What is meant by the time constant of an LR circuit? When the current flowing through a coil P decreases from 5 A to 0 in 0.2 seconds, an emf of 60 V is induced across the terminals of an adjacent coil Q. Calculate the coefficient of mutual inductance of the two coils P and Q.

Explanation

### Step-by-Step Solution:

1. **Understanding the Time Constant of an LR Circuit**:
- The time constant (τ) of an LR circuit is defined as the time taken for the current to rise to approximately 63.2% of its maximum value when a voltage is applied, or to fall to about 36.8% of its maximum value when the voltage is removed. It is given by the formula:
\[ \tau = \frac{L}{R} \]
where \(L\) is the inductance and \(R\) is the resistance in the circuit.
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