How to Solder (Electronics) This article mainly deals with the soldering of through-hole components into printed circuit boards (PCBs). Through-hole components are those which have leads (meaning wires or tabs) that pass through a hole in the board and are soldered to the pad (an area with metal plating) around the hole. The hole may be plated through or not. Soldering of other electrical items such as wires, lugs, have slightly different steps but the general principles are the same. Steps 1. Select the correct component. Many components look similar, so read the labels or check the color code carefully. 2. Bend leads correctly if required, discuss stress relief. To be completed... 3. Clinching leads. Discuss whether to cut leads before or after soldering based on whether heatsinking effect is required. To be completed... 4. Melt a small blob of solder on end of the soldering iron. This will be used to improve the transfer of heat to your work. 5. Carefully place the tip (with the blob) onto the interface of the lead and pad. The tip or blob must touch both the lead and the pad. The tip/blob should not be touching the nonmetallic pad area of the PCB (i.e the fibreglass area) as this area can be damaged by excessive heat. This should now heat the work area. 6. "Feed" the solder onto the interface between the pad and lead. Do not feed the solder onto the tip! The lead and pad should be heated enough for the solder to melt on it (see previous step). If the solder does not melt onto the area, the most likely cause is insufficient heat has been transfered to it. The molten solder should "cling" to the pad and lead together by way of surface tension This is commonly referred to as [[wikipedia:Wetting| wetting]. o with practice, you will learn how to heat the joint more efficiently with the way you hold the iron onto the work o flux from the solder wire is only active for about one second maximum after melting onto the joint as it is slowly "burnt off" by heat o solder will wet a surface only if: the surface is sufficiently heated and
Transcript
How to Solder (Electronics)This article mainly deals with the soldering of through-hole components into printed circuit boards (PCBs). Through-hole components are those which have leads (meaning wires or tabs) that pass through a hole in the board and are soldered to the pad (an area with metal plating) around the hole. The hole may be plated through or not.
Soldering of other electrical items such as wires, lugs, have slightly different steps but the general principles are the same.
Steps
1. Select the correct component. Many components look similar, so read the labels or check the color code carefully.
2. Bend leads correctly if required, discuss stress relief. To be completed... 3. Clinching leads. Discuss whether to cut leads before or after soldering based on whether
heatsinking effect is required. To be completed... 4. Melt a small blob of solder on end of the soldering iron. This will be used to improve the
transfer of heat to your work. 5. Carefully place the tip (with the blob) onto the interface of the lead and pad. The tip or
blob must touch both the lead and the pad. The tip/blob should not be touching the nonmetallic pad area of the PCB (i.e the fibreglass area) as this area can be damaged by excessive heat. This should now heat the work area.
6. "Feed" the solder onto the interface between the pad and lead. Do not feed the solder onto the tip! The lead and pad should be heated enough for the solder to melt on it (see previous step). If the solder does not melt onto the area, the most likely cause is insufficient heat has been transfered to it. The molten solder should "cling" to the pad and lead together by way of surface tension This is commonly referred to as [[wikipedia:Wetting|wetting]. o with practice, you will learn how to heat the joint more efficiently with the way you
hold the iron onto the work o flux from the solder wire is only active for about one second maximum after
melting onto the joint as it is slowly "burnt off" by heat o solder will wet a surface only if:
the surface is sufficiently heated and there is sufficient flux present to remove oxidation from the surface and the surface is clean and free of grease, dirt etc.
7. The solder should by itself, "run around" and fill in the interface. Stop feeding the solder when the correct amount of solder has been added the the joint. The correct amount of solder is determined by: o for non plated through hole (non-PTH) PCBs (most home made PCBs are of this
type) - stop feeding when the solder forms a flat fillet o for plated through hole (PTH) PCBs (most commercially manufactured PCBs) -
stop feeding when a solid concave fillet can be seen o too much solder will form a "bulbous" joint with a convex shape o too little solder will form a "very concave" joint.
Most soldering irons have replaceable tips. Soldering iron tips have a limited working life and also are available in different types of shapes and sizes, to suit the a variety of jobs.
