur, particularly in the surface, of conductors exposed to electromagnetic waves. When oscillating electric currents flow at the correct voltages within radio antennas, radio waves are generated. In electronics, other forms of electric current include the flow of electrons through resistors or through the vacuum in a vacuum tube, the flow of ions inside a battery or a neuron, and the flow of holes within a semiconductor. Current measurement Current can be measured using an ammeter. At the circuit level, there are various techniques that can be used to measure current: Shunt resistors Hall effect current sensor transducers Transformers (however DC cannot be measured) Magnetoresistive field sensors Resistive heating Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841.

Joule immersed a length of wire in a fixed mass of water and measured the temperature rise due to a known current flowing through the wire for a 30 minute period. By varying the current and the length of the wire he deduced that the heat produced was proportional to the square of the current multiplied by the electrical resistance of the wire. Q \propto I^2 \cdot R This relationship is known as Joule's First Law. The SI unit of energy was subsequently named the joule and given the symbol J. The commonly known unit of power, the watt, is equivalent to one joule per second. Electromagnetism According to Ampère's law, an electric current produces a magnetic field. Electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire that persists as long as the current flows. Magnetism can also produce electric currents. When a changing magnetic field is applied to a conductor, an EMF is produced, and when there is a suitable path, this causes current to flow. Electric current can be directly measured with a galvanometer, but this method involves breaking the electrical circuit, which is sometimes inconvenient.

Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. Devices used for this include Hall effect sensors, current clamps, current transformers, and Rogowski coils. The theory of Special Relativity allows one to transform the magnetic field into a static electric field for an observer moving at the same speed as the charge in the diagram. The amount of current is particular to a reference frame. Conventional Current vs Electron Flow Conventional Current vs Electron Flow Press a button at top. Sound Conventional Current assumes that current flows out of the positive terminal, through the circuit and into the negative terminal of the source. This was the convention chosen during the discovery of electricity. They were wrong! Electron Flow is what actually happens and electrons flow out of the negative terminal, through the circuit and into the positive terminal of the source. Both Conventional Current and Electron Flow are used. Many textbooks are available in both formats. Floyd, 1989, Principles of Electric Circuits, 5th edition, Conventional Current Version. Floyd, 1990, Principles of Electric Circuits, 4th edition, Electron Flow Version.

In fact, it makes no difference which way current is flowing as long as it is used consistently. The direction of current flow does not affect what the current does. In general, high school Physics and two year technician programs use Electron Flow. But three year technologist and university engineering programs use Conventional Current. Certain symbols (ex. diodes and transistors) and rules (ex. Right-hand rules) were created using Conventional Current. Changing from Conventional Current to Electron Flow would cause a degree of confusion for old and new students and errors would occur, so Conventional Current was kept to ensure there was no confusion with those already trained with Conventional Current. Two systems may seem confusing, but as long as usage is consistent, it really is not! You must realize what convention is being used because the rules change. Ex. Right-Hand rules in Conventional Current become Left-Hand rules in Electron Flow. Example Throughout this course, Conventional Current is used. Therefore always assume current flows out of the positive terminal of the source.

Conventional Versus Electron Flow : Basic Concepts Of Electricity - Electronics Textbook Conventional Versus Electron Flow Conventional Versus Electron Flow Chapter 1 - Basic Concepts Of Electricity “The nice thing about standards is that there are so many of them to choose from.” Andrew S. Tanenbaum, computer science professor When Benjamin Franklin made his conjecture regarding the direction of charge flow (from the smooth wax to the rough wool), he set a precedent for electrical notation that exists to this day, despite the fact that we know electrons are the constituent units of charge, and that they are displaced from the wool to the wax—not from the wax to the wool—when those two substances are rubbed together. This is why electrons are said to have a negative charge: because Franklin assumed electric charge moved in the opposite direction that it actually does, and so objects he called “negative” (representing a deficiency of charge) actually have a surplus of electrons. By the time the true direction of electron flow was discovered, the nomenclature of “positive” and “negative” had already been so well established in the scientific community that no effort was made to change it, although calling electrons “positive” would make more sense in referring to “excess” charge.

