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Rīgas Tehniska Universitāte Daugavpils filiāle Viktorija AGAFONOVA TEKSTU KRĀJUMS angļu valodā Metodiskie norādījumi Elektrozinību profila studentiem Daugavpils 2008 Contents 1. How batteries work ..................................................................................................................... 3 2. How flash memory works ........................................................................................................... 6 3. Electric field ............................................................................................................................... 8 4. Electrical conductor .................................................................................................................. 10 5. Electric current .......................................................................................................................... 12 2 How Batteries Work Batteries are all over the place - in our cars, our PCs, laptops, portable MP3 players and cell phones. A battery is essentially a can full of chemicals that produce electrons. Chemical reactions that produce electrons are called electrochemical reactions. In this article, you'll learn all about batteries -- the basic concept at work, the actual chemistry going on inside a battery, rechargeable versions, what the future holds for batteries and possible power sources that could replace them. If you look at any battery, you'll notice that it has two terminals. One terminal is marked (+), or positive, while the other is marked (-), or negative. In an AA, C or D cell (normal flashlight batteries), the ends of the battery are the terminals. In a large car battery, there are two heavy lead posts that act as the terminals. Electrons collect on the negative terminal of the battery. If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous, especially with large batteries, so it is not something you want to be doing). Normally, you connect some type of load to the battery using the wire. The load might be something like a light bulb, a motor or an electronic circuit like a radio. Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts. The ability to harness this sort of reaction started with the voltaic pile. Battery History The first battery was created by Alessandro Volta in 1800. To create his battery, he made a stack by alternating layers of zinc, blotting paper soaked in salt water, and silver. This arrangement was known as a voltaic pile. The top and bottom layers of the pile must be different metals. If you attach a wire to the top and bottom of the pile, you can measure a voltage and a current from the pile. The pile can be stacked as high as you like, and each layer will increase the voltage by a fixed amount. In the 1800s, before the invention of the electrical generator (the generator was not invented and perfected until the 1870s), the Daniell cell was extremely common for operating telegraphs and doorbells. The Daniell cell is also known by three other names: Crowfoot cell (because of the typical shape of the zinc electrode) Gravity cell (because gravity keeps the two sulfates separated) Wet cell (because it uses liquids for the electrolytes, as opposed to the modern dry cell) The Daniell cell is a wet cell consisting of copper and zinc plates and copper and zinc sulfates. To make the Daniell cell, the copper plate is placed at the bottom of a glass jar. Copper sulfate solution is poured over the plate to half-fill the jar. Then a zinc plate is hung in the jar and a zinc sulfate solution is poured very carefully into the jar. Copper sulfate is denser than zinc sulfate, so the zinc sulfate "floats" on top of the copper sulfate. Obviously, this arrangement does not work very well in a flashlight, but it works fine for stationary applications. 3 Battery Reactions and Chemistry In any battery, an electrochemical reaction occurs like the ones described on the previous page. This reaction moves electrons from one pole to the other. The actual metals and electrolytes used control the voltage of the battery -- each different reaction has a characteristic voltage. For example, here's what happens in one cell of a car's lead-acid battery: The cell has one plate made of lead and another plate made of lead dioxide, with a strong sulfuric acid electrolyte in which the plates are immersed. Lead combines with SO4 (sulfate) to create PbSO4 (lead sulfate), plus one electron. Lead dioxide, hydrogen ions and SO4 ions, plus electrons from the lead plate, create PbSO4 and water on the lead dioxide plate. As the battery discharges, both plates build up PbSO4 and water builds up in the acid. The characteristic voltage is about 2 volts per cell, so by combining six cells you get a 12-volt battery. A lead-acid battery has a nice feature -- the reaction is completely reversible. If you apply current to the battery at the right voltage, lead and lead dioxide form again on the plates so you can reuse the battery over and over. In a zinc-carbon battery, there is no easy way to reverse the reaction because there is no easy way to get hydrogen gas back into the electrolyte. Modern Battery Chemistry Modern batteries use a variety of chemicals to power their reactions. Typical battery chemistries include: Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte. Alkaline battery - Alkaline chemistry is used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte. Lithium-iodide battery - Lithium-iodide chemistry is used in pacemakers and hearing aides because of their long life. Lead-acid battery - Lead-acid chemistry is used in automobiles, the electrodes are made of lead and lead-oxide with a strong acidic electrolyte (rechargeable). Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with potassium-hydroxide as the electrolyte (rechargeable). Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable). Lithium-ion battery - With a very good power-to-weight ratio, this is often found in high-end laptop computers and cell phones (rechargeable). Zinc-air battery - This battery is lightweight and rechargeable. Zinc-mercury oxide battery - This is often used in hearing-aids. Silver-zinc battery - This is used in aeronautical applications because the power-toweight ratio is good. Rechargeable Batteries With the rise in portable devices such as laptops, cell phones, MP3 players and cordless power tools, the need for rechargeable batteries has grown substantially in recent years. The concept of the rechargeable battery has been around since 1859, when French physicist Gaston Plante 4 invented the lead acid cell, which would later become the world's first rechargeable battery. That same chemistry is still used in today's car battery. The basic idea behind the rechargeable battery is simple: when electrical energy is applied to the battery, the electron flow from negative to positive that occurs during discharge is reversed and power is restored. This requires an adapter in the case of devices with built-in batteries or for standard nickel-cadmium or nickel-metal hydride batteries, the most common multi-use rechargeable batteries used today in your remote control, flashlight or digital camera. Car batteries are one of the oldest kinds of rechargeable batteries and in fact, the electric car predates its gas-powered cousin. In a standard car, there is a single lead-acid SLI battery that supplies power to the starter, lights and ignition system. The battery charger in this case is the alternator, a clever device that converts gasoline power to electrical energy and distributes it where needed. In electric and hybrid cars, traction batteries are used to power the vehicle down the road. Traction batteries come in many varieties, from lead acid, to nickel-cadmium, nickel metal hydride and lithium ion. The recharging rate has improved substantially over the years and is broken down into three categories: Slow: 14-16 hours Quick: 3-6 hours Fast: Less than one hour The rate of charge is determined by how much electrical current is allowed into the battery by the charger. Some batteries can handle higher voltage in a shorter amount of time without overheating, while others need a lesser voltage applied over a longer period of time. The quicker the rate of charge, the more chance there is of over charging, which can ruin a battery's chance of holding its charge. The key in avoiding an over charge is the ability to dissipate the charging current once maximum power has been reached. Most chargers have built-in voltage regulators do this, allowing you to safely leave your cell phone or computer plugged in overnight. The speed and effectiveness of the charge depends largely on the quality of the charger itself. Chargers vary in performance based on the price tag and like most products you get what you pay for. Chargers are generally designed for specific cell chemistries, although newer universal chargers have sensors built in that identify the cell type and react appropriately. There are also smart chargers that use a microprocessor to monitor temperature, voltage and state of charge, which is the percentage of power available compared to its full capacity. One common problem in nickel-cadmium rechargeable batteries is something known as the memory effect. This is when the battery is continually recharged before it has discharged more than 50 percent of its power, causing it to essentially forget that it could fully discharge to begin with. Memory effect is caused by the formation of hard-to-dissolve cadmium crystals deep within the battery. Cadmium crystals are an unavoidable by-product of discharge; the trick is to keep them small enough to be reformed as cadmium during the charging process. When a battery is not fully discharged, the crystals deep within the battery are not affected by the influx of electrical current, so they are not reformed as cadmium and can grow into the troublesome larger cadmium crystals. The battery will still function normally, but is maxed out at 50 percent. The memory effect can be avoided by fully cycling the battery once every two to three weeks by allowing it to discharge completely, and then fully recharge. 5 It's important to remember that no battery, rechargeable or otherwise, will last forever. All batteries suffer from aging cells and the longer they are used, the less capacity they ultimately will have. Rechargeable batteries are still a great way to save money and reduce waste. How Flash Memory Works We store and transfer all kinds of files on our computers -- digital photographs, music files, word processing documents, PDFs and countless other forms of media. But sometimes your computer's hard drive isn't exactly where you want your information. Whether you want to make backup copies of files that live off of your systems or if you worry about your security, portable storage devices that use a type of electronic memory called flash memory may be the right solution. Electronic memory comes in a variety of forms to serve a variety of purposes. Flash memory is used for easy and fast information storage in computers, digital cameras and home video game consoles. It is used more like a hard drive than as RAM. In fact, flash memory is known as a solid state storage device, meaning there are no moving parts -- everything is electronic instead of mechanical. Here are a few examples of flash memory: Your computer's BIOS chip CompactFlash (most often found in digital cameras) SmartMedia (most often found in digital cameras) Memory Stick (most often found in digital cameras) PCMCIA Type I and Type II memory cards (used as solid-state disks in laptops) Memory cards for video game consoles Flash Memory: Tunneling and