Impedance wikipedia definition. Resonant circuits can generate very high voltages.

Impedance wikipedia definition. Examples of such applications include humans interacting with robots, where the force produced by the human relates to how fast the robot should move/stop. This can be a pronounced advantage in power system analysis where large numbers Parasitic impedance Parasitic elements of a typical electronic component package. [1] The symmetrical components corresponding to an asymmetrical set of three phasors are: [1 In the power systems analysis field of electrical engineering, a per-unit system is the expression of system quantities as fractions of a defined base unit quantity. The impedance is frequency-dependent because the reactance of the inductor and capacitor varies with a change in the frequency. Impedance is the extension of the concept of resistance to alternating current circuits. [1] This is typically caused when enough current is flowing in the connection between the two ground points to produce a voltage drop and cause the two points to be at different potentials. Simpler control methods Coaxial cable, or coax (pronounced / ˈkoʊ. It relates forces with velocities acting on a mechanical system. The formula is a combination of Ohm's law and Joule's law: where P is the power, R is the resistance, V is the voltage across the resistor, and I is the current through the resistor. The output impedance is a measure of the source's propensity to drop in voltage when the load draws current, the source network What is Impedance? Impedance is the amount of resistance that a component offers to current flow in a circuit at a specific frequency. Radiation resistance is that part of an antenna 's feedpoint electrical resistance caused by the emission of radio waves from the antenna. It is not a measure of eardrum or middle ear mobility Explore electrical impedance, its formula, and examples including resistors, capacitors, and inductors. When the power source delivers current, the measured voltage output is lower than the no- load voltage; the difference is the voltage drop (the product of current and resistance) caused by the internal resistance. Bei In-Vitro-Zellkulturen wird die elektrische Impedanz bei einer festen Frequenz als TEER-Wert angegeben. The term coaxial refers to the inner conductor and the outer shield sharing a geometric axis. Often, the desired value is selected to maximize power transfer or minimize signal reflection. It is not a hearing test, but rather a measure of energy transmission through the middle ear. In this article, we'll talk about how impedance is similar to and how it differs from just plain resistance. In electronics, a two-port network (a kind of four-terminal network or quadripole) is an electrical network (i. Object–relational impedance mismatch is a set of difficulties going between data in relational data stores and data in domain-driven object models. Sensitivity and input impedance When used for measuring voltage, the input impedance of the multimeter must be very high compared to the impedance of the circuit being measured; otherwise circuit operation may be affected and the reading will be inaccurate. Calculations are simplified because quantities expressed as per-unit do not change when they are referred from one side of a transformer to the other. It is measured in ohms. The characteristic impedance of a transmission line is the ratio of the amplitude of a single voltage wave to its current wave. This circuit does not have a discrete resistor, but all circuits have some intrinsic resistance, causing them to function as an RLC circuit. The term balanced comes from the method of balancing the impedance of each wire in the circuit; the The power dissipated by a resistor may be calculated from its resistance, and the voltage or current involved. The term is applied in a number of different fields, most often being encountered in respect of: The nominal value of the characteristic impedance of a cable or other form of transmission line. In SI units, susceptance is measured in siemens (S). Thermal resistance (or R-Value) is a thickness dependent property which describes its ability to resist heat flow. The rectangular devices are test connection blocks, used for testing and isolation of instrument transformer circuits. May 27, 2024 · Explore the essentials of electrical impedance, its analysis techniques, applications in electronics, and future trends in this comprehensive guide. The ability to manipulate these properties allows engineers to control temperature gradient, prevent thermal shock, and maximize the efficiency of Electromechanical protective relays at a hydroelectric generating plant. Oliver Heaviside coined the Image impedance is a concept used in electronic network design and analysis and most especially in filter design. [2] Balanced audio connections use a number of techniques to reduce noise. For DC systems, impedance and resistance are the same, defined as the voltage across an element divided by the current (R = V/I). This results in an excessive current flowing through the circuit. In an electric circuit consisting of a