# impedance

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## impedance,

in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals. Impedance is expressed as the ratio of the voltage impressed across a pair of terminals to the current flow between those terminals. When a circuit is supplied with steady direct current, the impedance equals the total resistanceresistance,
property of an electric conductor by which it opposes a flow of electricity and dissipates electrical energy away from the circuit, usually as heat. Optimum resistance is provided by a conductor that is long, small in cross section, and of a material that conducts
of the circuit. The resistance depends upon the number of electrons that are free to become part of the current and upon the difficulty that the electrons have in moving through the circuit. When a circuit is supplied with alternating current, the impedance is affected by the inductanceinductance,
quantity that measures the electromagnetic induction of an electric circuit component; it is a property of the component itself rather than of the circuit as a whole.
and capacitancecapacitance,
in electricity, capability of a body, system, circuit, or device for storing electric charge. Capacitance is expressed as the ratio of stored charge in coulombs to the impressed potential difference in volts.
in the circuit. When supplied with alternating current, elements of the circuit that contain inductance or capacitance build up voltages that act in opposition to the flow of current. This opposition is called reactance, and it must be combined with the resistance to find the impedance. The reactance produced by inductance is proportional to the frequency of the alternating current. The reactance produced by capacitance is inversely proportional to the frequency of the alternating current. In order for a source of electricity that has an internal impedance to transfer maximum power to a device that also has an impedance, the two impedances must be matched. For example, in the simple case of pure resistances, the resistance of the source must also equal the resistance of the device. Impedance matching is important in any electrical or electronic system in which power transfer must be maximized.

## impedance

[im′pēd·əns]
(electricity)
(physics)
The ratio of a sinusoidally varying quantity to a second quantity which measures the response of a physical system to the first, both being considered in complex notation; examples are electrical impedance, acoustic impedance, and mechanical impedance. Also known as complex impedance.
The ratio of the greatest magnitude of a sinusoidally varying quantity to the greatest magnitude of a second quantity which measures the response of a physical system to the first; equal to the magnitude of the quantity in the first definition.

## impedance

In alternating-current electric circuits, a quantitative measure of the opposition to the flow of current upon the application of voltage; measured in ohms.

## impedance

1. a measure of the opposition to the flow of an alternating current equal to the square root of the sum of the squares of the resistance and the reactance, expressed in ohms.
2. a component that offers impedance
3. the ratio of the sound pressure in a medium to the rate of alternating flow of the medium through a specified surface due to the sound wave.
4. the ratio of the mechanical force, acting in the direction of motion, to the velocity of the resulting vibration.

## impedance

(electronics, physics)
Opposition to flow of alternating current. Impedance consists of resistance plus reactance (capacitive or inductive). Measured in Ohms.

## impedance

(1) The opposition to the flow of alternating current in a circuit. Represented by the letter "Z" and measured in ohms, impedance is the combination of resistance, inductance and capacitance of the circuit. See ohm.

(2) The opposition that a speaker produces to the alternating current coming from an amplifier. The lower the impedance, the more power required. Most speakers have an impedance of four to eight ohms. See ohm.
References in periodicals archive ?
Crossvalidation of bioelectrical impedance analysis for the assessment of body composition in a representative sample of 6-to 13-year-old children.
Evaluation of a foot-to- foot bioelectrical impedance analyser in highly active, moderately active and less active young men.
Changes in bioelectrical impedance during 30 minutes of constant workloads on bicycle and rowing ergometers in sportsmen.
Resistance (R) and reactance (Xc) were measured using single-frequency portable bioelectrical impedance analyzers (Quantum II, RJL system, MI, USA).
The identification of sources of error illustrated here and subsequent adherence to a standardized protocol will offset the sources of error that may be present in bioelectrical impedance research and allow the technology to advance.
Fluid management in the Intensive Care Unit: Bioelectrical impedance vector analysis as a tool to assess hydration status and optimal fluid balance in critically ill patients.
Development and validation of bioelectrical impedance analysis equations for predicting total body water and fat-free mass in North-African adults.
The percentage fat mass (%FM) predicted by using each skinfold-thickness equations namely Durnin and Womersley, Jackson and Pollock and Sloan was compared with %FM measured using a bioelectrical impedance analyzer (BIA).
The BFP of children was assessed by bioelectrical impedance method using the body fat monitor (Omron-HBF-306).
One common method for estimating body composition is bioelectrical impedance analysis (BIA), which estimates body composition using a small undetectable electrical current passed through the body, but this has not been validated for use in individuals with SCI.
The small form-factor, comfortable-to-wear health patch has been optimized for low power consumption and is the first of its kind to track physical and cardiac activity while monitoring bioelectrical impedance.

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