Part A What is the phase relationship between current and voltage for a pure resistance? O current leads voltage by 90 O current lags voltage by 90° O current and voltage are in phase Submit Part B For an inductance? current leads voltage by 90 O current lags voltage by O current and voltage are in phase
The Correct Answer and Explanation is:
Correct Answers
Part A: For a pure resistance, current and voltage are in phase.
Part B: For a pure inductance, current lags voltage by 90°.
Explanation
In AC (alternating current) circuits, the phase relationship between current and voltage depends on the type of component—resistor, inductor, or capacitor.
Part A: Pure Resistance
In a circuit with only a resistor, the current and voltage are in phase. This means the peaks, zero crossings, and troughs of the current waveform occur at the same time as those of the voltage waveform.
This is because, according to Ohm’s Law (V = IR), the current is directly proportional to the voltage. When the voltage increases or decreases, the current does the same simultaneously. There is no time delay (or phase shift) between them. The power consumed in a resistive circuit is entirely real (not reactive), and it is converted into heat or work.
Part B: Pure Inductance
In a circuit with only an inductor, the current lags the voltage by 90 degrees (π/2 radians). This happens because an inductor resists changes in current due to its magnetic field. According to Faraday’s Law, the voltage across an inductor is proportional to the rate of change of current (V = L * di/dt).
So, when the voltage is at its maximum, the rate of change of current is also at its maximum—but the current itself is still increasing and has not peaked yet. The current reaches its peak a quarter cycle (90°) after the voltage does, hence it lags.
In a purely inductive circuit, no real power is consumed over time—the energy is temporarily stored in the magnetic field and then returned to the source.
These phase relationships are crucial in designing and analyzing AC circuits, power systems, and in understanding reactive power in inductive and capacitive loads.
