NEET 2019 · PhysicsDC vs AC behaviourPrevious Year Question
A circuit when connected to an AC source of 12 V gives a current of 0.2 A. The same circuit when connected to a DC source of 12 V, gives a current of 0.4 A. The circuit is
Answer: (A) series LR. Answer: (A) series LR Solution: On DC, only resistance limits current: R = V/I = 12/0.4 = 30 Ω.
- A.series LR✓
- B.series RC
- C.series LC
- D.series LCR
Correct Answer
(A) series LR
Solution & Explanation
Answer: (A) series LR Solution: On DC, only resistance limits current: R = V/I = 12/0.4 = 30 Ω. On AC, the total opposition is impedance: Z = V/I = 12/0.2 = 60 Ω. Since Z (60 Ω) > R (30 Ω), there must be a reactive element in series with R. A capacitor would block DC entirely (no steady DC current), but a steady DC current of 0.4 A does flow, so there is no capacitor. Hence the reactive element is an inductor: it has no effect on DC (X_L = 0 at DC) but adds reactance on AC. Therefore the circuit is a series LR circuit. Check: Z² = R² + X_L² ⇒ 60² = 30² + X_L² ⇒ X_L = 30√3 Ω.
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