Figure shows the circuit that contains three resistances ( 9Ω each) and two inductors ( 4 mH each). The reading of ammeter at the moment switch K is turned ON , is ____ A.

Solution

Physics sir · 3 steps

The moment K closes, each inductor acts like a broken wire, so only the lone 9 Ω branch carries current.

1. Inductors at switch-on. An inductor opposes any sudden change in current. The current through each inductor is zero just before K closes, so it is still zero just after. At t = 0 each inductor behaves as an open circuit.

   Branch	Contains	Current at t = 0
   Left	4 mH in series with 9 Ω	zero (inductor open)
   Middle	9 Ω only	flows
   Right	9 Ω in series with 4 mH	zero (inductor open)

2. Ohm's law. The full 9 V of the battery sits across the middle resistor, so I = (9 V)/(9 Ω) = 1 A.
3. So. The ammeter reads 1 A at the moment K is switched on, which is option (c).

The idea to keep. Right after switching on, an inductor is an open circuit. After a long time it is a plain wire. Decide which instant the question asks about before you redraw the circuit.

Answer from the NTA final answer key. Solution by PrepWiser.