A phasor is a complex number representing sinusoidal voltage or current in AC circuits. It encodes both magnitude and phase, allowing for easier analysis of AC circuits by converting time-varying signals into simpler sinusoidal forms.
What does impedance represent in an AC circuit?
A Power
B Voltage
C Opposition
D Current
Impedance is the total opposition to current flow in an AC circuit, comprising both resistance (R) and reactance (X). It limits the flow of alternating current and is measured in ohms.
How does impedance behave in a purely resistive circuit?
A Equal to resistance
B Depends on current
C Infinite
D Zero
In a purely resistive circuit, the impedance is equal to the resistance (R), since there is no reactive component (inductive or capacitive). This means the impedance directly limits the current according to Ohm’s law.
What is the unit of impedance in AC circuits?
A Watt
B Volt
C Ohm
D Ampere
The unit of impedance is the ohm (Ω), the same as for resistance. Impedance combines resistance and reactance, and it represents the total opposition to current in an AC circuit.
What does admittance measure in an AC circuit?
A Opposition
B Power flow
C Current flow
D Ease of current flow
Admittance is the reciprocal of impedance and measures how easily current can flow through a circuit. It is expressed in siemens (S) and helps analyze AC circuits by indicating the ease of current flow.
How does inductance affect impedance in an AC circuit?
A Increases with frequency
B Zero at high frequency
C Decreases with frequency
D Remains constant
The impedance of an inductor increases with frequency because inductive reactance (XL = 2πfL) is directly proportional to frequency. Higher frequencies result in greater opposition to current flow.
How is admittance calculated?
A Current divided by voltage
B Resistance divided by reactance
C Reciprocal of impedance
D Voltage divided by current
Admittance is calculated as the reciprocal of impedance (Y = 1/Z). It quantifies how easily current flows through a circuit, whereas impedance quantifies how much the circuit opposes current.
What is the behavior of impedance in a purely capacitive circuit?
A Remains constant
B Decreases with frequency
C Increases with frequency
D Zero at high frequency
In a purely capacitive circuit, the impedance decreases with frequency. The capacitive reactance (XC = 1/(2πfC)) is inversely proportional to the frequency, allowing more current to flow at higher frequencies.
How does impedance affect current flow in an AC circuit?
A Reduces current
B Changes direction
C No effect
D Increases current
Impedance opposes the flow of current in an AC circuit. The greater the impedance, the lower the current for a given voltage, following Ohm’s law (I = V/Z).
What does the phase angle in a phasor represent?
A Current magnitude
B Power factor
C Voltage magnitude
D Time shift between voltage and current
The phase angle in a phasor represents the time shift between the voltage and current waveforms in an AC circuit. A phase shift indicates that the current and voltage are not in sync, affecting power transfer.
How is the total impedance in a series R-L circuit determined?
A Difference of R and L
B Sum of R and reactance
C Square root of (R² + X_L²)
D Sum of R and L
In a series R-L circuit, the total impedance is calculated using the formula Z = √(R² + X_L²), where X_L is the inductive reactance. This gives the combined opposition to current from both resistance and inductance.
What happens to the impedance in a parallel LC circuit at resonance?
A Infinite impedance
B Minimum impedance
C Zero impedance
D Maximum impedance
At resonance in a parallel LC circuit, the impedance is minimized because the inductive reactance and capacitive reactance cancel each other out. This results in the maximum current flow and minimum opposition.
What is the relationship between impedance and power factor in an AC circuit?
A Inversely proportional
B No relationship
C Dependent on voltage
D Directly proportional
Impedance and power factor are inversely proportional. As impedance increases, the current decreases, leading to a lower power factor, which indicates less efficient use of the supplied power in the circuit.
What effect does a high impedance have on an AC circuit?
A Reduces power
B Increases current
C Reduces current
D Increases power
High impedance in an AC circuit reduces the amount of current that flows through the circuit. Since impedance limits the flow of current, higher impedance means less current for a given voltage.
How is impedance used in power analysis of AC circuits?
A To measure phase shift
B To determine power factor
C To calculate current
D To determine current and voltage relationship
Impedance is critical in power analysis of AC circuits as it helps determine the relationship between current and voltage. It affects both the magnitude and phase shift, influencing the total power consumed in the circuit.