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60-697: Electric currents that oscillate at radio frequencies ( RF currents ) have special properties not shared by direct current or lower audio frequency alternating current , such as the 50 or 60 Hz current used in electrical power distribution . The radio spectrum of frequencies is divided into bands with conventional names designated by the International Telecommunication Union (ITU): Frequencies of 1 GHz and above are conventionally called microwave , while frequencies of 30 GHz and above are designated millimeter wave . More detailed band designations are given by

120-430: A galvanometer , but this method involves breaking the electrical circuit , which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field associated with the current. Devices, at the circuit level, use various techniques to measure current: Joule heating, also known as ohmic heating and resistive heating , is the process of power dissipation by which

180-400: A molecule or atom with a surplus or deficit of electrons relative to protons are also charged particles. A plasma is a collection of charged particles, atomic nuclei and separated electrons, but can also be a gas containing a significant proportion of charged particles. Charged particles are labeled as either positive (+) or negative (-). The designations are arbitrary. Nothing

240-453: A rectifier . Direct current may flow in a conductor such as a wire, but can also flow through semiconductors , insulators , or even through a vacuum as in electron or ion beams . An old name for direct current was galvanic current . Natural observable examples of electric current include lightning , static electric discharge , and the solar wind , the source of the polar auroras . Man-made occurrences of electric current include

300-494: A wire . In semiconductors they can be electrons or holes . In an electrolyte the charge carriers are ions , while in plasma , an ionized gas, they are ions and electrons. In the International System of Units (SI), electric current is expressed in units of ampere (sometimes called an "amp", symbol A), which is equivalent to one coulomb per second. The ampere is an SI base unit and electric current

360-411: A circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention is needed for the direction of current that is independent of the type of charge carriers . Negatively charged carriers, such as the electrons (the charge carriers in metal wires and many other electronic circuit components), therefore flow in

420-405: A common lead-acid electrochemical cell, electric currents are composed of positive hydronium ions flowing in one direction, and negative sulfate ions flowing in the other. Electric currents in sparks or plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, the electric current is entirely composed of flowing ions. In a metal , some of

480-416: A definition of current independent of the type of charge carriers, conventional current is defined as moving in the same direction as the positive charge flow. So, in metals where the charge carriers (electrons) are negative, conventional current is in the opposite direction to the overall electron movement. In conductors where the charge carriers are positive, conventional current is in the same direction as

540-447: A localized high current. These regions may be initiated by field electron emission , but are then sustained by localized thermionic emission once a vacuum arc forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field. Vacuum tubes and sprytrons are some of the electronic switching and amplifying devices based on vacuum conductivity. Superconductivity

600-428: 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 charge carrier in a typical solid conductor. For a steady flow of charge through

660-404: 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 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

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720-408: A particular band called the valence band . Semiconductors and insulators are distinguished from metals because the valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few (semiconductor) or virtually none (insulator) of them are available in the conduction band , the band immediately above the valence band. The ease of exciting electrons in

780-475: 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

840-405: A surface, the current I (in amperes) can be calculated with the following equation: I = Q t , {\displaystyle I={Q \over t}\,,} where Q is the electric charge transferred through the surface over a time t . If Q and t are measured in coulombs and seconds respectively, I is in amperes. More generally, electric current can be represented as

900-603: Is I , which originates from the French phrase intensité du courant , (current intensity). Current intensity is often referred to simply as current . The I symbol was used by André-Marie Ampère , after whom the unit of electric current is named, in formulating Ampère's force law (1820). The notation travelled from France to Great Britain, where it became standard, although at least one journal did not change from using C to I until 1896. The conventional direction of current, also known as conventional current ,

960-551: Is a base quantity in the International System of Quantities (ISQ). Electric current is also known as amperage and is measured using a device called an ammeter . Electric currents create magnetic fields , which are used in motors, generators, inductors , and transformers . In ordinary conductors, they cause Joule heating , which creates light in incandescent light bulbs . Time-varying currents emit electromagnetic waves , which are used in telecommunications to broadcast information. The conventional symbol for current

1020-407: Is a flow of charged particles , such as electrons or ions , moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge carriers , which may be one of several types of particles, depending on the conductor . In electric circuits the charge carriers are often electrons moving through

1080-523: Is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature . It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden . Like ferromagnetism and atomic spectral lines , superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect ,