The tip of a soldering iron tends to get stuck with time (if frequently used), due to oxides that build up between the copper tip and the iron sleeve. Plated tips do not usually have this problem. If the copper tip is not removed now and then, it will get stuck permanently in the soldering iron! It is then destroyed. Therefore: every 20 - 50 or so hours of use, when cold, remove the tip and move it back and forth and around so the oxide scales can come out, before locking it in place again! Now you soldering iron will last for many years of use!
Warnings
Soldering irons are very hot. Do not touch the tip with your skin. Also, always use a suitable stand or holder to keep the tip up and off of your work surface.
Solders, especially lead-based solders, contain hazardous materials. Wash your hands after soldering, and be aware that items containing solder may require special handling if you dispose of them.
Things You'll Need
A soldering iron. Soldering irons are usually either:
Fixed power - e.g 25W (small jobs) to 100W (large jobs, heavy cabling etc)
Variable temperature - tip temperature can be controlled to suit the size of the job
Tongs, needle-nosed pliers, or tweezers to hold the component.
A clamp or stand to hold the board.
Flux-cored solder wire. o Solder alloys.
The most common solder alloy used in electronics is Lead-Tin 60/40. This alloy is recommended if you are new to soldering.
Various lead-free alloys are becoming popular recently. These require higher soldering temperatures and do not "wet" as well as Lead-Tin alloys. Lead-free solders require more skill to produce a good quality solder joint.
o Flux. Flux is an additive in solder that facilitates the soldering process by removing and preventing oxidation and by improving the wetting characteristics of the liquid solder. There are different types of flux cores available for solder wire. Rosin is most commonly used by hobbyists. After soldering, it leaves a
brown, sticky residue which is non-corrosive and non-conductive, but can be cleaned if desired with a solvent such as isopropanol (also called isopropyl alcohol or IPA). There are different grades of Rosin flux, the most commonly used is "RMA" (Rosin Mildly Activated).
No-clean fluxes leaves a clear residue after soldering, which is non-corrosive and non-conductive. This flux is designed to be left on the solder joint and surrounding areas.
Water-soluble fluxes usually have a higher activity (i.e is more aggressive) that leaves a residue which must be cleaned with water. The residue is
corrosive and may also damage the board or components if not cleaned correctly after use.
How to Remember Electrical Resistor Color CodesResistor color codes are something that every electronics hobbyist should remember. The old mnemonic was rather, well, disturbing, and a conscientious person would never recite it. Anyway, on with the better one! Write this down in a prominent place and you'll have it committed to permanent memory in no time!
Steps
1.
o Here's one mnemonic "Bright Boys Rave Over Young Girls But Veto Getting Wed. Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White <=> 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
o Alternatively, most of the colors are those from the traditional rainbow. Black is 0 (as in 'nothing'), Brown is 1, then Red through Violet, and finally Gray and White are 8 and 9.
2. The multiplier band goes by the same code and can be read as "followed by N zeros", plus Gold for "divide by 10" and Silver for "divide by 100".
3. Tolerances are something of a mess: Brown and Red are 1% and 2% (you usually spot them because they have an extra significant digit), Gold and Silver are 5% and 10%, and 20% doesn't even get a tolerance band (you will rarely, if ever, come across one of these).
4. To read the whole thing, put the tolerance band at your right, and go like this: "green-brown-red-gold = 5-1-00-5% = 5.1K 5%". You'll get used soon enough, and a bit later you'll be spotting what you want at once. It's also easy to classify them by decade: red is Ks, orange is 10Ks ...
Tips
The same code is used for inductors and capacitors, only that the "units" are uH and pF. That is, orange-white-red-gold is 3.9 mH or 3.9 nF, 5%.
WattFrom Wikipedia, the free encyclopediaJump to: navigation, searchFor other uses, see Watt (disambiguation).
The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second. It measures a rate of energy use or production.
A human climbing a flight of stairs is doing work at a rate of about 200 watts. A typical automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 33.5 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typically consume 5 to 30 watts.
Contents[hide]
1 Definition 2 Origin and adoption as an SI unit 3 Derived and qualified units for power distribution
o 3.1 Microwatt o 3.2 Milliwatt o 3.3 Kilowatt o 3.4 Megawatt o 3.5 Gigawatt o 3.6 Terawatt o 3.7 Electrical and thermal
4 Confusion of watts and watt-hours 5 See also 6 References
7 External links
Definition
One watt is the rate at which work is done when an object is moving at one meter per second against a force of one newton..
