Advanced AC Theory

If the frequency of the alternating supply to a pure capacitor reduces, its

capacitive reactance will

• reduce

• be unchanged

• increase

• become zero

A pure capacitor and a resistor are connected in series to an alternating supply.

The potential differences across the capacitor and across the resistor are

measured at 100 V and 140 V, respectively, so the supply voltage is:

• 240 V

• 98 V

• 172 V

• 40 V

If a coil of self-inductance 390 mH and resistance 1.9 kΩ is connected to a

24 V supply with a frequency of 1 kHz, the current will be

• 129 A

• 12.6 mA

• 77.4 μA

• 7.74 mA

If a pure inductor is connected to an AC supply, the phase relationship of the

current to the voltage is

• 90^o lagging

• in phase

• 45^o lagging

• 90^o leading

A circuit consisting of a capacitor connected in series with a resistor is

connected to an AC supply. If the potential differences across the capacitor

and resistor are measured as 56 V and 45 V, respectively, the supply

voltage is

• 101 V

• 33.3 V

• 71.8 V

• 11 V

The formula for the impedance of a series circuit containing resistance,

inductance and capacitance is

Resonance occurs in a series circuit containing resistance, inductance and

capacitance when

• circuit resistance is zero

• inductive and capacitive reactance's are equal

• circuit phase angle is 90^o

• R = 2πLC

One of the following statements is UNTRUE for a series resonant circuit.

The untrue statement is

• supply voltage is 45^o out of phase with current

• circuit impedance is equal to circuit resistance

• supply voltage and current are in phase

• circuit current will be a maximum at resonant frequency

Every circuit containing inductance and capacitance has a frequency at which

it will resonate. The foregoing statement is

• false

• true only if resistance is high

• true

• true only if capacitance is fixed

When a 5 V, 50 Hz supply is connected to a coil of resistance 80 Ω and self-inductance 191 mH which is in series with a 26.5 μF capacitor, the circuit current will be

• 62.5 mA

• 4.17 mA

• 50 mA

• 26.2 mA