Erasing Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10 to 13 volts, is applied to the floating gate. The charge comes from the column, or bitline, enters the floating gate and drains to a ground. This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A special device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is above the 50 percent threshold, it has a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. A blank EEPROM has all of the gates fully open, giving each cell a value of 1. The electrons in the cells of a flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip, which can then be rewritten. Flash memory works much faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block or the entire chip, and then rewrites it. 6 You may think that your car radio has flash memory, since you're able to program the presets and the radio remembers them. But it's actually using flash RAM. The difference is that flash RAM has to have some power to maintain its contents, while flash memory will maintain its data without any external source of power. Even though you've turned the power off, the car radio is pulling a tiny amount of current to preserve the data in the flash RAM. That is why the radio will lose its presets if your car battery dies or the wires are disconnected. Removable Flash Memory Cards While your computer's BIOS chip is the most common form of Flash memory, removable solidstate storage devices are also popular. SmartMedia and CompactFlash cards are both wellknown, especially as "electronic film" for digital cameras. Other removable flash-memory products include Sony's Memory Stick, PCMCIA memory cards, and memory cards for video game systems. We'll focus on SmartMedia and CompactFlash, but the essential idea is the same for all of these products -- every one of them is simply a form of flash memory. There are a few reasons to use flash memory instead of a hard disk: It has no moving parts, so it's noiseless. It allows faster access. It's smaller in size and lighter. So why don't we just use flash memory for everything? Because the cost per megabyte for a hard disk is drastically cheaper, and the capacity is substantially more. The solid-state floppy-disk card (SSFDC), better known as SmartMedia, was originally developed by Toshiba. SmartMedia cards are available in capacities ranging from 2 MB to 128 MB. The card itself is quite small, approximately 45 mm long, 37 mm wide and less than 1 mm thick. As shown below, SmartMedia cards are extremely simple. A plane electrode is connected to the flash-memory chip by bonding wires. The flash-memory chip, plane electrode and bonding wires are embedded in a resin using a technique called over-molded thin package (OMTP). This allows everything to be integrated into a single package without the need for soldering. The OMTP module is glued to a base card to create the actual card. Power and data is carried by the electrode to the Flash-memory chip when the card is inserted into a device. A notched corner indicates the power requirements of the SmartMedia card. Looking at the card with the electrode facing up, if the notch is on the left side, the card needs 5 volts. If the notch is on the right side, it requires 3.3 volts. SmartMedia cards erase, write and read memory in small blocks (256- or 512-byte increments). This approach means that they are capable of fast, reliable performance while allowing you to specify which data you wish to keep.They are less rugged than other forms of removable solidstate storage, so you should be very careful when handling and storing them. Because of newer, smaller cards with bigger storage capacities, such as xD-Picture Cards and Secure Digital cards, Toshiba has essentially discontinued the production of SmartMedia cards, so they're now difficult to find. CompactFlash cards were developed by Sandisk in 1994, and they're different from SmartMedia cards in two important ways: 7 They're thicker. They utilize a controller chip. CompactFlash consists of a small circuit board with flash-memory chips and a dedicated controller chip, all encased in a rugged shell that is thicker than a SmartMedia card. CompactFlash cards are 43 mm wide and 36 mm long, and come in two thicknesses: Type I cards are 3.3 mm thick, and Type II cards are 5.5 mm thick. CompactFlash cards support dual voltage and will operate at either 3.3 volts or 5 volts. The increased thickness of the card allows for greater storage capacity than SmartMedia cards. CompactFlash sizes range from 8 MB to as much as 100GB. The onboard controller can increase performance, particularly in devices that have slow processors. The case and controller chip add size, weight and complexity to the CompactFlash card when compared to the SmartMedia card. Electric field In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field (that can also be equated to electric flux density). This electric field exerts a force on other electrically charged objects. The concept of an electric field was introduced by Michael Faraday. The electric field is a vector field with SI units of newtons per coulomb (N C−1) or, equivalently, volts per meter (V m−1). The strength of the field at a given point is defined as the force that would be exerted on a positive test charge of +1 coulomb placed at that point; the direction of the field is given by the direction of that force. Electric fields contain electrical energy with energy density proportional to the square of the field intensity. The electric field is to charge as gravitational acceleration is to mass and force density is to volume. A moving charge has not just an electric field but also a magnetic field, and in general the electric and magnetic fields are not completely separate phenomena; what one observer perceives as an electric field, another observer in a different frame of reference perceives