capacitor and an inductor in parallel, antiresonance occurs when the alternating current line voltage and the resultant current are in phase. When calculating the impedance, resistance represents the real part, while reactance is the Vascular resistance is the resistance that must be overcome for blood to flow through the circulatory system. Il vient du verbe anglais to impede signifiant « retenir Line level is the specified strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles. The key difference between resistance and impedance is the word "change", the rate of change affects the impedance. First, we'll not the similarities of the two. Usually a two-port network is implied but the concept can be extended to networks with more than two ports. In some circuits, this happens when the impedance between the input and output of the circuit is almost zero and the transfer function is SWR of a vertical HB9XBG Antenna for the 40m-band as a function of frequency In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. when there are no sound waves travelling in it). Measured in ohms, it influences voltage, current, and power flow. Image impedance Image impedance is a concept used in electronic network design and analysis and most especially in filter design. There are weaker signals such as those from microphones (Mic Level/Microphone Level) and Transformer equivalent circuit impedance and transformer ratio parameters can be derived from the following tests: open-circuit test, short-circuit test, winding resistance test, and transformer ratio test. The complex generalization of resistance is impedance, usually denoted Z; it can be shown that for an inductor, and for a capacitor, We can now write, where V and I are the complex scalars in the voltage and current respectively and Z is the complex impedance. Current In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber. In der Biologie kann mittels Electric Cell-Substrate Impedance Sensing die Impedanz genutzt werden, um Formveränderungen bei tierischen Zellen nachzuweisen. Sep 26, 2025 · Electrical impedance, measure of the total opposition that a circuit or a part of a circuit presents to electric current. Characteristic acoustic impedance The rayl is also used for the characteristic (acoustic) impedance of a medium, which is an inherent property of a medium: [6] Here, is the characteristic impedance, and and are the density and speed of sound in the unperturbed medium (i. In a high-impedance grounded system, the fault current is limited to a few amperes (exact values depend on the voltage class of the system); a low-impedance grounded system will permit several hundred amperes to flow on a fault. g. I have known electronic enthusiasts who still couldn't even mentally visualise the concept of impedance even after 25 years This impedance is termed the internal resistance of the source. Dielectric spectroscopy (which falls in a subcategory of the impedance spectroscopy) measures the dielectric properties of a medium as a function of frequency. . [a][1][2] A radio transmitter applies a radio frequency alternating current to an antenna, which radiates the energy of the current as radio waves. [1] It's measured in Ω (Ohms). It is commonly used to characterize materials made by semiconductor doping, metal deposition, resistive paste Tree limbs cause a short circuit during a storm, triggering an electrical arc. The term image impedance applies to the impedance seen looking in to the ports of a network. It is usually expressed as a ratio in decibels (dB): RL ( dB ) = 10 log 10 ⁡ P i P r Impedance cardiography (ICG; also called electrical impedance plethysmography, EIP, or thoracic electrical bioimpedance, TEB) is a non-invasive technology measuring total electrical conductivity of the thorax and its changes over time. Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area, also called the sound power density and the sound energy flux density. The reciprocal of admittance is impedance (Z = R + jX), where the imaginary part is reactance (X) and the real part is resistance (R). What is impedance? Learn how it combines resistance and reactance to oppose AC in circuits. Impedance, just like resistance, is a value which show the amount of resistance that a component has to What is Impedance? Impedance is the amount of resistance that a component offers to current flow in a circuit at a specific frequency. Oct 19, 2018 · This page is about the impedance of an electrical circuit. Impedance includes both resistance and reactance. These parameters are essential for effective communication. Impedance control is an approach to dynamic control relating force and position. Jun 11, 2023 · Impedance is a complex value representing the opposition a circuit element presents to the flow of AC or complex voltage, including resistance and reactance. Inductors and capacitors have "reactance", and impedance is the combination of the effects of resistance and reactance. At these frequencies, the physical length of passive components is a In microwave and radio-frequency engineering, a stub or resonant stub is a transmission line or waveguide connected at one end only. Definition of Impedance In AC electric circuits, the impedance is defined as the total electrical opposition offered by the circuit elements to the flow of alternating current through the circuit. The page shows the basic definition of impedance, physical significance of impedances and representation of different forms of impedance, including impedances of series and parallel RL, RC and RLC circuits. The relays are in round glass cases. Impedance is typically measured at 1 kHz. Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. [citation needed] High impedance headphones require more voltage, and the loudness of the headphones for a given voltage decreases. The definition of image impedance for a two - port The driving point impedance is a mathematical representation of the input impedance of a filter in the frequency domain using one of a number of notations such as Laplace transform (s-domain) or Fourier transform (jω-domain). In the case of impedance, an inductor resists changes to the current and the capacitor resists changes to the voltage. Complex resistivity and conductivity When analyzing the response of materials to alternating electric fields (dielectric spectroscopy), [60] in applications such as electrical impedance tomography, [61] it is convenient to replace resistivity with a complex quantity called impedivity (in analogy to electrical impedance). For A pneumograph, also known as a pneumatograph or spirograph, is a device for recording velocity and force of chest movements during respiration. Neglecting transmission line losses, the input impedance of the stub is purely reactive; either capacitive or inductive, depending on the electrical Electrical impedance tomography (EIT) is a noninvasive type of medical imaging in which the electrical conductivity, permittivity, and impedance of a part of the body is inferred from surface electrode measurements and used to form a tomographic image of that part. [1] It is usually a measurement of electrical resistance of thin films that are uniform in thickness. L'exemple le plus simple est lorsqu'un enfant pousse un autre sur une In materials science Giant Magnetoimpedance (GMI) is the effect that occurs in some materials where an external magnetic field causes a large variation in the electrical impedance of the material. Regulations for earthing systems vary among countries, though most follow the Antenna measurement techniques refer to the testing of antennas to ensure that they meet specifications or simply to characterize them. The resistance offered by the systemic circulation is known as the systemic vascular resistance or may sometimes be called by another term total peripheral resistance, while the resistance caused by the pulmonary circulation is known as the pulmonary vascular resistance The impedance to ground of both legs is the same and the line remains balanced. In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static (resistance) and dynamic (reactance), into the load network being connected that is internal to the electrical source. The electric current produces a magnetic field around the conductor. Aux fréquences de résonance, l'impédance mécanique sera plus faible, ce qui signifie qu'une quantité moindre de force est nécessaire pour déplacer une structure à une vitesse donnée. The antenna pattern is the response of the antenna to a plane wave incident from a given Impedance is an extension of the concept of resistance that includes the effects of capacitance and inductance. The definition of image impedance for a two-port network is the impedance, Zi 1, seen Impedance parameters or Z-parameters (the elements of an impedance matrix or Z-matrix) are properties used in electrical engineering, electronic engineering, and communication systems engineering to describe the electrical behavior of linear electrical networks. Electrical resonance occurs in an electric circuit at a particular resonant frequency when the impedances or admittances of circuit elements cancel each other. Impedance What is Impedance? Definition Impedance, represented by the symbol Z, is a measure of the opposition to electrical flow. This discontinuity can be caused by a mismatch between the termination or load connected to the line and the characteristic impedance of the line. Usually a two - port network is implied but the concept is capable of being extended to networks with more than two ports. In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. In heat transfer, thermal engineering, and thermodynamics, thermal conductance and thermal resistance are fundamental concepts that describe the ability of materials or systems to conduct heat and the opposition they offer to the heat current. Transformer equivalent circuit impedance and transformer ratio parameters can be derived from the following tests: open-circuit test, short-circuit test, winding resistance test, and transformer ratio test. Impedance Definition: 1) The total opposing force to the flow of current in an ac circuit. In some circuits, this happens when the impedance between the input and output of the circuit is almost zero and the transfer function is This leads to the concept of impedance matching, the design of the overall system of antenna and transmission line so the impedance is as close as possible, thereby reducing these losses. La définition de l'impédance est une généralisation de la loi d'Ohm au courant alternatif. It is essential in analyzing AC circuits, as it affects how voltage and current behave when sinusoidal sources are applied. It is often used in applications where a manipulator interacts with its environment and the force position relation is of concern. From Faraday's law of induction, any change in magnetic field Aug 30, 2022 · Thermal impedance is a measure of the sum of a) thermal resistance and b) thermal contact resistance of a material. 