You see, the terms “positive” and “negative” are human inventions, and as such have no absolute meaning beyond our own conventions of language and scientific description. Franklin could have just as easily referred to a surplus of charge as “black” and a deficiency as “white,” in which case scientists would speak of electrons having a “white” charge (assuming the same incorrect conjecture of charge position between wax and wool). However, because we tend to associate the word “positive” with “surplus” and “negative” with “deficiency,” the standard label for electron charge does seem backward. Because of this, many engineers decided to retain the old concept of electricity with “positive” referring to a surplus of charge, and label charge flow (current) accordingly. This became known as conventional flow notation: Others chose to designate charge flow according to the actual motion of electrons in a circuit. This form of symbology became known as electron flow notation: In conventional flow notation, we show the motion of charge according to the (technically incorrect) labels of + and -. This way the labels make sense, but the direction of charge flow is incorrect.

In electron flow notation, we follow the actual motion of electrons in the circuit, but the + and - labels seem backward. Does it matter, really, how we designate charge flow in a circuit? Not really, so long as we’re consistent in the use of our symbols. You may follow an imagined direction of current (conventional flow) or the actual (electron flow) with equal success insofar as circuit analysis is concerned. Concepts of voltage, current, resistance, continuity, and even mathematical treatments such as Ohm’s Law (chapter 2) and Kirchhoff’s Laws (chapter 6) remain just as valid with either style of notation. You will find conventional flow notation followed by most electrical engineers, and illustrated in most engineering textbooks. Electron flow is most often seen in introductory textbooks (this one included) and in the writings of professional scientists, especially solid-state physicists who are concerned with the actual motion of electrons in substances. These preferences are cultural, in the sense that certain groups of people have found it advantageous to envision electric current motion in certain ways. Being that most analyses of electric circuits do not depend on a technically accurate depiction of charge flow, the choice between conventional flow notation and electron flow notation is arbitrary . . . almost.

Many electrical devices tolerate real currents of either direction with no difference in operation. Incandescent lamps (the type utilizing a thin metal filament that glows white-hot with sufficient current), for example, produce light with equal efficiency regardless of current direction. They even function well on alternating current (AC), where the direction changes rapidly over time. Conductors and switches operate irrespective of current direction, as well. The technical term for this irrelevance of charge flow is nonpolarization. We could say then, that incandescent lamps, switches, and wires are nonpolarized components. Conversely, any device that functions differently on currents of different direction would be called a polarized device. There are many such polarized devices used in electric circuits. Most of them are made of so-called semiconductor substances, and as such aren’t examined in detail until the third volume of this book series. Like switches, lamps, and batteries, each of these devices is represented in a schematic diagram by a unique symbol. As one might guess, polarized device symbols typically contain an arrow within them, somewhere, to designate a preferred or exclusive direction of current. This is where the competing notations of conventional and electron flow really matter.

Because engineers from long ago have settled on conventional flow as their “culture’s” standard notation, and because engineers are the same people who invent electrical devices and the symbols representing them, the arrows used in these devices’ symbols all point in the direction of conventional flow, not electron flow. That is to say, all of these devices’ symbols have arrow marks that point against the actual flow of electrons through them. Perhaps the best example of a polarized device is the diode. A diode is a one-way “valve” for electric current, analogous to a check valve for those familiar with plumbing and hydraulic systems. Ideally, a diode provides unimpeded flow for current in one direction (little or no resistance), but prevents flow in the other direction (infinite resistance). Its schematic symbol looks like this: Placed within a battery/lamp circuit, its operation is as such: When the diode is facing in the proper direction to permit current, the lamp glows. Otherwise, the diode blocks all electron flow just like a break in the circuit, and the lamp will not glow.

If we label the circuit current using conventional flow notation, the arrow symbol of the diode makes perfect sense: the triangular arrowhead points in the direction of charge flow, from positive to negative: On the other hand, if we use electron flow notation to show the true direction of electron travel around the circuit, the diode’s arrow symbology seems backward: For this reason alone, many people choose to make conventional flow their notation of choice when drawing the direction of charge motion in a circuit. If for no other reason, the symbols associated with semiconductor components like diodes make more sense this way. However, others choose to show the true direction of electron travel so as to avoid having to tell themselves, “just remember the electrons are actually moving the other way” whenever the true direction of electron motion becomes an issue. In this series of textbooks, I have committed to using electron flow notation. Ironically, this was not my first choice. I found it much easier when I was first learning electronics to use conventional flow notation, primarily because of the directions of semiconductor device symbol arrows. Later, when I began my first formal training in electronics, my instructor insisted on using electron flow notation in his lectures.

