IMPEDENCE AND RESISTANCE

• Impedance and resistance are terms used to describe the opposition to electrical current flow.
• Resistance is used to describe opposition to flow in DC circuits, whereas impedance is used for AC circuits.
• Unlike direct currents, alternating currents exhibit reactance (capacitive and inductive) because they have frequency and are phase associated.
• In AC circuits, impedance (Z) consists of a real part (ohmic resistance) and an imaginary part (reactance of an inductor or capacitor). It is measured in ohms, and is the total opposition to current flow
• Capacitors placed in DC circuitscreate an increasing resistance to current flow. This is because the charge at the negative plate of the capacitor accumulates, until maximum capacitance is reached and current flow ceases.
• Capacitors in AC circuitsallow current to flow, as the alternating direction of current flow prohibits a significant build up of charge on one of the plates.
• Reactanceis the resistance to AC that a capacitor or inductor exhibits and is inversely proportional to frequency.
• This principle is used in filters to screen out DC currents and low frequency AC.
• Inductors are coils of conducting wire wound around a ferrous or air core.
• An increasing current flowing through an inductor generates a magnetic field around it. This magnetic field in turn creates an electromagnetic force, which opposes the current flow, known as back-emf.This effect is known as inductance, and its SI unit is the henry (H).
• In a circuit where the rate of current change is 1 A/s, an inductance of one henry would generate one volt across the inductor.
• Henry (H) = Voltage (V) × Time (s) / Amperes (A)
• When the power is switched off, collapse of the magnetic field induces flow of electrons in the inductor and circuit, prolonging the flow of current for a short period.
• Inductors placed in DC circuits will initially encounter transient resistance while the magnetic field is established. Once a steady state is reached, the reactance is negligible.
• Conversely, inductors in AC circuits encounter increasing reactance proportional to the frequency. This is because the creation and subsequent reversal of magnetic field development produces a constant back-emf resisting current flow. Therefore, high-frequency AC will cause a high reactance in the inductor.
• Inductors are used to filter out high-frequency alternating currents, or to smooth out the effect of power surges in monitoring equipment
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