Power in AC circuits; power factor and wattless current — practice 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 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 6 · 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 7 · 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

Question 8 · difficulty L4 · analysis

If φ changes from 60° to 0° while Vrms and Irms remain fixed, average power

• A. halves
• B. doubles
• C. is unchanged
• D. becomes zero

Question 9 · difficulty L5 · analysis

A load takes 2 kW at 200 V RMS with power factor 0.5. The RMS current is

• A. 5 A
• B. 10 A
• C. 20 A
• D. 40 A

Question 10 · difficulty L5 · analysis

Why are low power factors undesirable in power transmission for a fixed delivered real power?

• A. They eliminate all current from lines
• B. They make transformer action impossible
• C. They force frequency to zero
• D. They require larger current and increase I²R losses