By the definition of the units of ampere and volt, work is done at a rate of one watt when one ampere flows through a potential difference of one volt. [1]
Origin and adoption as an SI unit
The watt is named after James Watt for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th General Conference on Weights and Measures in 1960 as the unit of power incorporated in the International System of Units (or "SI").
This SI unit is named after James Watt. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (W). When an SI unit is spelled out in English, it should always begin with a lowercase letter (watt), except where any word would be capitalized, such as at the beginning
of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
— Based on The International System of Units, section 5.2.
Derived and qualified units for power distribution
Microwatt
The microwatt (symbol:μW) is equal to one millionth (10-6) of a watt.
Milliwatt
The milliwatt (symbol:mW) is equal to one thousandth (10-3) of a watt.
Kilowatt
The kilowatt (symbol: kW), equal to one thousand watts, is typically used to state the power output of engines and the power consumption of tools and machines. A kilowatt is approximately equivalent to 1.34 horsepower. An electric heater with one heating-element might use 1 kilowatt.
Megawatt
The megawatt (symbol: MW) is equal to one million (106) watts.
Many things can sustain the transfer or consumption of energy on this scale; some of these events or entities include: lightning strikes, large electric motors, naval craft (such as aircraft carriers and submarines), engineering hardware, and some scientific research equipment (such as the supercollider and large lasers). A large residential or retail building may consume several megawatts in electric power and heating energy.
The productive capacity of electrical generators operated by utility companies is often measured in MW. Modern high-powered diesel-electric railroad locomotives typically have a peak power output of 3 to 5 MW, whereas U.S. nuclear power plants have net summer capacities between about 500 and 1300 MW.[2]
According to the Oxford English Dictionary, the earliest citing for "megawatt" is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November 1847, article in Science (506:2).
[edit] Gigawatt
The gigawatt (symbol: GW) is equal to one billion (109) watts. This unit is sometimes used with large power plants or power grids.
The terawatt (symbol: TW) is equal to one trillion (1012) watts. The average energy usage by humans (about 15 TW) is commonly measured in these units. The most powerful lasers from the mid 1960s to the mid 1990s produced power in terawatts, but only for nanoseconds.
[edit] Electrical and thermal
In the electric power industry, Megawatt electrical (abbreviation: MWe[citation needed] or
MWe[3]) is a term that refers to electric power, while megawatt thermal (abbreviations: MWt, MWth, MWt, or MWth) refers to thermal power produced. Other SI prefixes are sometimes used, for example gigawatt electrical (GWe). [4]
For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MWt of heat, which creates steam to drive a turbine, which generates 648 MWe of electricity. The difference is heat lost to the surroundings.
[edit] Confusion of watts and watt-hours
Power and energy are frequently confused in the general media. Power is the rate at which energy is used. A watt is one joule of energy per second. For example, if a 100 watt light bulb is turned on for one hour, the energy used is 100 watt-hours or 0.1 kilowatt-hour, or 360,000 joules. This same quantity of energy would light a 40 watt bulb for 2.5 hours. A power station would be rated in watts, but its annual energy sales would be in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour:
Volt-ampere Conversion of units James Watt Declared net capacity (power plants) Orders of magnitude (power) Power factor Root mean square (RMS) Watt balance Watt-hour Wattmeter
[edit] References1. ^ "Amps, Volts, Watts, Ohms". Retrieved on 2007-04-17.
2. ^ Nuclear Regulatory Commission. (2007). 2007–2008 Information Digest. Retrieved on 2008-01-27. Appendix A.
3. ^ How Many? A Dictionary of Units of Measurement 4. ^ 'Megawatt electrical' and 'megawatt thermal' are not SI units,Taylor 1995, Guide for the
Use of the International System of Units (SI), NIST Special Publication SP811 The International Bureau of Weights and Measures states that unit symbols should not use subscripts to provide additional information about the quantity being measured, and regards these symbols as incorrect. International Bureau of Weights
OhmFrom Wikipedia, the free encyclopediaJump to: navigation, searchThis article is about the SI derived unit. For other meanings, see Ohm (disambiguation).