In fact, he asked that we take our textbooks (which were illustrated using conventional flow notation) and use our pens to change the directions of all the current arrows so as to point the “correct” way! His preference was not arbitrary, though. In his 20-year career as a U.S. Navy electronics technician, he worked on a lot of vacuum-tube equipment. Before the advent of semiconductor components like transistors, devices known as vacuum tubes or electron tubes were used to amplify small electrical signals. These devices work on the phenomenon of electrons hurtling through a vacuum, their rate of flow controlled by voltages applied between metal plates and grids placed within their path, and are best understood when visualized using electron flow notation. When I graduated from that training program, I went back to my old habit of conventional flow notation, primarily for the sake of minimizing confusion with component symbols, since vacuum tubes are all but obsolete except in special applications. Collecting notes for the writing of this book, I had full intention of illustrating it using conventional flow. Years later, when I became a teacher of electronics, the curriculum for the program I was going to teach had already been established around the notation of electron flow.

Oddly enough, this was due in part to the legacy of my first electronics instructor (the 20-year Navy veteran), but that’s another story entirely! Not wanting to confuse students by teaching “differently” from the other instructors, I had to overcome my habit and get used to visualizing electron flow instead of conventional. Because I wanted my book to be a useful resource for my students, I begrudgingly changed plans and illustrated it with all the arrows pointing the “correct” way. Oh well, sometimes you just can’t win! On a positive note (no pun intended), I have subsequently discovered that some students prefer electron flow notation when first learning about the behavior of semiconductive substances. Also, the habit of visualizing electrons flowing against the arrows of polarized device symbols isn’t that difficult to learn, and in the end I’ve found that I can follow the operation of a circuit equally well using either mode of notation.

Still, I sometimes wonder if it would all be much easier if we went back to the source of the confusion—Ben Franklin’s errant conjecture—and fixed the problem there, calling electrons “positive” and protons “negative.” Physics equations/Current and current density - Wikiversity Physics equations/Current and current density From Wikiversity Electric current [1] [ edit ] A simple electric circuit, where current is represented by the letter i. The relationship between the voltage (V), resistance (R), and current (I) is V=IR; this is known as Ohm's Law . The SI unit for measuring an electric current is the ampere , which is the flow of electric charges through a surface at the rate of one coulomb per second.

Electric current can be measured using an ammeter .More generally, electric current can be represented as the rate at which charge flows through a given surface as: I {\displaystyle I={\frac {\mathrm {d} Q}{\mathrm {d} t}}\,.} The conventional symbol for current is I , which originates from the French phrase intensité de courant, The I {\displaystyle I} symbol was used by André-Marie Ampère , after whom the unit of electric current is named. The electrons , the charge carriers in an electrical circuit, flow in the opposite direction of the conventional electric current. The symbol for a battery in a circuit diagram . The direction of the current is defined so the arrow would indicate the flow of positive charges. In metals, which make up the wires and other conductors in most electrical circuits , the positive charges are immobile, and the charge carriers are electrons . Because the electrons carry negative charge, their motion in a metal conductor is in the direction opposite to that of conventional (or electric) current. Ohm's law[ edit ] Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points.

Introducing the constant of proportionality, the resistance , I R {\displaystyle I={\frac {V}{R}}} where I is the current through the conductor in units of amperes , V is the potential difference measured across the conductor in units of volts , and R is the resistance of the conductor in units of ohms . More specifically, Ohm's law states that the R in this relation is constant, independent of the current. AC and DC[ edit ] The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage . Direct current (DC) is the unidirectional flow of electric charge . Direct current is produced by sources such as batteries , thermocouples , solar cells , and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors , insulators , or even through a vacuum as in electron or ion beams . The electric charge flows in a constant direction, distinguishing it from alternating current (AC). In alternating current (AC, also ac), the movement of electric charge periodically reverses direction.

In direct current (DC, also dc), the flow of electric charge is only in one direction. AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave . In certain applications, different waveforms are used, such as triangular or square waves . Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated ) onto the AC signal. Current density and Ohm's law[ edit ] Main source: Current density Current density is a measure of the density of an electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI units , the current density is measured in amperes per square metre. I being the elementary charge and n {\displaystyle n} the electron density. The carriers move in the direction of decreasing concentration, so for electrons a positive current results for a positive density gradient. If the carriers are holes, replace electron density n by the negative of the hole density p . In linear anisotropic materials, σ, ρ and D are tensors .

