Whenever the magnet flux linked with a coil changes, then is an induced emf in the circuit. This emf lasts

Options

A. For a short time
B. For a long time
C. Forever
D. So long as the change in the flux takes place

Answer

### Step-by-Step Solution:

1. **Understanding the Concept**: The question revolves around the principle of electromagnetic induction, specifically Faraday's law of electromagnetic induction. This law states that an electromotive force (EMF) is induced in a coil when there is a change in magnetic flux linked with it.

2. **Identifying the Conditions for Induced EMF**: According to Faraday's law, the induced EMF (ε) is directly proportional to the rate of change of magnetic flux (Φ) through the coil. Mathematically, this can be expressed as:
\[ \varepsilon = -\frac{d\Phi}{dt} \]
Here, \( \frac{d\Phi}{dt} \) represents the change in magnetic flux over time.

3. **Analyzing the Duration of Induced EMF**: The induced EMF will only exist as long as there is a change in magnetic flux. If the magnetic flux becomes constant (i.e., there is no change), the induced EMF will cease to exist.

4. **Evaluating the Options**: The options given are:
- A) for a short time
- B) for a long time
- C) forever
- D) so long as the change in the flux takes place

Based on our understanding, the induced EMF will last **only while the magnetic flux is changing**. Therefore, option D is the most accurate.

5. **Conclusion**: The correct answer is option D: "so long as the change in the flux takes place."