What is the only difference between positive and negative voltage in an AC circuit?

• A. Frequency
• B. Phase
• C. Magnitude
• D. Direction of current flow

Answer: (D)

In AC, the voltage swings between positive and negative with the same magnitude and at the same frequency. The impedance of a linear load doesn’t change when the polarity flips, so the current follows v(t) through the circuit as i(t) = v(t)/Z. When the voltage goes negative, the current simply flows in the opposite direction, while its magnitude and the phase relationship with voltage (set by the impedance) stay the same. So the only real difference between the positive and negative halves is which way the current is flowing.