In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform. In such conditions, Ohm's law states that the current is directly proportional to the potential difference between two ends (across) of that metal (ideal) resistor (or other ohmic device ): I is the current, measured in amperes; V is the potential difference , measured in volts ; and R {\displaystyle R} is the resistance , measured in ohms . For alternating currents , especially at higher frequencies, skin effect causes the current to spread unevenly across the conductor cross-section, with higher density near the surface, thus increasing the apparent resistance. Drift speed[ edit ] The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas . In order for there to be a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the opposite direction of the electric field.

The speed at which they drift can be calculated from the equation: I Electric Current Current Electricity - Lesson 2 - Electric Current Electric Current Common Misconceptions Regarding Electric Circuits If the two requirements of an electric circuit are met, then charge will flow through the external circuit. It is said that there is a current - a flow of charge. Using the word current in this context is to simply use it to say that something is happening in the wires - charge is moving. Yet current is a physical quantity that can be measured and expressed numerically. As a physical quantity, current is the rate at which charge flows past a point on a circuit. As depicted in the diagram below, the current in a circuit can be determined if the quantity of charge Q passing through a cross section of a wire in a time t can be measured. The current is simply the ratio of the quantity of charge and time. Current is a rate quantity. There are several rate quantities in physics. For instance, velocity is a rate quantity - the rate at which an object changes its position. Mathematically, velocity is the position change per time ratio. Acceleration is a rate quantity - the rate at which an object changes its velocity. Mathematically, acceleration is the velocity change per time ratio.

And power is a rate quantity - the rate at which work is done on an object. Mathematically, power is the work per time ratio. In every case of a rate quantity, the mathematical equation involves some quantity over time. Thus, current as a rate quantity would be expressed mathematically as Note that the equation above uses the symbol I to represent the quantity current. As is the usual case, when a quantity is introduced in The Physics Classroom, the standard metric unit used to express that quantity is introduced as well. The standard metric unit for current is the ampere. Ampere is often shortened to Amp and is abbreviated by the unit symbol A. A current of 1 ampere means that there is 1 coulomb of charge passing through a cross section of a wire every 1 second. 1 ampere = 1 coulomb / 1 second To test your understanding, determine the current for the following two situations. Note that some extraneous information is given in each situation. Click the Check Answer button to see if you are correct. A 2 mm long cross section of wire is isolated and 20 C of charge is determined to pass through it in 40 s.

A 1 mm long cross section of wire is isolated and 2 C of charge is determined to pass through it in 0.5 s. I = _____ Ampere Conventional Current Direction The particles that carry charge through wires in a circuit are mobile electrons. The electric field direction within a circuit is by definition the direction that positive test charges are pushed. Thus, these negatively charged electrons move in the direction opposite the electric field. But while electrons are the charge carriers in metal wires, the charge carriers in other circuits can be positive charges, negative charges or both. In fact, the charge carriers in semiconductors, street lamps and fluorescent lamps are simultaneously both positive and negative charges traveling in opposite directions. Ben Franklin, who conducted extensive scientific studies in both static and current electricity, envisioned positive charges as the carriers of charge. As such, an early convention for the direction of an electric current was established to be in the direction that positive charges would move. The convention has stuck and is still used today. The direction of an electric current is by convention the direction in which a positive charge would move. Thus, the current in the external circuit is directed away from the positive terminal and toward the negative terminal of the battery.

Electrons would actually move through the wires in the opposite direction. Knowing that the actual charge carriers in wires are negatively charged electrons may make this convention seem a bit odd and outdated. Nonetheless, it is the convention that is used worldwide and one that a student of physics can easily become accustomed to. Current versus Drift Speed Current has to do with the number of coulombs of charge that pass a point in the circuit per unit of time. Because of its definition, it is often confused with the quantity drift speed. Drift speed refers to the average distance traveled by a charge carrier per unit of time. Like the speed of any object, the drift speed of an electron moving through a wire is the distance to time ratio. The path of a typical electron through a wire could be described as a rather chaotic, zigzag path characterized by collisions with fixed atoms. Each collision results in a change in direction of the electron. Yet because of collisions with atoms in the solid network of the metal conductor, there are two steps backwards for every three steps forward. With an electric potential established across the two ends of the circuit, the electron continues to migrate forward. Progress is always made towards the positive terminal.

