Question
What is impedance?
Answer Options
- A) The ratio of current to voltage
- B) The product of current and voltage
- C) The ratio of voltage to current
- D) The product of current and reactance
Correct Answer: C
Explanation
In a pure DC circuit in which voltage and current are not changing, Ohm’s law is expressed as the ratio of DC voltage (V) to DC current (I): R=V/I. In an AC circuit, impedance (Z) is the total opposition to the flow of Alternating Current (AC) in a circuit. It is defined as the ratio of voltage to current in an AC circuit (Z=V/I).
Impedance is crucial in RF work for matching transmitters to antennas and feed lines, and, like resistance, it is measured in ohms.
More detail:
Electrical impedance is the attribute that establishes (limits) the current flow because of an applied voltage. It is comprised of two parts, a so-called “real” part and an “imaginary” part.
The “real” part, called resistance, is the result of a resistor that has a voltage across it and a current through it. The product V\times{}I represents power shed as heat, never to return to the circuit. Ohms law expresses the current as a function of voltage and resistance, I=V/R where I is the current and R is the resistance (V=I\times{}R). Time plays no part in this expression. V and I appear at the same instant and are said to be “in phase” with one another.
The “imaginary” part, called reactance, is the result of a capacitor, inductor, or a combination of both. The capacitor stores energy in an electric field while an inductor stores energy in a magnetic field. This power is not dissipated as heat, and can be returned to the circuit.
Energy increases in a capacitor as current flows into it. If a constant current is applied into a capacitor its voltage will linearly increase from its initial voltage. The current must flow into the capacitor before the voltage increases. V=(I\times{}t)/C, plus the initial capacitor voltage before the current was applied. Apply a current and soon you get voltage. The current leads the voltage in a capacitor.
Energy increases in an inductor as voltage is applied across it. If a constant voltage is applied across an inductor the current will linearly increase from its initial current. The voltage must be applied to the inductor before the current increases. I=(V\times{}t)/L, plus the initial inductor current before the voltage was applied. Apply a voltage and soon you get current. The voltage leads the current in an inductor.
Impedance is expressed in ohms and has the designator capital Z. Since resistance R never gives energy back, and capacitance C or inductance L can return energy back to the circuit, they cannot be simply added together. This is where the mathematical operator lower case i enters the picture. In simple terms it keeps “apples” and “oranges” separated. The impedance of capacitance and inductance are both known as reactance, inductance being a positive reactance and capacitance being a negative reactance. The reactance of a capacitor is designated X_C and the reactance of an inductor is designated X_L. The impedance of a series RLC circuit is the sum of its resistance R and its reactance X_L - X_C
Z = R + i (X_L – X_C)
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