2) The combination of resistance and reactance affecting the flow of an alternating current generally expressed in ohms. In electrical circuits, reactance is the opposition presented to alternating current by inductance and capacitance. Resistor based on the sheet resistance of carbon film Sheet resistance is the resistance of a square piece of a thin material with contacts made to two opposite sides of the square. An RLC circuit is an electrical circuit consisting of a resistor (R), an inductor (L), and a capacitor (C), connected in series or in Learn about characteristic impedance (Z₀), its definition, and equations for calculating it in various transmission lines, including parallel, coaxial, and striplines. Impedance is the measure of opposition to alternating current in electrical circuits, combining resistance, inductive reactance, and capacitive reactance. [2][3][4][5] It is based on the interaction of an external field with the electric dipole moment of the sample, often expressed by permittivity. The reactance of the inductor is inversely proportional to the Electrical susceptance In electrical engineering, susceptance (B) is the imaginary part of admittance (Y = G + jB), where the real part is conductance (G). The receiving amplifier still rejects any common-mode noise as it has a differential input. Generally, line-level signals sit in the middle of the hierarchy of signal levels in audio engineering. The opposite of a short circuit is an open circuit, which In an electrical system, a ground loop or earth loop occurs when two points of a circuit are intended to have the same ground reference potential but instead have a different potential between them. In AC systems, the "reactance" enters the equation due to the frequency-dependent contributions of capacitance HDMI (High-Definition Multimedia Interface) is a brand of proprietary digital interface used to transmit high-quality video and audio signals between devices. Crucial in electronics and electrical transmission. Impedance, just like resistance, is a value which show the amount of resistance that a component has to High impedance In electronics, high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. It is a complex quantity A series RLC network (in order): a resistor, an inductor, and a capacitor Tuned circuit of a shortwave radio transmitter. This form of Ohm's law, with Z taking the place of R, generalizes the simpler form. Tympanometry is an objective test of middle-ear function. Mar 28, 2017 · Elastic impedance, , is an effective impedance dependent on the incident angle, . Understanding impedance helps in calculating power consumption and behavior in circuits containing resistors Figure 1: Example two-port network with symbol definitions. [10] Iterative impedance is the input impedance of an infinite chain of identical networks. High impedance circuits are low current and potentially high voltage, whereas low impedance circuits are the opposite (low voltage and potentially high current). [1][2] Mechanical impedance is the inverse of mechanical admittance or mobility. In electrical engineering, admittance is a measure of how easily a circuit or device will allow a current to flow. It is commonly used to connect devices such as televisions, computer monitors, projectors, gaming consoles, and personal computers. When representing the electrical circuit parameters as vectors in a complex plane, known as phasors, a capacitor's loss tangent is equal to the tangent of the angle between the capacitor's impedance vector and the negative reactive The characteristic impedance of the line changes slightly with frequency (again, even with a non-dispersive substrate material). The free end of the stub is either left open-circuit, or short-circuited (as is always the case for waveguides). The term image impedance applies to the impedance seen looking into a port of a network. The characteristic impedance of non-TEM modes is not uniquely defined, and depending on the precise definition used, the impedance of microstrip either rises, falls, or falls then rises with increasing frequency. In accelerator physics, impedance is a quantity that characterizes the self interaction of a charged particle beam, mediated by the beam environment, such as the vacuum chamber, RF cavities, and other elements encountered along the accelerator or storage ring. It should not be confused with the separate physical phenomenon of Giant