Question:

A coil of resistance \(12 \Omega\) and inductance 18 H is suddenly connected to a dc supply of 30 V. Calculate time constant

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The time constant (\(\tau\)) for an RL circuit is \(\tau = L/R\).
The time constant for an RC circuit is \(\tau = RC\).
Updated On: May 22, 2025
  • 6.7 sec
  • 4.5 sec
  • 1.5 sec
  • 0.11 sec
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The Correct Option is C

Solution and Explanation

For a series RL circuit, the time constant (\(\tau\)) is given by the formula: \[ \tau = \frac{L}{R} \] Where: L = Inductance in Henries (H) R = Resistance in Ohms (\(\Omega\)) Given: Resistance (R) = \(12 \Omega\) Inductance (L) = 18 H The DC supply voltage (30 V) is not needed to calculate the time constant itself. Calculate the time constant: \[ \tau = \frac{18 \text{ H}}{12 \Omega} = \frac{18}{12} \text{ sec} = \frac{3}{2} \text{ sec} = 1.5 \text{ sec} \] The time constant is 1.5 seconds. \[ \boxed{1.5 \text{ sec}} \]
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