as a mixture of electric and magnetic fields. For this reason, one speaks of "electromagnetism" or "electromagnetic fields." In quantum mechanics, disturbances in the electromagnetic fields are called photons, and the energy of photons is quantized. Definition A stationary charged particle in an electric field experiences a force proportional to its charge given by the equation where the magnetic flux density is given by and where is the Coulomb force. Electric charge is a characteristic of some subatomic particles, and is quantized when expressed as a multiple of the so-called elementary charge e. Electrons by convention have a charge of -1, while protons have the opposite charge of +1. Quarks have a fractional charge of −1/3 or +2/3. The antiparticle equivalents of these have the opposite charge. There are other charged particles. In general, same-sign charged particles repel one another, while different-sign charged particles attract. This is expressed quantitatively in Coulomb's law, which states the magnitude of the 8 repelling force is proportional to the product of the two charges, and weakens proportionately to the square of the distance. The electric charge of a macroscopic object is the sum of the electric charges of its constituent particles. Often, the net electric charge is zero, since naturally the number of electrons in every atom is equal to the number of the protons, so their charges cancel out. Situations in which the net charge is non-zero are often referred to as static electricity. Furthermore, even when the net charge is zero, it can be distributed non-uniformly (e.g., due to an external electric field), and then the material is said to be polarized, and the charge related to the polarization is known as bound charge (while the excess charge brought from outside is called free charge). An ordered motion of charged particles in a particular direction (in metals, these are the electrons) is known as electric current. The discrete nature of electric charge was proposed by Michael Faraday in his electrolysis experiments, then directly demonstrated by Robert Millikan in his oil-drop experiment. The SI unit for quantity of electricity or electric charge is the coulomb, which represents approximately 1.60 × 1019 elementary charges (the charge on a single electron or proton). The coulomb is defined as the quantity of charge that has passed through the cross-section of an electrical conductor carrying one ampere within one second. The symbol Q is often used to denote a quantity of electricity or charge. The quantity of electric charge can be directly measured with an electrometer, or indirectly measured with a ballistic galvanometer. Formally, a measure of charge should be a multiple of the elementary charge e (charge is quantized), but since it is an average, macroscopic quantity, many orders of magnitude larger than a single elementary charge, it can effectively take on any real value. Furthermore, in some contexts it is meaningful to speak of fractions of a charge; e.g. in the charging of a capacitor. If the charged particle can be considered a point charge, the electric field is defined as the force it experiences per unit charge: where is the electric force experienced by the particle q is its charge is the electric field wherein the particle is located Taken literally, this equation only defines the electric field at the places where there are stationary charges present to experience it. Furthermore, the force exerted by another charge q will alter the source distribution, which means the electric field in the presence of q differs from itself in the absence of q. However, the electric field of a given source distribution remains defined in the absence of any charges with which to interact. This is achieved by measuring the force exerted on successively smaller test charges placed in the vicinity of the source distribution. By this process, the electric field created by a given source distribution is defined as the limit as the test charge approaches zero of the force per unit charge exerted thereupon. This allows the electric field to be dependent on the source distribution alone. As is clear from the definition, the direction of the electric field is the same as the direction of the force it would exert on a positively-charged particle, and opposite the direction of the force on a negatively-charged particle. Since like charges repel and opposites attract (as quantified below), the electric field tends to point away from positive charges and towards negative charges. is the vacuum permittivity. Coulomb's law is actually a special case of Gauss's Law, a more fundamental description of the relationship between the distribution of electric charge in space and the resulting electric field. Gauss's law is one of Maxwell's equations, a set of four laws governing electromagnetics. 9 Time-varying fields Charges do not only produce electric fields. As they move, they generate magnetic fields, and if the magnetic field changes, it generates electric fields. A changing magnetic field gives rise to an electric field, which yields Faraday's law of induction, where indicates the curl of the electric field, represents the vector rate of decrease of magnetic field with time. This means that a magnetic field changing in time produces a curled electric field, possibly also changing in time. The situation in which electric or magnetic fields change in time is no longer electrostatics, but rather electrodynamics or electromagnetics. Energy in the electric field The electric field stores energy. The energy density of the electric field is given by where is the permittivity of the medium in which the field exists is the electric field vector. The total energy stored in the electric field in a given volume V is therefore where dV is the differential volume element. Electrical