A multimeter can be used to measure resistance in ohms. It can also be used to measure capacitance, voltage, current, and other electrical characteristics.
Several resistors. Their resistance, in ohms, is marked using a color code.
The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Ohm.
1 Definition 2 Conversions 3 References and notes 4 See also
5 External links
[edit] Definition
The ohm is the electric resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.[1]
[edit] Conversions A measurement in ohms is the reciprocal of a measurement in siemens, the SI unit
of electrical conductance. Note that 'siemens' is both singular and plural. The non-SI unit, the mho (ohm written backwards), is equivalent to siemens but is mostly obsolete and rarely used.
Ohms to watts: The power dissipated by a resistor may be calculated using resistance and voltage. The formula is a combination of Ohm's law and Joule's law:
where P is the power in watts, R is the resistance in ohms and V is the voltage across the resistor. This method is not reliable for determining the power of an incandescent light bulb, resistance heater or electric short since all these devices operate at high temperatures and the resistance measured using a meter will not represent the operating resistance. For these conditions instead multiply V by current in amperes to get power in watts.
[edit] References and notes1. ^ BIPM SI Brochure: Appendix 1, p. 144
[edit] See also Ohm's law Resistor Resistivity abohm
[edit] External links Scanned books of Georg Simon Ohm at the library of the University of Applied
Official SI brochure NIST Special Publication 811 History of the ohm at sizes.com
Retrieved from "http://en.wikipedia.org/wiki/Ohm"
VoltageFrom Wikipedia, the free encyclopediaJump to: navigation, search
International safety symbol "Caution, risk of electric shock" (ISO 3864), colloquially known as high voltage symbol.
Voltage (sometimes also called electric or electrical tension) is the difference of electrical potential between two points of an electrical or electronic circuit, expressed in volts.[1] It measures the potential energy of an electric field to cause an electric current in an electrical conductor. Depending on the difference of electrical potential it is called extra low voltage, low voltage, high voltage or extra high voltage. Specifically Voltage is equal to energy per unit charge. It is analogous to fluid pressure in a hydraulic or pneumatic system.
1 Explanation o 1.1 Voltage with respect to a common point o 1.2 Voltage between two stated points o 1.3 Addition of voltages o 1.4 Hydraulic analogy o 1.5 Mathematical definition
2 Useful formulas o 2.1 DC circuits o 2.2 AC circuits o 2.3 AC conversions o 2.4 Total voltage o 2.5 Voltage drops
3 Measuring instruments 4 Safety 5 See also 6 References
7 External links
[edit] Explanation
Between two points in an electric field, such as exists in an electrical circuit, the difference in their electrical potentials is known as the electrical potential difference. This difference is directly proportional to the force that tends to push electrons or other charge-carriers from one point to the other. Electrical potential difference can be thought of as the ability to move electrical charge through a resistance. At a time in physics when the word force was used loosely, the potential difference was named the electromotive force or EMF—a term which is still used in certain contexts.
Voltage is a property of an electric field, not individual electrons.[citation needed]An electron moving across a voltage difference experiences a net change in energy, often measured in electron-volts. This effect is analogous to a mass falling through a given height difference in a gravitational field. When using the term 'potential difference' or voltage, one must be clear about the two points between which the voltage is specified or measured. There are two ways in which the term is used. This can lead to some confusion.
[edit] Voltage with respect to a common point
One way in which the term voltage is used is when specifying the voltage of a point in a circuit. When this is done, it is understood that the voltage is usually being specified or measured with respect to a stable and unchanging point in the circuit that is known as ground or common. This voltage is really a voltage difference, one of the two points being the reference point, which is ground. A voltage can be positive or negative. "High"
or "low" voltage may refer to the magnitude (the absolute value relative to the reference point). Thus, a large negative voltage may be referred to as a high voltage. Other authors may refer to a voltage that is more negative as being "lower".
[edit] Voltage between two stated points
Another usage of the term "voltage" is in specifying how many volts are across an electrical device (such as a resistor). In this case, the "voltage," or, more accurately, the "voltage across the device," is really the first voltage taken, relative to ground, on one terminal of the device minus a second voltage taken, relative to ground, on the other terminal of the device. In practice, the voltage across a device can be measured directly and safely using a voltmeter that is isolated from ground, provided that the maximum voltage capability of the voltmeter is not exceeded.
