Reactance (X) is defined as the ratio of the AC voltage to the AC current in an AC circuit, as caused by the presence of a reactive component in the circuit, i.e., a capacitor or an inductor. The reactance of a capacitor or inductor is therefore like the resistance of a resistor, except that the reactance of a capacitor or inductor is not constant - it varies with the frequency of the signal across the capacitor or through the inductor. Reactance is also expressed in ohms.
In an AC circuit where both resistance and reactance exist, the effective ratio of voltage to current is known as the impedance Z of the circuit, which is given by the equation Z = R + jX, or |Z| = SQRT(R2 + X2). Impedance is therefore the over-all ability of a circuit to 'impede' the flow of current for a given voltage, and it consists of two components: resistance and reactance.
The presence of a capacitor or an inductor in a circuit impedes changes in voltage or current within the circuit. Specifically, the voltage across a capacitor can not change instantaneously, in the same way that current flowing through an inductor can not change instantaneously. As such, the presence of a capacitor or inductor in an AC circuit introduces a phase shift between the voltage and current signals in the circuit.
Assume that a sinusoidal voltage with frequency f (in Hz) is applied across a capacitor with capacitance C. The reactance XC of the capacitor is then given by the equation: XC = 1 / (2πfC) = 1 / ωC where ω = 2πf is the angular frequency in radians per second. Thus, the reactance XC of a capacitor decreases as the frequency f increases, and increases as f decreases. This is why a capacitor is used to block the DC component of an AC signal. Furthermore, the current sine wave through a capacitor is 90 degrees ahead of the voltage sine wave across it.
The reactance XL of an inductor is given by the equation XL = 2πfL = ωL, which means that the reactance of an inductor increases as the frequency of the signal through it increases. Furthermore, the current sine wave through an inductor lags the voltage sine wave across it by 90 degrees.
Because the reactance exhibited by a capacitor decreases as frequency increases, its reactance is considered negative. Thus, a reactance that's less than 0 means that it is capacitive. Similarly, a reactance that's greater than 0 means that it is inductive. Reactance equals zero in a purely resistive circuit.
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Reactance (X) is defined as the ratio of the AC voltage to the AC current in an AC circuit, as caused by the presence of a reactive component in the circuit, i.e., a capacitor or an inductor. The reactance of a capacitor or inductor is therefore like the resistance of a resistor, except that the reactance of a capacitor or inductor is not constant - it varies with the frequency of the signal across the capacitor or through the inductor. Reactance is also expressed in ohms.
In an AC circuit where both resistance and reactance exist, the effective ratio of voltage to current is known as the impedance Z of the circuit, which is given by the equation Z = R + jX, or |Z| = SQRT(R2 + X2). Impedance is therefore the over-all ability of a circuit to 'impede' the flow of current for a given voltage, and it consists of two components: resistance and reactance.
The presence of a capacitor or an inductor in a circuit impedes changes in voltage or current within the circuit. Specifically, the voltage across a capacitor can not change instantaneously, in the same way that current flowing through an inductor can not change instantaneously. As such, the presence of a capacitor or inductor in an AC circuit introduces a phase shift between the voltage and current signals in the circuit.
Assume that a sinusoidal voltage with frequency f (in Hz) is applied across a capacitor with capacitance C. The reactance XC of the capacitor is then given by the equation: XC = 1 / (2πfC) = 1 / ωC where ω = 2πf is the angular frequency in radians per second. Thus, the reactance XC of a capacitor decreases as the frequency f increases, and increases as f decreases. This is why a capacitor is used to block the DC component of an AC signal. Furthermore, the current sine wave through a capacitor is 90 degrees ahead of the voltage sine wave across it.
The reactance XL of an inductor is given by the equation XL = 2πfL = ωL, which means that the reactance of an inductor increases as the frequency of the signal through it increases. Furthermore, the current sine wave through an inductor lags the voltage sine wave across it by 90 degrees.
Because the reactance exhibited by a capacitor decreases as frequency increases, its reactance is considered negative. Thus, a reactance that's less than 0 means that it is capacitive. Similarly, a reactance that's greater than 0 means that it is inductive. Reactance equals zero in a purely resistive circuit.
Susceptance (B) is the reciprocal of reactance, i.e., B = 1 / X.
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