conductor In science and engineering, a conductor is a material which contains movable electric charges. In metallic conductors, such as copper or aluminium, the movable charged particles are electrons (See electrical conduction). Positive charges may also be mobile in the form of atoms in a lattice missing electrons (called "holes") or ions, such as in the electrolyte of a battery. The following applies to direct current only. When the direction of voltage/current alternates, other effects (inductive and capacitive reactance) come into play also. All conductors contain electric charges which will move when an electric potential difference (measured in volts) is applied across separate points on the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the rate of current is proportional to the voltage (Ohm's law,) provided the temperature remains constant and the material remains in the same shape and state. The ratio between the voltage and the current is called the resistance (measured in ohms) of the object between the points where the voltage was applied. The resistance across a standard mass (and shape) of a material at a given temperature is called the resistivity of the material. The inverse of resistance and resistivity is conductance and conductivity. Some good examples of conductors are metal. Most familiar conductors are metallic. Copper is the most common material for electrical wiring (silver is the best but expensive), and gold for high-quality surface-to-surface contacts. However, 10 there are also many non-metallic conductors, including graphite, solutions of salts, and all plasmas. Non-conducting materials lack mobile charges, and so resist the flow of electric current, generating heat. In fact, all materials offer some resistance and warm up when a current flows. Thus, proper design of an electrical conductor takes into account the temperature that the conductor needs to be able to endure without damage, as well as the quantity of electrical current. The motion of charges also creates an electromagnetic field around the conductor that exerts a mechanical radial squeezing force on the conductor. A conductor of a given material and volume (length x cross-sectional area) has no real limit to the current it can carry without being destroyed as long as the heat generated by the resistive loss is removed and the conductor can withstand the radial forces. This effect is especially critical in printed circuits, where conductors are relatively small and close together, and inside an enclosure: the heat produced, if not properly removed, can cause fusing (melting) of the tracks. Since all conductors have some resistance, and all insulators will carry some current, there is no theoretical dividing line between conductors and insulators. However, there is a large gap between the conductance of materials that will carry a useful current at working voltages and those that will carry a negligible current for the purpose in hand, so the categories of insulator and conductor do have practical utility. Thermal and electrical conductivity often go together (for instance, most metals are both electrical and thermal conductors). However, some materials are practical electrical conductors without being a good thermal conductor. In power engineering, an electrical wire is a length of metal, usually surrounded by an insulating sheath, that is used to conduct electricity. Conductor materials Of the metals commonly used for conductors, copper has a high conductivity. Silver is more conductive, but due to cost it is not practical in most cases. However, it is used in specialized equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high frequencies. Because of its ease of connection by soldering or clamping, copper is still the most common choice for most light-gauge wires. Aluminum has been used as a conductor in housing applications for cost reasons. It is actually more conductive than copper when compared by unit weight, but it has technical problems related to heat and its coefficient of thermal expansion, which tends to loosen connections over time. Conductor voltage The voltage on a conductor is determined by the connected circuitry and has nothing to do with the conductor itself. Conductors are usually surrounded by and/or supported by insulators and the insulation determines the maximum voltage that can be applied to any given conductor. Voltage of a conductor "V" is given by V = IR where I is the current, measured in amperes V is the potential difference measured in volts R is the resistance measured in ohms 11 Electric Current 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 which 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 are 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. Conventional Current Direction The particles which carry charge through wires in a circuit are mobile electrons. The electric field direction within a circuit is by definition the direction which 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 12 convention for the direction of an electric current was established to be in the direction which 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 which is used world wide 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 which 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 affect 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 13 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 which informs them to start moving travels at a fraction of the speed of light. The electrons which 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 which light the bulb immediately after the switch is turned to on are the electrons which 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 which 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 which 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. 14 15