Difference Between Impedance and Resistance

Impedance vs Resistance

Resistance and impedance are two very important properties of components in circuit theory. This article will look into the key differences between impedance and resistance.

Resistance

Resistance is a very important property in the field of electricity and electronics. The resistance in a qualitative definition tells us how hard it is for an electrical current to flow. In the quantitative sense, the resistance between two points can be defined as the voltage difference that is required to take a unit current across the defined two points. Electrical resistance is the inverse of electrical conduction. The resistance of an object is defined as the ratio of the voltage across the object, to the current flowing through it. The resistance in a conductor depends on the amount of free electrons in the medium. The resistance of a semiconductor mostly depends on the number of doping atoms used (impurity concentration). The Ohm’s law is the single most influential law when the topic resistance is discussed. It states that for a given temperature, the ratio of voltage across two points, to the current passing through those points, is constant. This constant is known as the resistance between those two points. The resistance is measured in ohms.

Impedance

There are two types of devices classified according to their impedance response. These two types are active components and passive components. Active components change their resistance according to the input voltage or current. A passive component has a fixed resistance. Components such as capacitors and inductors are active components. A resistor is a passive component. Active components have another property of changing the phase of the incoming signal. If the phase difference of incoming voltage and current is zero, the output through a capacitor or an inductor will cause the current to either lag or lead the voltage. However, it must be noted that if these devices are ideal the resistance will be zero. A part of the impedance does not occur due to the same reasons the resistance occurs. Imagine an inductor coil. When a current starts to run through a magnetic field is created. The magnetic field itself is trying to minimize the current increment, thus creating the impedance. However, not all the components are ideal in practice; every component has an impedance value, which is not purely resistive. A circuit with the combination of inductors (L), capacitors (C), and resistors (R) is known as LCR circuit. Combinations having maximum impedance (in the impedance vs. input frequency plot) are frequency cut off filters, and a circuit having minimum impedance can be used as a tuner circuit or a frequency pass filter.