Two points in an electric circuit that are connected by an "ideal conductor," that is, a conductor without resistance and not within a changing magnetic field, have a potential difference of zero. However, other pairs of points may also have a potential difference of zero. If two such points are connected with a conductor, no current will flow through the connection.
[edit] Addition of voltages
Voltage is additive in the following sense: the voltage between A and C is the sum of the voltage between A and B and the voltage between B and C. The various voltages in a circuit can be computed using Kirchhoff's circuit laws.
When talking about alternating current (AC) there is a difference between instantaneous voltage and average voltage. Instantaneous voltages can be added as for direct current (DC), but average voltages can be meaningfully added only when they apply to signals that all have the same frequency and phase.
If one imagines water circulating in a network of pipes, driven by pumps in the absence of gravity, as an analogy of an electrical circuit, then the potential difference corresponds to the fluid pressure difference between two points. If there is a pressure difference between two points, then water flowing from the first point to the second will be able to do work, such as driving a turbine.
This hydraulic analogy is a useful method of teaching a range of electrical concepts. In a hydraulic system, the work done to move water is equal to the pressure multiplied by the volume of water moved. Similarly, in an electrical circuit, the work done to move electrons or other charge-carriers is equal to 'electrical pressure' (an old term for voltage) multiplied by the quantity of electrical charge moved. Voltage is a convenient way of
quantifying the ability to do work. In relation to electric current, the larger the gradient (voltage or hydraulic) the greater the current (assuming resistance is constant).
[edit] Mathematical definition
The electrical potential difference is defined as the amount of work needed to move a unit electric charge from the second point to the first, or equivalently, the amount of work that a unit charge flowing from the first point to the second can perform. The potential difference between two points a and b is the line integral of the electric field E:
[edit] Useful formulas
[edit] DC circuits
where V = potential difference (volts), I = current intensity (amps), R = resistance (ohms), P = power (watts).
[edit] AC circuits
Where V=voltage, I=current, R=resistance, P=true power, Z=impedance, φ=phasor angle between I and V
[edit] AC conversions
Where Vpk=peak voltage, Vppk=peak-to-peak voltage, Vavg=average voltage over a half-cycle, Vrms=effective (root mean square) voltage, and we assumed a sinusoidal wave of the form Vpksin(ωt − kx), with a period T = 2π / ω, and where the angle brackets (in the root-mean-square equation) denote a time average over an entire period.
[edit] Total voltage
Voltage sources and drops in series:
Voltage sources and drops in parallel:
Where is the nth voltage source or drop
[edit] Voltage drops
Across a resistor (Resistor R):
Across a capacitor (Capacitor C):
Across an inductor (Inductor L):
Where V=voltage, I=current, R=resistance, X=reactance.
Instruments for measuring potential differences include the voltmeter, the potentiometer (measurement device), and the oscilloscope. The voltmeter works by measuring the current through a fixed resistor, which, according to Ohm's Law, is proportional to the potential difference across the resistor. The potentiometer works by balancing the unknown voltage against a known voltage in a bridge circuit. The cathode-ray oscilloscope works by amplifying the potential difference and using it to deflect an electron beam from a straight path, so that the deflection of the beam is proportional to the potential difference.
Safety
Electrical safety is discussed in the articles on High voltage (note that even low voltage, e. g. of 50 Volts, can lead to a lethal electric shock) and Electric shock.
Electric currentFrom Wikipedia, the free encyclopedia (Redirected from Current (electricity))Jump to: navigation, search
Electric current is the flow (movement) of electric charge. The SI unit of electric current is the ampere, and electric current is measured using an ammeter. For the definition of the ampere, see the Ampere article.
Contents[hide]
1 Current in a metal wire 2 Current density 3 The drift speed of electric charges 4 Ohm's law 5 Conventional current 6 Examples 7 Electromagnetism 8 Reference direction 9 Electrical safety 10 See also 11 References
12 External links
[edit] Current in a metal wire
A solid conductive metal contains a large population of mobile, or free, electrons. These electrons are bound to the metal lattice but not to any individual atom. Even with no external electric field applied, these electrons move about randomly due to thermal energy but, on average, there is zero net current within the metal. Given an imaginary plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is on average equal to the number passing in the opposite direction.
