Power in AC circuits; power factor and wattless current — NEET & JEE Questions

Question 1 · difficulty L1 · recall

Average power in an AC circuit is

• A. Vrms Irms sinφ
• B. V0I0
• C. Vrms Irms cosφ
• D. Vrms/Irms

Question 2 · difficulty L1 · understanding

The power factor of a purely resistive AC circuit is

• A. 0
• B. -1
• C. 1/2
• D. 1

Question 3 · difficulty L2 · understanding

The average power consumed by an ideal pure inductor is

• A. zero
• B. VrmsIrms
• C. VrmsIrms/2
• D. I²XL

Question 4 · difficulty L2 · application

The average power consumed by an ideal pure capacitor is

• A. VrmsIrms
• B. zero
• C. V²/XC as irreversible heat
• D. maximum at low frequency

Question 5 · difficulty L2 · understanding

In an AC circuit containing only a pure inductor or only a pure capacitor, the current is said to be 'wattless'. This means that

• A. no current flows in the circuit
• B. current flows, but the average power consumed over a cycle is zero because the phase difference between current and voltage is 90$^\circ$
• C. the current is very small
• D. the power is dissipated as heat but not measured

Question 6 · difficulty L3 · application

A circuit with power factor 0.8 draws 10 A from 100 V RMS. Its average power is

• A. 1000 W
• B. 80 W
• C. 800 W
• D. 1250 W

Question 7 · difficulty L3 · application

Wattless current refers to AC current for which

• A. instantaneous current is always zero
• B. resistance is infinite
• C. voltage is zero at all times
• D. average power consumption is zero

Question 8 · difficulty L3 · application

A series circuit of R = 30 $\Omega$ and $X_L$ = 40 $\Omega$ is connected to a 200 V (rms) supply. The power factor and the average power consumed are

• A. 0.8 and 640 W
• B. 1.0 and 800 W
• C. 0.6 and 480 W
• D. 0.6 and 800 W

Question 9 · difficulty L3 · application

A pure inductor of 0.70 H is connected to a 220 V, 50 Hz supply. The rms current and the average power dissipated are approximately

• A. 1.0 A and 220 W
• B. 0.5 A and 110 W
• C. 0 A and 0 W
• D. 1.0 A and 0 W

Question 10 · difficulty L4 · analysis

For a fixed real power and supply voltage, improving power factor toward 1 causes the required current to

• A. decrease
• B. increase
• C. remain unchanged
• D. reverse direction