NEET Alternating Current MCQs & PYQs

When frequency \(f > f_r\) in a series LCR circuit, the inductive reactance \(X_L\) is greater than the capacitive reactance \(X_C\). This makes the circuit inductive. In an inductive circuit, the resultant voltage leads the current. Hence, both assertion and reason are true, and the reason correctly explains the assertion.

Moving coil instruments measure the average value of current. For AC, the average value over a full cycle is zero, resulting in zero average torque. Hence, moving coil ammeters/voltmeters cannot measure AC. Both assertion and reason are true, and the reason correctly explains the assertion.

Our eyes cannot detect flickers above \(10-15 \text{ Hz}\). Household AC supply (\(50/60 \text{ Hz}\)) changes too rapidly for us to perceive fluctuations in bulbs. Thus, Assertion (A) is true. However, \(50/60 \text{ Hz}\) is not considered a "very low frequency", thus Reason (R) is false.

\(220V\) AC represents the RMS voltage, not the instantaneous or peak voltage. The instantaneous voltage varies sinusoidally, reaching a peak of \(V_{\text{peak}} = V_{\text{RMS}} \sqrt{2} = 220 \sqrt{2} \approx 311 \text{V}\). So, Assertion (A) is false. Appliances are usually specified by their RMS operating voltage, not peak tolerable voltage. So, Reason (R) is false. Both assertion and reason are false.

Transformers operate on the principle of mutual induction, requiring a changing magnetic flux. \( text{dc} \) current produces a constant magnetic field, thus no change in flux and no induced EMF. Hence, (A) is true. \( text{dc} \) current indeed has constant magnitude and direction, so (R) is also true and correctly explains (A).

Choke coils reduce current in \( text{ac} \) circuits with minimal power loss, as power dissipation \( P = V_{rms} I_{rms} cosphi \) is low due to the phase angle approaching \( 90^circ \) for a purely inductive component. Thus, \( cosphi approx 0 \) for an ideal inductor. So, (A) and (R) are true, and (R) correctly explains (A).

\( \text{ac} \) ammeters (hot wire type) measure current based on the heating effect, where heat produced is \( H \propto I^2 \). This quadratic relationship results in a non-linear scale, not equally marked. Thus (A) is false. Heat produced is proportional to the square of the current, not directly, so (R) is also false.

Capacitive reactance is given by \( X_C = frac{1}{omega C} \). As capacitance \( C \) increases, \( X_C \) decreases (R is true). A decrease in \( X_C \) leads to a decrease in the total circuit impedance \( Z \). With constant voltage \( V \), a lower \( Z \) results in higher current \( I = V/Z \), thus increasing the lamp's brightness (A is true). (R) correctly explains (A).

In a series \( text{RL} \) circuit, the inductive reactance \( X_L \) causes the voltage to lead the current, so (A) is true. In a series \( text{LCR} \) circuit, if \( X_C > X_L \), the circuit is capacitive, and current leads the voltage, so (R) is true. Both statements are true, but (R) is about a different circuit type and does not explain (A).

The formulae are \(X_C = \frac{1}{2\pi f C}\) (inversely proportional) and \(X_L = 2\pi f L\) (directly proportional). Hence, both statements are correct.