Yet the overall effect of the countless collisions and the high between-collision speeds is that the overall drift speed of an electron in a circuit is abnormally low. A typical drift speed might be 1 meter per hour. That is slow! One might then ask: How can there by a current on the order of 1 or 2 ampere in a circuit if the drift speed is only about 1 meter per hour? The answer is: there are many, many charge carriers moving at once throughout the whole length of the circuit. Current is the rate at which charge crosses a point on a circuit. A high current is the result of several coulombs of charge crossing over a cross section of a wire on a circuit. If the charge carriers are densely packed into the wire, then there does not have to be a high speed to have a high current. That is, the charge carriers do not have to travel a long distance in a second, there just has to be a lot of them passing through the cross section. Current does not have to do with how far charges move in a second but rather with how many charges pass through a cross section of wire on a circuit.

To illustrate how densely packed the charge carriers are, we will consider a typical wire found in household lighting circuits - a 14-gauge copper wire. In a 0.01 cm-long (very thin) cross-sectional slice of this wire, there would be as many as 3.51 x 1020 copper atoms. Each copper atom has 29 electrons; it would be unlikely that even the 11 valence electrons would be in motion as charge carriers at once. If we assume that each copper atom contributes just a single electron, then there would be as much as 56 coulombs of charge within a thin 0.01-cm length of the wire. With that much mobile charge within such a small space, a small drift speed could lead to a very large current. To further illustrate this distinction between drift speed and current, consider this racing analogy. Suppose that there was a very large turtle race with millions and millions of turtles on a very wide race track. Turtles do not move very fast - they have a very low drift speed.

Suppose that the race was rather short - say 1 meter in length - and that a large percentage of the turtles reached the finish line at the same time - 30 minutes after the start of the race. In such a case, the current would be very large - with millions of turtles passing a point in a short amount of time. In this analogy, speed has to do with how far the turtles move in a certain amount of time; and current has to do with how many turtles cross the finish line in a certain amount of time. The Nature of Charge Flow Once it has been established that the average drift speed of an electron is very, very slow, the question soon arises: Why does the light in a room or in a flashlight light immediately after the switched is turned on? Wouldn't there be a noticeable time delay before a charge carrier moves from the switch to the light bulb filament? The answer is NO! and the explanation of why reveals a significant amount about the nature of charge flow in a circuit. As mentioned above , charge carriers in the wires of electric circuits are electrons. These electrons are simply supplied by the atoms of copper (or whatever material the wire is made of) within the metal wire.

Once the switch is turned to on, the circuit is closed and there is an electric potential difference is established across the two ends of the external circuit. The electric field signal travels at nearly the speed of light to all mobile electrons within the circuit, ordering them to begin marching. As the signal is received, the electrons begin moving along a zigzag path in their usual direction. Thus, the flipping of the switch causes an immediate response throughout every part of the circuit, setting charge carriers everywhere in motion in the same net direction. While the actual motion of charge carriers occurs with a slow speed, the signal that informs them to start moving travels at a fraction of the speed of light. The electrons that light the bulb in a flashlight do not have to first travel from the switch through 10 cm of wire to the filament. Rather, the electrons that light the bulb immediately after the switch is turned to on are the electrons that are present in the filament itself. As the switch is flipped, all mobile electrons everywhere begin marching; and it is the mobile electrons present in the filament whose motion are immediately responsible for the lighting of its bulb. As those electrons leave the filament, new electrons enter and become the ones that are responsible for lighting the bulb.

The electrons are moving together much like the water in the pipes of a home move. When a faucet is turned on, it is the water in the faucet that emerges from the spigot. One does not have to wait a noticeable time for water from the entry point to your home to travel through the pipes to the spigot. The pipes are already filled with water and water everywhere within the water circuit is set in motion at the same time. The picture of charge flow being developed here is a picture in which charge carriers are like soldiers marching along together, everywhere at the same rate. Their marching begins immediately in response to the establishment of an electric potential across the two ends of the circuit. There is no place in the electrical circuit where charge carriers become consumed or used up. While the energy possessed by the charge may be used up (or a better way of putting this is to say that the electric energy is transformed to other forms of energy), the charge carriers themselves do not disintegrate, disappear or otherwise become removed from the circuit. And there is no place in the circuit where charge carriers begin to pile up or accumulate.

The rate at which charge enters the external circuit on one end is the same as the rate at which charge exits the external circuit on the other end. Current - the rate of charge flow - is everywhere the same. Charge flow is like the movement of soldiers marching in step together, everywhere at the same rate. F. top of the slide Answer: B Current is the rate at which something flows. Electric current is the rate at which electric charge flows past a point on the electric circuit. Water current is the rate at which water flows past a point on the water circuit. As such, current is analogous to the number of gallons of water flowing into, along, and out of a slide per unit of time. 5. The diagram at the right depicts a conducting wire. Two cross-sectional areas are located 50 cm apart. Every 2.0 seconds, 10 C of charge flow through each of these areas. The current in this wire is ____ A. a. 0.10 Answer: E Current is the ratio of charge to time. The quantity of charge passing through a cross section in 2 seconds is 10 C.

