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Electrical impedance measures how easily electricity flows through an AC circuit, combining resistance and reactance. It is used in medical diagnostics and imaging techniques, such as electrical impedance tomography, which has shown promise in the early detection of breast cancer.
Electrical impedance, often simply called impedance, is a measurement used to describe how easily electricity flows through an alternating current (AC) circuit. Describes the total amount of opposition a circuit presents to current. In a direct current (DC) circuit, resistance is used to describe the same properties, but in an AC system, additional factors come into play. Electrical impedance is a function of both resistance and reactance, due to the cyclic nature of an AC circuit.
Generally, the loop resistance is determined by the number of free electrons available within conductors to carry electric current. A large diameter copper wire, for example, will have more free electrons than a smaller diameter copper wire and will have lower resistance. The type of metal used for the conductor also plays a role, because a less conductive metal has fewer free electrons and offers greater resistance to current.
The second component of electrical impedance, reactance, is due to the cycling of an alternating electric current. A typical household electric current reverses direction 60 times per second or at a rate of 60 Hertz. Each cycle causes an additional electric and magnetic opposition to the current. This opposition is called reactance.
Like resistance, the unit of electrical impedance is the ohm. Impedance is usually expressed as the potential voltage difference across a circuit, divided by the maximum amount of current flowing through the circuit. The simplest expression of electrical impedance is shown in the mathematical equation Z=V/I, where Z represents impedance, V is voltage, and I represents current.
Electrical measurements are not only useful for copper wires and circuit boards. The human body also possesses a certain amount of conductivity, resistance and even impedance. In fact, the medical community has used electrical impedance as a diagnostic tool since the early 1900s. Medical tests that rely on impedance take advantage of the fact that different tissue types have different magnitudes of impedance.
Electrical impedance imaging techniques, such as electrical impedance tomography (EIT), have been around for many years and have continued to improve and gain popularity. Because malignant tissue typically shows lower impedance than healthy tissue, this type of test has shown great promise in the early detection of breast cancer. A typical EIT test involves placing electrodes on the part of the body to be tested. A weak electric current is applied to two or more electrodes and the voltage differences are measured. The test can be repeated many times and on many combinations of electrodes to obtain the most accurate results.
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