When the power factor is unity (cosφ = 1), which statement about reactive power Q is true?

When the power factor is unity (cosφ = 1), which statement about reactive power Q is true?

• A. Q equals zero
• B. Q is maximal
• C. Q equals P
• D. Q equals V_L I_L

Answer: (B)

Unity power factor means the voltage and current are in phase, so there is no reactive power. Reactive power Q comes from a phase difference between voltage and current, and when φ = 0 degrees (cosφ = 1), the sine part is zero. In RMS terms, Q = VI sinφ, which becomes zero, while real power is P = VI cosφ = VI and apparent power is S = VI. So P equals S and Q equals zero. The other statements would require a phase angle different from zero (for example, Q being maximal would need a 90-degree phase shift, which is not the case here), so the correct takeaway is that reactive power is zero when the power factor is unity.

Unity power factor means the voltage and current are in phase, so there is no reactive power. Reactive power Q comes from a phase difference between voltage and current, and when φ = 0 degrees (cosφ = 1), the sine part is zero. In RMS terms, Q = VI sinφ, which becomes zero, while real power is P = VI cosφ = VI and apparent power is S = VI. So P equals S and Q equals zero. The other statements would require a phase angle different from zero (for example, Q being maximal would need a 90-degree phase shift, which is not the case here), so the correct takeaway is that reactive power is zero when the power factor is unity.