The ratio of charge to time is I = Q / t = ( 10 C) / ( 2 s) = 5 C/s = 5 Ampere 6. Use the diagram at the right to complete the following statements: a. A current of one ampere is a flow of charge at the rate of _______ coulomb per second. b. When a charge of 8 C flows past any point along a circuit in 2 seconds, the current is ________ A. c. If 5 C of charge flow past point A (diagram at right) in 10 seconds, then the current is _________ A. d. If the current at point D is 2.0 A, then _______ C of charge flow past point D in 10 seconds. e. If 12 C of charge flow past point A in 3 seconds, then 8 C of charge will flow past point E in ________ seconds. f. True or False: The current at point E is considerably less than the current at point A since charge is being used up in the light bulbs.

Conventional Versus Electron Flow Conventional Versus Electron Flow Home > Magnet University > Electricity > Conventional Versus Electron Flow Conventional Versus Electron Flow "The nice thing about standards is that there are so many of them to choose from." Andres S. Tannenbaum, computer science professor When Benjamin Franklin made his conjecture regarding the direction of charge flow (from the smooth wax to the rough wool), he set a precedent for electrical notation that exists to this day, despite the fact that we know electrons are the constituent units of charge, and that they are displaced from the wool to the wax -- not from the wax to the wool -- when those two substances are rubbed together. This is why electrons are said to have a negative charge: because Franklin assumed electric charge moved in the opposite direction that it actually does, and so objects he called "negative" (representing a deficiency of charge) actually have a surplus of electrons. By the time the true direction of electron flow was discovered, the nomenclature of "positive" and "negative" had already been so well established in the scientific community that no effort was made to change it, although calling electrons "positive" would make more sense in referring to "excess" charge.

You see, the terms "positive" and "negative" are human inventions, and as such have no absolute meaning beyond our own conventions of language and scientific description. Franklin could have just as easily referred to a surplus of charge as "black" and a deficiency as "white," in which case scientists would speak of electrons having a "white" charge (assuming the same incorrect conjecture of charge position between wax and wool). However, because we tend to associate the word "positive" with "surplus" and "negative" with "deficiency," the standard label for electron charge does seem backward. Because of this, many engineers decided to retain the old concept of electricity with "positive" referring to a surplus of charge, and label charge flow (current) accordingly. This became known as conventional flow notation: Others chose to designate charge flow according to the actual motion of electrons in a circuit. This form of symbology became known as electron flow notation: In conventional flow notation, we show the motion of charge according to the (technically incorrect) labels of + and -. This way the labels make sense, but the direction of charge flow is incorrect.

In electron flow notation, we follow the actual motion of electrons in the circuit, but the + and - labels seem backward. Does it matter, really, how we designate charge flow in a circuit? Not really, so long as we're consistent in the use of our symbols. You may follow an imagined direction of current (conventional flow) or the actual (electron flow) with equal success insofar as circuit analysis is concerned. Concepts of voltage, current, resistance, continuity, and even mathematical treatments such as Ohm's Law (chapter 2) and Kirchhoff's Laws (chapter 6) remain just as valid with either style of notation. You will find conventional flow notation followed by most electrical engineers, and illustrated in most engineering textbooks. Electron flow is most often seen in introductory textbooks (this one included) and in the writings of professional scientists, especially solid-state physicists who are concerned with the actual motion of electrons in substances. These preferences are cultural, in the sense that certain groups of people have found it advantageous to envision electric current motion in certain ways. Being that most analyses of electric circuits do not depend on a technically accurate depiction of charge flow, the choice between conventional flow notation and electron flow notation is arbitrary . . . almost.

Many electrical devices tolerate real currents of either direction with no difference in operation. Incandescent lamps (the type utilizing a thin metal filament that glows white-hot with sufficient current), for example, produce light with equal efficiency regardless of current direction. They even function well on alternating current (AC), where the direction changes rapidly over time. Conductors and switches operate irrespective of current direction, as well. The technical term for this irrelevance of charge flow is nonpolarization. We could say then, that incandescent lamps, switches, and wires are nonpolarized components. Conversely, any device that functions differently on currents of different direction would be called a polarized device. There are many such polarized devices used in electric circuits. Most of them are made of so-called semiconductor substances, and as such aren't examined in detail until the third volume of this book series. Like switches, lamps, and batteries, each of these devices is represented in a schematic diagram by a unique symbol. As one might guess, polarized device symbols typically contain an arrow within them, somewhere, to designate a preferred or exclusive direction of current. This is where the competing notations of conventional and electron flow really matter.