A typical metal wire for electrical conduction is the stranded copper wire.
When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electrons are therefore the current carrier in a typical solid conductor. For an electric current of 1 ampere, 1 coulomb of electric charge (which consists of about 6.242 × 1018 electrons) drifts every second through any imaginary plane through which the conductor passes.
The current I in amperes can be calculated with the following equation:
where
is the electric charge in coulombs (ampere seconds) is the time in seconds
It follows that:
and
More generally, electric current can be represented as the time rate of change of charge, or
.
[edit] Current densityMain article: Current density
Current density is a measure of the density of electrical 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 meter.
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 direction of the electric field. The speed at which they drift can be calculated from the equation:
where
is the electric current is number of charged particles per unit volume is the cross-sectional area of the conductor is the drift velocity, and is the charge on each particle.
Electric currents in solids typically flow very slowly. For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light.
Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wave that propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction of the speed of light, as can be deduced from Maxwell's Equations, and is therefore many times faster than the drift velocity of the electrons. For example, in AC power lines, the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance.
The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the velocity factor, and depends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on their shape and size.
The nature of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases. The low drift velocity of charge carriers is analogous to air motion; in other words, winds. The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas; while the random motion of charges is analogous to heat - the thermal velocity of randomly vibrating gas particles.
[edit] Ohm's law
Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the applied voltage divided by resistance:
I is the current, measured in amperes V is the potential difference measured in volts R is the resistance measured in ohms
[edit] Conventional current
Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charge carriers are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron current is in the direction opposite to that of conventional (or electric) current.
Diagram showing conventional current notation. Electric charge moves from the positive side of the power source to the negative.
In other conductive materials, the electric current is due to the flow of charged particles in both directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chloride ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands.
There are also materials where the electric current is due to the flow of electrons and yet it is conceptually easier to think of the current as due to the flow of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor.
[edit] Examples
Natural examples include lightning and the solar wind, the source of the polar auroras (the aurora borealis and aurora australis). The artificial form of electric current is 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. 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, and the flow of holes within a semiconductor.
According to Ampère's law, an electric current produces a magnetic field.
[edit] Electromagnetism
Electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire.
Electric current can be directly measured with a galvanometer, but this method involves breaking the 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.
[edit] Reference direction
When solving 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.
[edit] Electrical safety
The most obvious hazard is electrical shock, where a current passes through part of the body. It is the amount of current passing through the body that determines the effect, and this depends on the nature of the contact, the condition of the body part, the current path through the body and the voltage of the source. While a very small amount can cause a slight tingle, too much can cause severe burns if it passes through the skin or even cardiac arrest if enough passes through the heart. The effect also varies considerably from individual to individual. (For approximate figures see Shock Effects under electric shock.)
Due to this and the fact that passing current cannot be easily predicted in most practical circumstances, any supply of over 50 volts should be considered a possible source of dangerous electric shock. In particular, note that 110 volts (a minimum voltage at which AC mains power is distributed in much of the Americas, and 4 other countries, mostly in Asia) can certainly cause a lethal amount of current to pass through the body.
Electric arcs, which can occur with supplies of any voltage (for example, a typical arc welding machine has a voltage between the electrodes of just a few tens of volts), are very hot and emit ultra-violet (UV) and infra-red radiation (IR). Proximity to an electric arc can therefore cause severe thermal burns, and UV is damaging to unprotected eyes and skin.
Accidental electric heating can also be dangerous. An overloaded power cable is a frequent cause of fire. A battery as small as an AA cell placed in a pocket with metal coins can lead to a short circuit heating the battery and the coins which may inflict burns. NiCad, NiMh cells, and lithium batteries are particularly risky because they can deliver a very high current due to their low internal resistance.
[edit] See alsoElectronics portal
Alternating current Current density Direct current Electrical conduction for more information on the physical mechanism of current
flow in materials Four-current History of electrical engineering Hydraulic analogy SI electromagnetism units
AmpereFrom Wikipedia, the free encyclopediaJump to: navigation, searchFor other uses, see Ampere (disambiguation).
Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the current.
The ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current, or amount of electric charge per second. The ampere is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.