1140-411: Is arbitrarily defined as the direction in which positive charges flow. In a conductive material , the moving charged particles that constitute the electric current are called charge carriers . In metals, which make up the wires and other conductors in most electrical circuits , the positively charged atomic nuclei of the atoms are held in a fixed position, and the negatively charged electrons are

1200-683: Is current. Magnetic fields can also be used to make electric currents. When a changing magnetic field is applied to a conductor, an electromotive force (EMF) is induced, which starts an electric current, when there is a suitable path. When an electric current flows in a suitably shaped conductor at radio frequencies , radio waves can be generated. These travel at the speed of light and can cause electric currents in distant conductors. In metallic solids, electric charge flows by means of electrons , from lower to higher electrical potential . In other media, any stream of charged objects (ions, for example) may constitute an electric current. To provide

1260-423: Is in a nanowire , for every energy there is a state with electrons flowing in one direction and another state with the electrons flowing in the other. For a net current to flow, more states for one direction than for the other direction must be occupied. For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap. Often this is stated as: full bands do not contribute to

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1320-468: Is low, gases are dielectrics or insulators . However, once the applied electric field approaches the breakdown value, free electrons become sufficiently accelerated by the electric field to create additional free electrons by colliding, and ionizing , neutral gas atoms or molecules in a process called avalanche breakdown . The breakdown process forms a plasma that contains enough mobile electrons and positive ions to make it an electrical conductor. In

1380-455: Is opposite that of the chosen reference direction. Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance , one arrives at the usual mathematical equation that describes this relationship: I = V R , {\displaystyle I={\frac {V}{R}},} where I

1440-658: Is opposite to the velocity of the charges. In SI units , current density (symbol: j) is expressed in the SI base units of amperes per square metre. In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform. In such conditions, Ohm's law states that the current is directly proportional to the potential difference between two ends (across) of that metal (ideal) resistor (or other ohmic device ): I = V R , {\displaystyle I={V \over R}\,,} where I {\displaystyle I}

1500-433: Is the current through the conductor in units of amperes , V is the potential difference measured across the conductor in units of volts , and R is the resistance of the conductor in units of ohms . More specifically, Ohm's law states that the R in this relation is constant, independent of the current. In alternating current (AC) systems, the movement of electric charge periodically reverses direction. AC

1560-400: Is the current, measured in amperes; V {\displaystyle V} is the potential difference , measured in volts ; and R {\displaystyle R} is the resistance , measured in ohms . For alternating currents , especially at higher frequencies, skin effect causes the current to spread unevenly across the conductor cross-section, with higher density near

1620-441: Is the form of electric power most commonly delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave , though certain applications use alternative waveforms, such as triangular or square waves . Audio and radio signals carried on electrical wires are also examples of alternating current. An important goal in these applications is recovery of information encoded (or modulated ) onto

1680-461: Is the rate at which charge passes through a chosen unit area. It is defined as a vector whose magnitude is the current per unit cross-sectional area. As discussed in Reference direction , the direction is arbitrary. Conventionally, if the moving charges are positive, then the current density has the same sign as the velocity of the charges. For negative charges, the sign of the current density

1740-403: Is zero net current within the metal. At room temperature, the average speed of these random motions is 10 metres per second. Given a surface through which a metal wire passes, electrons move in both directions across the surface at an equal rate. As George Gamow wrote in his popular science book, One, Two, Three...Infinity (1947), "The metallic substances differ from all other materials by

1800-405: The electrical conductivity . However, as a semiconductor's temperature rises above absolute zero , there is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band. The current-carrying electrons in the conduction band are known as free electrons , though they are often simply called electrons if that is clear in context. Current density

1860-458: The reference direction of the current I {\displaystyle I} . When analyzing electrical circuits , the actual direction of current through a specific circuit element is usually unknown until the analysis is completed. Consequently, the reference directions of currents are often assigned arbitrarily. When the circuit is solved, a negative value for the current implies the actual direction of current through that circuit element

Radio frequency - Misplaced Pages Continue

1920-418: The watt (symbol: W), is equivalent to one joule per second. In an electromagnet a coil of wires behaves like a magnet when an electric current flows through it. When the current is switched off, the coil loses its magnetism immediately. Electric current produces a magnetic field . The magnetic field can be visualized as a pattern of circular field lines surrounding the wire that persists as long as there