Magnetoresistance. [1] Impedance is the complex-valued generalization of resistance. Typical antenna parameters include gain, bandwidth, radiation pattern, beamwidth, polarization, and impedance. Two terminals constitute a port if the Tympanometry is an acoustic evaluation of the condition of the middle ear [1] eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal. Impedance is a vector (two-dimensional) quantity consisting of two independent scalar (one-dimensional) phenomena: resistance and reactance. In electrical networks, a parasitic impedance is a circuit element (resistance, inductance or capacitance) which is not desirable in an electrical component for its intended purpose. The nominal value of the input, output or image Mechanical impedance is a measure of how much a structure resists motion when subjected to a harmonic force. In electrical engineering, the input impedance of an electrical network is the measure of the opposition to current (impedance), both static (resistance) and dynamic (reactance), into a load network or circuit that is external to the electrical source network. Relational Database Management Systems (RDBMS) is the standard method for storing data in a dedicated database, while object-oriented (OO) programming is the default method for business-centric Er kann mittels Zeitbereichsreflektometrie bestimmt werden. e. While spirometry measures respiratory rate and other markers of respiratory health via analysis of the airflow from the lungs during inhalation and exhalation, pneumography measures the respiratory rate via chest motion. Learn about reactance and Ohm's Law. A linear resistor has a constant resistance value over all applied voltages or currents; many The impedance of free space (that is, the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability μ0 and the speed of light in vacuum c0. The Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The mechanical impedance of a point on a structure is the ratio of the force applied at a point to the resulting velocity at that point. L' impédance électrique mesure l'opposition d'un circuit électrique au passage d'un courant alternatif sinusoïdal. Set of three unbalanced phasors, and the necessary symmetrical components that sum up to the resulting plot at the bottom. [2] The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m 2). Distributed-element filters are mostly used at frequencies above the VHF (Very High Frequency) band (30 to 300 MHz). The magnetic field strength depends on the magnitude of the electric current, and therefore follows any changes in the magnitude of the current. Impedance Headphones are available with high or low impedance. Impedance matching is accomplished by a circuit called an antenna tuner or impedance matching network between the transmitter and antenna. Coaxial Impedance is one of the most confusing aspects of electronics - I will de-mystify impedance by taking an extremely casual approach. Understanding Impedance Impedance is a fundamental concept in electrical and electronic engineering, describing the opposition a circuit element presents to the flow of alternating current (AC) or complex voltage. Figure 2. Here, the thermal impedance comes into play: it describes the sum of the thermal resistance and the contact resistance, i. In electrical engineering, antiresonance is the condition for which the reactance vanishes but the resistive impedance of an electrical circuit is none the less very high, approaching infinity. [1]: 4 The first protective relays were The reference impedance used is typically the characteristic impedance of a transmission line that's involved, but one can speak of reflection coefficient without any actual transmission line being present. A tesla coil is a high-Q resonant circuit. A typical balanced cable contains two identical wires, which are twisted together and then wrapped with a third conductor (foil or braid) that acts as a shield. [1] Under these Impedance is a measure of how much a circuit resists the flow of alternating current (AC), combining both resistance and reactance into a single complex quantity. Notice the port condition is satisfied: the same current flows into each port as leaves that port. Thus, the far field "impedance of free space" is resistive and is given by: An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the equipment's conductive surface, for safety and functional purposes. This is achieved by adding or subtracting excessive varying voltage in series to the voltage drop across an equivalent positive impedance. Nominal impedance in electrical engineering and audio engineering refers to the approximate designed impedance of an electrical circuit or device. Bioelectrical impedance analysis (BIA) is a method for estimating body composition, in particular body fat and muscle mass, where a weak electric current flows through the body, and the voltage is measured in order to calculate impedance (resistance and reactance) of the body. Impedance matching Source and load impedance circuit In electrical engineering, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Resonant circuits can generate very high voltages. æks /), is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric (insulating material); many coaxial cables also have a protective outer sheath or jacket. [1] Along with resistance, it is one of two elements of impedance; however, while both elements involve transfer of electrical energy, no dissipation of electrical energy as heat occurs in reactance; instead, the reactance stores energy until a In accelerator physics, shunt impedance is a measure of the strength with which an eigenmode of a resonant radio frequency structure (e. It is therefore the impedance equivalent of angle dependent reflectivity ; if is the elastic impedance above an interface and the elastic impedance below then the standard impedance/reflectivity relationship applies; L'impédance mécanique est une fonction de la fréquence de la force appliquée et peut varier considérablement selon la fréquence. One application is the noise measurement of sound intensity in the air Negative impedance converter The negative impedance converter (NIC) is an active circuit which injects energy into circuits in contrast to an ordinary load that consumes energy from them. , in a microwave cavity) interacts with charged particles on a given straight line, typically along the axis of rotational symmetry. It may refer to: Acoustic impedance, a constant related to the propagation of sound waves in an acoustic medium Electrical impedance, the ratio of the voltage phasor to the electric current phasor, a measure of the opposition to time-varying electric current in an electric circuit The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a wave travelling in one direction along the line in the absence of reflections in the other direction. it is the total opposition that an assembly, which is the material and the material interfaces, presents to the flow of heat. The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields (the transverse components being those at right angles to the direction of propagation). A parallel-coupled lines filter in microstrip construction The symbol λ is used to mean the wavelength of the signal being transmitted on the line or a section of line of that electrical length. Le mot impédance fut inventé par Oliver Heaviside en juillet 1886. The two wires form a circuit that can carry an audio signal. It is related to the image impedance used in filter design, but has a simpler, more straightforward definition. A short circuit (sometimes abbreviated to "short" or "s/c") is an electrical circuit that allows an electric current to travel along an unintended path with no or very low electrical impedance. Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. Sep 13, 2024 · Impedance, denoted as Z, is an expression of the opposition that an electronic component, circuit or system offers to alternating or direct electric current. On passe de à , mais avec et de formes sinusoïdales. Mar 9, 2020 · Impedance is the opposition to electrical flow in AC circuits, it depends on resistance, inductance and capacitance, and varies frequently. Treating it as a one-port network, the expression is expanded using continued fraction or partial fraction expansions. n00b introduction: Essentially, it lets you think about capacitors and inductors as if they were resistors, making calculations simpler and more intuitive. The SI unit of admittance is the siemens (symbol S); the older, synonymous unit is mho, and its symbol is ℧ (an upside-down uppercase omega Ω). a circuit) or device with two pairs of terminals to connect to external circuits. This is the same type of cable used in analog composite video setups, potentially allowing for easier "drop-in" equipment upgrades (although, at high bitrates and/or long distances, it may be necessary for older, oxidising, or lower-grade cable to be The wave impedance is the ratio of the strength of the electric and magnetic fields, which in the far field are in phase with each other. Electrical impedance From Wikipedia, the free encyclopedia Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current (AC). Because the antenna is absorbing the energy it is radiating from the transmitter, the antenna's input The various serial digital interface standards all use (one or more) coaxial cables with BNC connectors, with a nominal impedance of 75 ohms. It is defined as the reciprocal of impedance, analogous to how conductance and resistance are defined. Since most transmission lines also have a reflected wave, the characteristic impedance is generally not the impedance that is measured on the line. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave The loss tangent is defined by the angle between the capacitor's impedance vector and the negative reactive axis. In electrical engineering, the method of symmetrical components simplifies the analysis of a three-phase power system exhibiting an electrical fault or other unbalanced condition. The impedance may be a resistor, or an inductor (coil). [1] The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Low-impedance headphones are in the range 16 to 32 ohms and high-impedance headphones are 100 to 600 ohms. jyud1j 7vaeb p7er ssx14 dqz tan 88d mvx5 uz afx2