Electromagnetism

Electricity · Magnetism Magnetostatics  · Ampère’s law · Electric current · Magnetic field · Magnetic flux · Biot–Savart law · Magnetic dipole moment · Gauss’s law for magnetism ·

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Electric current is the flow (movement) of electric charge. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. Magnetostatics is the study of static Magnetic fields In Electrostatics the charges are stationary whereas here the currents are stationary or dc(direct In Classical electromagnetism, Ampère's circuital law, discovered by André-Marie Ampère, relates the integrated Magnetic field around a closed In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Magnetic flux, represented by the Greek letter Φ ( Phi) is a measure of quantity of Magnetism, taking into account the strength and the extent of a Magnetic The Biot–Savart Law is an equation in electromagnetism that describes the Magnetic field B generated by an Electric current. In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. The SI unit of electric current is the ampere, and electric current is measured using an ammeter. The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second An ammeter is a Measuring instrument used to measure the Electric current in a circuit. For the definition of the ampere, see the Ampere article. The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second

Current in a metal wire

A solid conductive metal contains a large population of mobile, or free, electrons. A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. In Science and engineering, a conductor is a material which contains movable Electric charges. In Solid-state physics, the free electron model is a simple model for the behaviour of Valence electrons in a Crystal structure of a Metallic 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. Thermal energy is the sum of the sensible energy and latent energy. 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. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J

A typical metal wire for electrical conduction is the stranded copper wire. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 A wire is a single usually cylindrical, elongated string of drawn Metal.

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. Direct current ( DC) is the unidirectional flow of Electric charge. A voltage source is any device or system that produces an Electromotive force between its terminals OR derives a secondary voltage from a primary In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy 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. In Solid-state physics, the free electron model is a simple model for the behaviour of Valence electrons in a Crystal structure of a Metallic The free electrons are therefore the current carrier in a typical solid conductor. In Physics, a charge carrier denotes a free (mobile unbound particle carrying an Electric charge. For an electric current of 1 ampere, 1 coulomb of electric charge (which consists of about 6. The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. 242 × 10¹⁸ electrons) drifts every second through any imaginary plane through which the conductor passes. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units

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

Current density

Main article: Current density

Current density is a measure of the density of electrical current. The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units Current density is a measure of the Density of flow of a conserved charge. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different 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 ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second M^2 redirects here For other uses see M². CM2 redirects here

The drift speed of electric charges

The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas. This page is about the physical properties of gas as a state of matter 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 M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can 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. The drift velocity is the average Velocity that a particle such as an Electron, attains due to an Electric field.

Electric currents in solids typically flow very slowly. For example, in a copper wire of cross-section 0. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 A wire is a single usually cylindrical, elongated string of drawn Metal. 5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. The drift velocity is the average Velocity that a particle such as an Electron, attains due to an Electric field. 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. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or

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. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of 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. In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric 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. Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power If an electric circuit has a well-defined output terminal the circuit connected to this terminal (or its Input impedance) is the load.

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.

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

Conventional current

Conventional current was defined early in the history of electrical science as a flow of positive charge. Ohm's law applies to Electrical circuits it states that the current through a conductor between two points is directly proportional to the |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | An ohmic contact is a region on a Semiconductor device that has been prepared so that the current-voltage (I-V curve of the device is linear and symmetric Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second In Physics, the potential difference or pd between two points is the difference of the points' Scalar potential, equivalent to the line integral The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is The ohm (symbol Ω) is the SI unit of Electrical impedance or in the Direct current case Electrical resistance, In solid metals, like wires, the positive charge carriers are immobile, and only the negatively charged electrons flow. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J 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. An electrolyte is any substance containing free Ions that behaves as an electrically conductive medium An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge 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. Electrochemistry is a branch of Chemistry that studies Chemical reactions which take place in a Solution at the interface of an electron conductor For sodium chloride in the diet see Salt. Sodium chloride, also known as common salt, table salt, or Halite, is a 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 Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound In ice and in certain solid electrolytes, flowing protons constitute the electric current. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive 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). An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. This is the case in a p-type semiconductor. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that

Examples

Natural examples include lightning and the solar wind, the source of the polar auroras (the aurora borealis and aurora australis). Lightning is an atmospheric discharge of Electricity, which typically occurs during Thunderstorms and sometimes during volcanic eruptions or The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. 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. Electrical conduction is the movement of electrically charged particles through a Transmission medium ( Electrical conductor) The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J A wire is a single usually cylindrical, elongated string of drawn Metal. Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power 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. Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that

According to Ampère's law, an electric current produces a magnetic field. In Classical electromagnetism, Ampère's circuital law, discovered by André-Marie Ampère, relates the integrated Magnetic field around a closed In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges

Electromagnetism

Electric current produces a magnetic field. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges 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. A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Devices used for this include Hall effect sensors, current clamps, current transformers, and Rogowski coils. The Hall effect refers to the Potential difference ( Hall voltage) on the opposite sides of an Electrical conductor through which there is an Electric A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument In electrical and electronic engineering a current clamp or current probe is an electrical device having two jaws which open to allow clamping around an electrical conductor A current transformer ( CT) is a type of instrument Transformer designed to provide a current in its secondary winding proportional to the alternating current A Rogowski coil, named after Walter Rogowski, is an electrical device for measuring Alternating current (AC or high speed current pulses

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.

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. A burn is a type of Injury that may be caused by Heat, cold, Electricity, Chemicals, Light, Radiation, or A cardiac arrest, also known as cardiorespiratory arrest, cardiopulmonary arrest or circulatory arrest, is the abrupt cessation of normal circulation of The effect also varies considerably from individual to individual. (For approximate figures see Shock Effects under electric shock. "Electrocute" redirects here For the band see Electrocute (band. )

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. Mains is the general-purpose Alternating current (AC Electric power supply (as in "I've connected the appliance to the mains" This article includes a list of countries and territories with the plugs voltages and frequencies they use for providing electrical power to Small appliances

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). An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive Arc welding uses a Welding power supply to create an Electric arc between an electrode and the base material to melt the metals at the welding point An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of 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 power cable is an Assembly of two or more Electrical conductors, usually held together with an overall Sheath. 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. A AA battery (double a or /ˈdʌbəl eɪ/ is a dry cell-type battery commonly used in portable Electronic devices NiCad, NiMh cells, and lithium batteries are particularly risky because they can deliver a very high current due to their low internal resistance. The nickel-cadmium battery (commonly abbreviated NiCd and ˈnɑɪˌkæd "nye-cad" is a type of Rechargeable battery using Nickel oxide hydroxide A nickel-metal hydride battery, abbreviated NiMH, is a type of Rechargeable battery similar to a nickel-cadmium ( Ni[[Cadmium Cd]] battery Lithium batteries are not to be confused with lithium-ion batteries which are high energy-density rechargeable batteries Lithium batteries are Any linear electronic circuit or device which supplies a current may be modelled as an Ideal voltage source in series with an impedance.

See also

• Alternating current
• Current density
• Direct current
• Electrical conduction for more information on the physical mechanism of current flow in materials
• Four-current
• Hydraulic analogy
• SI electromagnetism units

References

External links

• All about circuits - a useful site introducing electricity and electronics
• Electric current and Ohm's law
• Electric current and power