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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 resistance resistance, property of an electric conductor by which it opposes a flow of electricity and dissipates electrical energy away from the circuit, usually as heat. ..... Click the link for more information. 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 inductance inductance, 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. ..... Click the link for more information. and capacitance capacitance, 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. ..... Click the link for more information. 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. (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.
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| Key statement: Measuring of curing conditions by dielectric or impedance means to produce a process curve which defines the correlation between the curve and the desired part properties. Although numerous authors have provided evidence of the usefulness of ASSR testing, their reports have concerned patients whose middle ear impedance measures were normal We report the cases of 2 patients who, following improvement of abnormal middle ear impedance values, experienced a marked improvement in measurable thresholds by ASSR testing. Some of these methods include back-EMF sensing, the addition of a sense winding in the motor, or measuring the impedance change of the motor windings by injecting current pulses or a high frequency sense carrier. |
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impedance
