Elektrik

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.
8. More to come...

Tips

• 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 Codes
Resistor 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%.

Watt
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For 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.

[edit] Terawatt

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:

(1 kW·h)(1000 W/kW)(3600 s/h) = 3,600,000 W·s = 3,600,000 J = 3.6 MJ.

[edit] See also
Energy portal
• 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] References
1. ^ "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

Ohm
This 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.

Contents
[hide]
• 1 Definition
• 2 Conversions
• 3 References and notes
• 4 See also


[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 notes
1. ^ 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
Sciences Nuernberg
• Official SI brochure
• NIST Special Publication 811
• History of the ohm at sizes.com

Retrieved from "http://en.wikipedia.org/wiki/Ohm"

Voltage

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.

Contents
[hide]
• 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


[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.

[edit] Hydraulic analogy
Main article: Hydraulic analogy

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.

[edit] Measuring instruments

A multimeter set to measure voltage.

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 current
(Redirected from Current (electricity))

Electromagnetism

Electricity · Magnetism

[show]Electrostatics
[hide]Magnetostatics

· Ampère’s law · Electric current ·
Magnetic field · Magnetic flux · Biot–Savart
law · Magnetic dipole moment · Gauss’s law
for magnetism ·
[show]Electrodynamics
[show]Electrical Network
[show]Covariant formulation
[show]Scientists

This box: view • talk • edit

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


[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 density
Main 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.

[edit] The drift speed of electric charges

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:

where

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 also
Electronics 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

Ampere
For 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.

Contents
[hide]
• 1 Definition
• 2 History
• 3 Realization
• 4 Proposed future definition
o 4.1 CIPM recommendation
• 5 See also
• 6 References


[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
• Electric shock
• Ampère's force law
• Ammeter
• Coulomb
• Magnetic constant

[edit] References
1. ^ 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 current

Electromagnetism

Electricity · Magnetism

[show]Electrostatics
[hide]Magnetostatics
· Ampère’s law · Electric current ·
Magnetic field · Magnetic flux · Biot–Savart
law · Magnetic dipole moment · Gauss’s law

for magnetism ·
[show]Electrodynamics
[show]Electrical Network
[show]Covariant formulation
[show]Scientists

This box: view • talk • edit

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


[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.

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 also
Electronics 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
• SI electromagnetism units

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