1 Definition 2 History 3 Realization 4 Proposed future definition
o 4.1 CIPM recommendation 5 See also 6 References
7 External links
[edit] Definition
One ampere is defined to be the constant current which will produce a force of 2×10–7 newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum.[1][2] The definition is based on Ampère's force law .[3] . The ampere is a base unit, along with the metre, kelvin, second, mole, candela and the kilogram: it is defined without reference to the quantity of electric charge.
The S.I. unit of charge, the coulomb, is defined to be the quantity of charge displaced by a one ampere per second.[4] Conversely, an ampere is one coulomb of charge going past a given point in the duration of one second; that is, in general, charge Q is determined by steady current I flowing per unit time t as:
[edit] History
The ampere was originally defined as one tenth of the CGS system electromagnetic unit of current (now known as the abampere), the amount of current which generates a force of two dynes per centimetre of length between two wires one centimetre apart. [5] - the size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized.
The "international ampere" was an early realization of the ampere, defined as the current that would deposit 0.001118000 grams of silver per second from a silver nitrate solution. [6] Later, more accurate measurements revealed that this current is 0.99985 A.
[edit] Realization
The ampere is most accurately realized using a watt balance, but is in practice maintained via Ohm's Law from the units of EMF and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively. The official realization of a
standard ampere is discussed in NIST Special publication 330 Barry N Taylor (editor) Appendix 2, p. 56.
[edit] Proposed future definition
Since a coulomb is approximately equal to 6.24150948×1018 elementary charges, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as electrons, moving past a boundary in one second.
As with other SI base units, there have been proposals to redefine the kilogram in such a way as to define some presently measured physical constants to fixed values. One proposed definition of the kilogram is:
The kilogram is the mass which would be accelerated at precisely 2×10-7 m/s2 if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6 241 509 479 607 717 888 elementary charges per second.
This redefinition of the kilogram has the effect of fixing the elementary charge to be e = 1.60217653×10-19 C and would result in a functionally equivalent definition for the coulomb as being the sum of exactly 6 241 509 479 607 717 888 elementary charges and the ampere as being the electrical current of exactly 6 241 509 479 607 717 888 elementary charges per second. This is consistent with the current 2002 CODATA value for the elementary charge which is 1.60217653×10-19 ± 0.00000014×10-19 C.
[edit] CIPM recommendation
International Committee for Weights and Measures (CIPM) Recommendation 1 (CI-2005): Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants
The International Committee for Weights and Measures (CIPM),
approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre Convention in time for possible adoption by the 24th CGPM in 2011;
give consideration to the possibility of redefining, at the same time, the mole in terms of a fixed value of the Avogadro constant;
prepare a Draft Resolution that may be put to the 23rd CGPM in 2007 to alert Member States to these activities;
This SI unit is named after André-Marie Ampère. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is
uppercase (A). When an SI unit is spelled out in English, it should always begin with a lowercase letter (ampere), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
— Based on The International System of Units, section 5.2.
[edit] See also International System of Units Ohm's Law Hydraulic analogy Electric shock Ampère's force law Ammeter Coulomb Magnetic constant
[edit] References1. ^ BIPM official definition 2. ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical World, John
Wiley and Sons, 2004 online. 3. ^ Raymond A Serway & Jewett JW (2006). Serway's principles of physics: a calculus
based text, Fourth Edition, Belmont, CA: Thompson Brooks/Cole, p. 746. ISBN 053449143X.
4. ^ BIPM Table 3 5. ^ A short history of the SI units in electricity 6. ^ History of the ampere
Electric currentFrom Wikipedia, the free encyclopediaJump to: navigation, search
Electromagnetism
Electricity · Magnetism
[show]Electrostatics
[hide]Magnetostatics · Ampère’s law · Electric current ·
Electric current is the flow (movement) of electric charge. The SI unit of electric current is the ampere, and electric current is measured using an ammeter. For the definition of the ampere, see the Ampere article.
Contents[hide]
1 Current in a metal wire 2 Current density 3 The drift speed of electric charges 4 Ohm's law 5 Conventional current 6 Examples 7 Electromagnetism 8 Reference direction 9 Electrical safety 10 See also 11 References
12 External links
[edit] Current in a metal wire
A solid conductive metal contains a large population of mobile, or free, electrons. These electrons are bound to the metal lattice but not to any individual atom. Even with no external electric field applied, these electrons move about randomly due to thermal energy but, on average, there is zero net current within the metal. Given an imaginary plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is on average equal to the number passing in the opposite direction.