1980-449: The AC signal. In contrast, direct current (DC) refers to a system in which the movement of electric charge in only one direction (sometimes called unidirectional flow). Direct current is produced by sources such as batteries , thermocouples , solar cells , and commutator -type electric machines of the dynamo type. Alternating current can also be converted to direct current through use of

2040-1051: The body. That being said, there is limited studies on how effective these devices are. Test apparatus for radio frequencies can include standard instruments at the lower end of the range, but at higher frequencies, the test equipment becomes more specialized. While RF usually refers to electrical oscillations, mechanical RF systems are not uncommon: see mechanical filter and RF MEMS . ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm Electric current An electric current

2100-412: The charge carriers, free to move about in the metal. In other materials, notably the semiconductors , the charge carriers can be positive or negative, depending on the dopant used. Positive and negative charge carriers may even be present at the same time, as happens in an electrolyte in an electrochemical cell . A flow of positive charges gives the same electric current, and has the same effect in

2160-402: The charge carriers. In a vacuum , a beam of ions or electrons may be formed. In other conductive materials, the electric current is due to the flow of both positively and negatively charged particles at the same time. In still others, the current is entirely due to positive charge flow . For example, the electric currents in electrolytes are flows of positively and negatively charged ions. In

2220-534: The complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics . In a semiconductor it is sometimes useful to think of the current as due to the flow of positive " holes " (the mobile positive charge carriers that are places where

2280-544: The correct voltages within radio antennas , radio waves are generated. In electronics , other forms of electric current include the flow of electrons through resistors or through the vacuum in a vacuum tube , the flow of ions inside a battery , and the flow of holes within metals and semiconductors . A biological example of current is the flow of ions in neurons and nerves, responsible for both thought and sensory perception. Current can be measured using an ammeter . Electric current can be directly measured with

2340-520: The current proliferation of radio frequency wireless telecommunications devices such as cellphones . Medical applications of radio frequency (RF) energy, in the form of electromagnetic waves ( radio waves ) or electrical currents, have existed for over 125 years, and now include diathermy , hyperthermy treatment of cancer, electrosurgery scalpels used to cut and cauterize in operations, and radiofrequency ablation . Magnetic resonance imaging (MRI) uses radio frequency fields to generate images of

2400-410: The energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material. The energy bands each correspond to many discrete quantum states of the electrons, and most of the states with low energy (closer to the nucleus) are occupied, up to

2460-458: The fact that the outer shells of their atoms are bound rather loosely, and often let one of their electrons go free. Thus the interior of a metal is filled up with a large number of unattached electrons that travel aimlessly around like a crowd of displaced persons. When a metal wire is subjected to electric force applied on its opposite ends, these free electrons rush in the direction of the force, thus forming what we call an electric current." When

Radio frequency - Misplaced Pages Continue

2520-459: The flow of conduction electrons in metal wires such as the overhead power lines that deliver electrical energy across long distances and the smaller wires within electrical and electronic equipment. Eddy currents are electric currents that occur in conductors exposed to changing magnetic fields. Similarly, electric currents occur, particularly in the surface, of conductors exposed to electromagnetic waves . When oscillating electric currents flow at

2580-410: The heavier positive ions, and hence carry the bulk of the current. The free ions recombine to create new chemical compounds (for example, breaking atmospheric oxygen into single oxygen [O 2 → 2O], which then recombine creating ozone [O 3 ]). Since a " perfect vacuum " contains no charged particles, it normally behaves as a perfect insulator. However, metal electrode surfaces can cause a region of

2640-412: The human body. Radio Frequency or RF energy is also being used in devices that are being advertised for weight loss and fat removal. The possible effects RF might have on the body and whether RF can lead to fat reduction needs further study. Currently, there are devices such as trusculpt ID , Venus Bliss and many others utilizing this type of energy alongside heat to target fat pockets in certain areas of

2700-440: The length of the wire he deduced that the heat produced was proportional to the square of the current multiplied by the electrical resistance of the wire. P ∝ I 2 R . {\displaystyle P\propto I^{2}R.} This relationship is known as Joule's Law . The SI unit of energy was subsequently named the joule and given the symbol J . The commonly known SI unit of power,

