Question

In an AC series circuit, the instantaneous current is maximum when the instantaneous voltage is maximum. The circuit element connected to the source will be:
(A) Pure inductor
(B) Pure capacitor
(C) Pure resistor
(D) Combination of a capacitor and an inductor

Answer 1

Hint As mentioned in the question, instantaneous current will be maximum only when instantaneous voltage is maximum. The AC circuit is connected in series. We know that any component that stores energy like an inductor or a capacitor gives a phase difference in the current and voltage. This means that if there is a net non-zero effect of either of these components in the circuit we will not have a maximum of the instantaneous voltage at the maximum if instantaneous current.

Complete step by step answer
Pure capacitor: When current leads the potential difference of the capacitor by ${90^0}$. It shows a phase difference of ${90^0}$ ahead. Less amount of current will flow.
Pure inductor: When current lags the potential difference of the inductor by ${90^0}$. It shows a phase difference of ${90^0}$. Less amount of current will flow.
Pure resistor: when current and potential difference are in the same phase. Or in other words instantaneous current will maximum/ minimum only when instantaneous voltage is maximum.
Combination of inductor and capacitor: It is known as a tank circuit, resonant circuit and tuned circuit. It is used to generate or pick up signals. Its application: oscillator, filter, frequency mixer etc. Sinusoidal ac is produced. It will act as a band-pass filter. It had zero impedance at resonant frequency.

Option B (pure resistor) is the correct solution.

Note
It is vital to note that instantaneous means that the quantity is dependent on the “instant” of time when it is measured. One must not confuse this quantity (like instantaneous current) with average quantity (average current). If the current and voltage are in different phases then instantaneous current and instantaneous voltage cannot be maximum simultaneously. Hence it cannot be a pure capacitor or pure inductor or a combination of two. Therefore, we are left with only one option which satisfies the given conditions I.e. pure resistor (B).