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The obstacle that the voltage faces while driving a current through a conductor is called resistance, whereas reactance is the resistance to a variable current. Resistance is affected by the temperature of the conductor; reactance is affected by the frequency of the alternating current flowing through an inductor or capacitor. Resistance exists on both direct and alternating currents, whereas reactance exists only on variable currents, resulting in opposition to the current change in the component.
Resistance
The obstacle that the voltage faces while driving a current through a conductor is called resistance. To pass a large current, the applied voltage to the conductor’s ends must be high. That is, as per Ohm’s law, the applied voltage (V) must be proportional to the current (I) flowing through the conductor and the constant of proportionality is the conductor’s resistance (R).
V=I x R
Regardless of whether the current is steady or varying, conductors always have the same resistance. Resistance for alternating current can be calculated utilising Ohm’s law and instantaneous voltage and current. The resistance in Ohms () is determined by the resistivity (), length (l) and cross-section area of the conductor (A).
Resistance is also affected by the temperature of the conductor, where 0denotes the resistivity stated at the standard temperature T0, which is typically room temperature and is the temperature coefficient.
Reactance
The opposition to a current change is called reactance. Reactance occurs in inductors and capacitors when the current is changed. As a result, reactance is affected by the alternating current frequency flowing through an inductor or capacitor. Its unit is Ohm.
When a voltage is supplied to the two terminals of the capacitor, it accumulates charges until the capacitor voltage fits the source voltage. If the voltage is from an alternating current source, the accumulated charges are brought back to the source at the voltage’s negative cycle. Because of the constant charging and discharging time, as the frequency increases, the amount of charges stored in the capacitor for a short time decreases. Hence, as the frequency increases, the capacitor’s opposition to the flow of current in the circuit decreases. That is, the capacitor’s reactance is inversely proportional to the angular frequency () of the alternating current.
C is the capacitor’s capacitance, and f is the frequency in Hertz.On the other hand, a capacitor’s impedance is a negative number.
When the current flowing through an inductor changes, a potential difference is created. This potential difference is proportional to the current’s rate of variation. As a result, the potential difference precedes the current by a quarter cycle for inductors.
XL=ωL=2πfL
XL denotes inductive reactance; the inductance of the inductor is L and the frequency of the alternating current be f.
As a result of the inductors’ and capacitors’ resistance to current variation, the voltage variation will follow a different pattern than the current variation. This means that the phase of the alternating current voltage differs from the phase of the alternating current. Inductive reactance causes the current phase to lag behind the voltage phase, whereas capacitive reactance causes the current phase to lead. This lead and lag have a magnitude of 90 degrees in ideal components.
Differences Between Resistance And Reactance
Resistance | Reactance |
The obstacle that the voltage faces while driving a current through a conductor is called resistance. It is the true impedance of a component. | In an inductor or a capacitor, reactance is the resistance to a variable current. It is the imaginary impedance. |
Resistance depends on the dimensions, resistivity and temperature of the conductor. It is unaffected by the frequency of the alternating current voltage. | The frequency of the alternating current determines reactance. It is proportional to frequency for inductors and inversely proportional to frequency for capacitors. |
The phase difference between the voltage and current passing through a resistor is zero. | The current phase lags behind the voltage phase due to inductive reactance. Current is the driving force in capacitive reactance. The phase difference should be 90 degrees in an ideal situation. |
Power consumption is real power and it is the product of voltage and current. | Because of lagging or leading current, power supplied to a reactive device is not fully utilised by the device. |
Conclusion
Electrical components like resistors, capacitors and inductors create an impedance, which is a complex value, for current to flow through them. Pure resistors have a real-valued impedance called resistance, whereas ideal inductors and ideal capacitors have an imaginary-valued impedance known as reactance. Resistance exists on both direct and alternating currents, whereas reactance exists only on variable currents, resulting in an opposition to the current change in the component. While resistance is independent of AC frequency, reactance varies with AC frequency. Reactance also causes a phase shift between the current and voltage phases. This is the distinction between resistance and reaction.