2760-424: The metal into the vacuum. Externally heated electrodes are often used to generate an electron cloud as in the filament or indirectly heated cathode of vacuum tubes . Cold electrodes can also spontaneously produce electron clouds via thermionic emission when small incandescent regions (called cathode spots or anode spots ) are formed. These are incandescent regions of the electrode surface that are created by

2820-424: The moving electrons in metals. In certain electrolyte mixtures, brightly coloured ions are the moving electric charges. The slow progress of the colour makes the current visible. In air and other ordinary gases below the breakdown field, the dominant source of electrical conduction is via relatively few mobile ions produced by radioactive gases, ultraviolet light, or cosmic rays. Since the electrical conductivity

2880-400: The opposite direction of conventional current flow in an electrical circuit. A current in a wire or circuit element can flow in either of two directions. When defining a variable I {\displaystyle I} to represent the current, the direction representing positive current must be specified, usually by an arrow on the circuit schematic diagram . This is called

2940-403: The opposite direction of the electric field. The speed they drift at can be calculated from the equation: I = n A v Q , {\displaystyle I=nAvQ\,,} where Typically, electric charges in solids flow slowly. For example, in a copper wire of cross-section 0.5 mm , carrying a current of 5 A, the drift velocity of the electrons is on the order of

3000-502: The outer electrons in each atom are not bound to the individual molecules as they are in molecular solids , or in full bands as they are in insulating materials, but are free to move within the metal lattice . These conduction electrons can serve as charge carriers , carrying a current. Metals are particularly conductive because there are many of these free electrons. With no external electric field applied, these electrons move about randomly due to thermal energy but, on average, there

3060-413: The passage of an electric current through a conductor increases the internal energy of the conductor, converting thermodynamic work into heat . The phenomenon was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature rise due to a known current through the wire for a 30 minute period. By varying the current and

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3120-468: The process, it forms a light emitting conductive path, such as a spark , arc or lightning . Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their molecules or atoms. A plasma can be formed by high temperature , or by application of a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than

3180-401: The rate at which charge flows through a given surface as: I = d Q d t . {\displaystyle I={\frac {\mathrm {d} Q}{\mathrm {d} t}}\,.} Electric currents in electrolytes are flows of electrically charged particles ( ions ). For example, if an electric field is placed across a solution of Na and Cl (and conditions are right)

3240-501: The semiconductor crystal is missing a valence electron). This is the case in a p-type semiconductor. A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator . This means a conductivity roughly in the range of 10 to 10 siemens per centimeter (S⋅cm ). In the classic crystalline semiconductors, electrons can have energies only within certain bands (i.e. ranges of levels of energy). Energetically, these bands are located between

3300-503: The semiconductor from the valence band to the conduction band depends on the band gap between the bands. The size of this energy band gap serves as an arbitrary dividing line (roughly 4 eV ) between semiconductors and insulators . With covalent bonds, an electron moves by hopping to a neighboring bond. The Pauli exclusion principle requires that the electron be lifted into the higher anti-bonding state of that bond. For delocalized states, for example in one dimension – that

3360-419: The sodium ions move towards the negative electrode (cathode), while the chloride ions move towards the positive electrode (anode). Reactions take place at both electrode surfaces, neutralizing each ion. Water-ice and certain solid electrolytes called proton conductors contain positive hydrogen ions (" protons ") that are mobile. In these materials, electric currents are composed of moving protons, as opposed to

3420-519: 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. Charged particle In physics , a charged particle is a particle with an electric charge . For example, some elementary particles , like the electron or quarks are charged. Some composite particles like protons are charged particles. An ion , such as

3480-467: The standard IEEE letter- band frequency designations and the EU/NATO frequency designations. Radio frequencies are used in communication devices such as transmitters , receivers , computers , televisions , and mobile phones , to name a few. Radio frequencies are also applied in carrier current systems including telephony and control circuits. The MOS integrated circuit is the technology behind

3540-443: The surface, thus increasing the apparent resistance. The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas . (More accurately, a Fermi gas .) To create a net flow of charge, the particles must also move together with an average drift rate. Electrons are the charge carriers in most metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in

3600-405: The vacuum to become conductive by injecting free electrons or ions through either field electron emission or thermionic emission . Thermionic emission occurs when the thermal energy exceeds the metal's work function , while field electron emission occurs when the electric field at the surface of the metal is high enough to cause tunneling , which results in the ejection of free electrons from

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