A typical metal wire for electrical conduction is the stranded copper wire.
When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electrons are therefore the current carrier in a typical solid conductor. For an electric current of 1 ampere, 1 coulomb of electric charge (which consists of about 6.242 × 1018 electrons) drifts every second through any imaginary plane through which the conductor passes.
The current I in amperes can be calculated with the following equation:
where
is the electric charge in coulombs (ampere seconds) is the time in seconds
It follows that:
and
More generally, electric current can be represented as the time rate of change of charge, or
.
[edit] Current densityMain article: Current density
Current density is a measure of the density of electrical 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 meter.
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 direction of the electric field. The speed at which they drift can be calculated from the equation:
where
is the electric current is number of charged particles per unit volume is the cross-sectional area of the conductor is the drift velocity, and is the charge on each particle.
Electric currents in solids typically flow very slowly. For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light.
Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wave that propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction of the speed of light, as can be deduced from Maxwell's Equations, and is therefore many times faster than the drift velocity of the electrons. For example, in AC power lines, the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance.
The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the velocity factor, and depends on the electromagnetic properties of the conductor and the insulating materials surrounding it, and on their shape and size.
The nature of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases. The low drift velocity of charge carriers is analogous to air motion; in other words, winds. The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas; while the random motion of charges is analogous to heat - the thermal velocity of randomly vibrating gas particles.
[edit] Ohm's law
Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the applied voltage divided by resistance:
I is the current, measured in amperes V is the potential difference measured in volts R is the resistance measured in ohms
[edit] Conventional current
Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charge carriers are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron current is in the direction opposite to that of conventional (or electric) current.
Diagram showing conventional current notation. Electric charge moves from the positive side of the power source to the negative.
In other conductive materials, the electric current is due to the flow of charged particles in both directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chloride ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands.
There are also materials where the electric current is due to the flow of electrons and yet it is conceptually easier to think of the current as due to the flow of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor.
[edit] Examples
Natural examples include lightning and the solar wind, the source of the polar auroras (the aurora borealis and aurora australis). The artificial form of electric current is 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. 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, and the flow of holes within a semiconductor.
According to Ampère's law, an electric current produces a magnetic field.
[edit] Electromagnetism
Electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire.
Electric current can be directly measured with a galvanometer, but this method involves breaking the 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.
[edit] Reference direction
When solving 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.
[edit] Electrical safety
The most obvious hazard is electrical shock, where a current passes through part of the body. It is the amount of current passing through the body that determines the effect, and this depends on the nature of the contact, the condition of the body part, the current path through the body and the voltage of the source. While a very small amount can cause a slight tingle, too much can cause severe burns if it passes through the skin or even cardiac arrest if enough passes through the heart. The effect also varies considerably from individual to individual. (For approximate figures see Shock Effects under electric shock.)
Due to this and the fact that passing current cannot be easily predicted in most practical circumstances, any supply of over 50 volts should be considered a possible source of dangerous electric shock. In particular, note that 110 volts (a minimum voltage at which AC mains power is distributed in much of the Americas, and 4 other countries, mostly in Asia) can certainly cause a lethal amount of current to pass through the body.
Electric arcs, which can occur with supplies of any voltage (for example, a typical arc welding machine has a voltage between the electrodes of just a few tens of volts), are very hot and emit ultra-violet (UV) and infra-red radiation (IR). Proximity to an electric arc can therefore cause severe thermal burns, and UV is damaging to unprotected eyes and skin.
Accidental electric heating can also be dangerous. An overloaded power cable is a frequent cause of fire. A battery as small as an AA cell placed in a pocket with metal coins can lead to a short circuit heating the battery and the coins which may inflict burns. NiCad, NiMh cells, and lithium batteries are particularly risky because they can deliver a very high current due to their low internal resistance.
[edit] See alsoElectronics portal
Alternating current Current density Direct current Electrical conduction for more information on the physical mechanism of current
flow in materials Four-current History of electrical engineering Hydraulic analogy SI electromagnetism units