Because engineers from long ago have settled on conventional flow as their "culture's" standard notation, and because engineers are the same people who invent electrical devices and the symbols representing them, the arrows used in these devices' symbols all point in the direction of conventional flow, not electron flow. That is to say, all of these devices' symbols have arrow marks that point against the actual flow of electrons through them. Perhaps the best example of a polarized device is the diode. A diode is a one-way "valve" for electric current, analogous to a check valve for those familiar with plumbing and hydraulic systems. Ideally, a diode provides unimpeded flow for current in one direction (little or no resistance), but prevents flow in the other direction (infinite resistance). Its schematic symbol looks like this: Placed within a battery/lamp circuit, its operation is as such: When the diode is facing in the proper direction to permit current, the lamp glows. Otherwise, the diode blocks all electron flow just like a break in the circuit, and the lamp will not glow.

If we label the circuit current using conventional flow notation, the arrow symbol of the diode makes perfect sense: the triangular arrowhead points in the direction of charge flow, from positive to negative: On the other hand, if we use electron flow notation to show the true direction of electron travel around the circuit, the diode's arrow symbology seems backward: For this reason alone, many people choose to make conventional flow their notation of choice when drawing the direction of charge motion in a circuit. If for no other reason, the symbols associated with semiconductor components like diodes make more sense this way. However, others choose to show the true direction of electron travel so as to avoid having to tell themselves, "just remember the electrons are actually moving the other way" whenever the true direction of electron motion becomes an issue. In this series of textbooks, I have committed to using electron flow notation. Ironically, this was not my first choice. I found it much easier when I was first learning electronics to use conventional flow notation, primarily because of the directions of semiconductor device symbol arrows. Later, when I began my first formal training in electronics, my instructor insisted on using electron flow notation in his lectures.

In fact, he asked that we take our textbooks (which were illustrated using conventional flow notation) and use our pens to change the directions of all the current arrows so as to point the "correct" way! His preference was not arbitrary, though. In his 20-year career as a U.S. Navy electronics technician, he worked on a lot of vacuum-tube equipment. Before the advent of semiconductor components like transistors, devices known as vacuum tubes or electron tubes were used to amplify small electrical signals. These devices work on the phenomenon of electrons hurtling through a vacuum, their rate of flow controlled by voltages applied between metal plates and grids placed within their path, and are best understood when visualized using electron flow notation. When I graduated from that training program, I went back to my old habit of conventional flow notation, primarily for the sake of minimizing confusion with component symbols, since vacuum tubes are all but obsolete except in special applications. Collecting notes for the writing of this book, I had full intention of illustrating it using conventional flow. Years later, when I became a teacher of electronics, the curriculum for the program I was going to teach had already been established around the notation of electron flow.

Oddly enough, this was due in part to the legacy of my first electronics instructor (the 20-year Navy veteran), but that's another story entirely! Not wanting to confuse students by teaching "differently" from the other instructors, I had to overcome my habit and get used to visualizing electron flow instead of conventional. Because I wanted my book to be a useful resource for my students, I begrudgingly changed plans and illustrated it with all the arrows pointing the "correct" way. Oh well, sometimes you just can't win! On a positive note (no pun intended), I have subsequently discovered that some students prefer electron flow notation when first learning about the behavior of semiconductive substances. Also, the habit of visualizing electrons flowing against the arrows of polarized device symbols isn't that difficult to learn, and in the end I've found that I can follow the operation of a circuit equally well using either mode of notation.

Still, I sometimes wonder if it would all be much easier if we went back to the source of the confusion -- Ben Franklin's errant conjecture -- and fixed the problem there, calling electrons "positive" and protons "negative." Lessons In Electric Circuits copyright (C) 2000-2002 Tony R. Kuphaldt, under the terms and conditions of the Design Science License . Customer Service Circuit Symbols and Circuit Diagrams Current Electricity - Lesson 4 - Circuit Connections Circuit Symbols and Circuit Diagrams Circuit Symbols and Circuit Diagrams Combination Circuits Thus far, this unit of The Physics Classroom tutorial has focused on the key ingredients of an electric circuit and upon the concepts of electric potential difference, current and resistance. Conceptual meaning of terms have been introduced and applied to simple circuits. Mathematical relationships between electrical quantities have been discussed and their use in solving problems has been modeled. Lesson 4 will focus on the means by which two or more electrical devices can be connected to form an electric circuit. Our discussion will progress from simple circuits to mildly complex circuits. Former principles of electric potential difference, current and resistance will be applied to these complex circuits and the same mathematical formulas will be used to analyze them.

Electric circuits, whether simple or complex, can be described in a variety of ways. An electric circuit is commonly described with mere words. Saying something like "A light bulb is connected to a D-cell" is a sufficient amount of words to describe a simple circuit. On many occasions in Lessons 1 through 3, words have been used to describe simple circuits. Upon hearing (or reading) the words, a person grows accustomed to quickly picturing the circuit in their mind. But another means of describing a circuit is to simply draw it. Such drawings provide a quicker mental picture of the actual circuit. Circuit drawings like the one below have been used many times in Lessons 1 through 3. Describing Circuits with Words "A circuit contains a light bulb and a 1.5-Volt D-cell." Describing Circuits with Drawings A final means of describing an electric circuit is by use of conventional circuit symbols to provide a schematic diagram of the circuit and its components. Some circuit symbols used in schematic diagrams are shown below. A single cell or other power source is represented by a long and a short parallel line. A collection of cells or battery is represented by a collection of long and short parallel lines.

In both cases, the long line is representative of the positive terminal of the energy source and the short line represents the negative terminal. A straight line is used to represent a connecting wire between any two components of the circuit. An electrical device that offers resistance to the flow of charge is generically referred to as a resistor and is represented by a zigzag line. An open switch is generally represented by providing a break in a straight line by lifting a portion of the line upward at a diagonal. These circuit symbols will be frequently used throughout the remainder of Lesson 4 as electric circuits are represented by schematic diagrams. It will be important to either memorize these symbols or to refer to this short listing frequently until you become accustomed to their use. As an illustration of the use of electrical symbols in schematic diagrams, consider the following two examples. Example 1: Description with Words: Three D-cells are placed in a battery pack to power a circuit containing three light bulbs. Using the verbal description, one can acquire a mental picture of the circuit being described. This verbal description can then be represented by a drawing of three cells and three light bulbs connected by wires. Finally, the circuit symbols presented above can be used to represent the same circuit.

Note that three sets of long and short parallel lines have been used to represent the battery pack with its three D-cells. And note that each light bulb is represented by its own individual resistor symbol. Straight lines have been used to connect the two terminals of the battery to the resistors and the resistors to each other. The above circuits presumed that the three light bulbs were connected in such a way that the charge flowing through the circuit would pass through each one of the three light bulbs in consecutive fashion. The path of a positive test charge leaving the positive terminal of the battery and traversing the external circuit would involve a passage through each one of the three connected light bulbs before returning to the negative terminal of the battery. But is this the only way that three light bulbs can be connected? Do they have to be connected in consecutive fashion as shown above? Absolutely not! In fact, example 2 below contains the same verbal description with the drawing and the schematic diagrams being drawn differently. Example 2: Description with Words: Three D-cells are placed in a battery pack to power a circuit containing three light bulbs. Using the verbal description, one can acquire a mental picture of the circuit being described.

But this time, the connections of light bulbs is done in a manner such that there is a point on the circuit where the wires branch off from each other. The branching location is referred to as a node. Each light bulb is placed in its own separate branch. These branch wires eventually connect to each other to form a second node. A single wire is used to connect this second node to the negative terminal of the battery. These two examples illustrate the two common types of connections made in electric circuits. When two or more resistors are present in a circuit, they can be connected in series or in parallel. The remainder of Lesson 4 will be devoted to a study of these two types of connections and the effect that they have upon electrical quantities such as current, resistance and electric potential. The next part of Lesson 4 will introduce the distinction between series and parallel connections. 2. Use the concept of conventional current to draw an unbroken line on the schematic diagram at the right that indicates the direction of the conventional current. Place an arrowhead on your unbroken line. See Answer The conventional current in the external circuit is the direction that a positive charge would move. It is directed from the positive terminal to the negative terminal.

The positive terminal of a cell is the long line and the negative terminal is the short line. If there is a two-cell battery, then the long line on the end is the positive terminal of the battery. The short line on the opposite end is the negative terminal of the battery.

Question: The conventional symbol for electrical current is?
Answer: i