A bulb and an air-cored coil are connected in series with a 20 V DC source. Now a soft-iron core is inserted inside the coil. During the time interval when iron core is being inserted

Options

A. brightness of bulb remains same.
B. brightness of bulb increases.
C. brightness of bulb decreases.
D. Nothing can be said about the change in brightness of bulb .

Explanation

To solve the problem step by step, we need to analyze the situation when a soft-iron core is inserted into an air-cored coil that is connected in series with a bulb and a DC source.

### Step-by-Step Solution:

1. **Understanding the Circuit**:
- We have a series circuit consisting of a bulb and an air-cored coil connected to a 20 V DC source.
- The air-cored coil has an inductance \( L \).

**Hint**: Identify the components of the circuit and their arrangement.

2. **Effect of Inserting the Iron Core**:
- When the soft-iron core is inserted into the coil, the inductance \( L \) of the coil increases. This is because the presence of the iron core enhances the magnetic field within the coil, thereby increasing the inductance.

**Hint**: Recall that inductance depends on the core material; ferromagnetic materials like iron increase inductance.

3. **Inductance and Current Relationship**:
- The inductance \( L \) is related to the current \( i \) in the circuit by the formula \( \Phi = L \cdot i \), where \( \Phi \) is the magnetic flux.
- As the inductance increases, for a constant voltage, the current \( i \) must adjust according to the relationship defined by the inductance.

**Hint**: Remember that in a DC circuit, the current is influenced by the inductance when it changes.

4. **Behavior of the Bulb**:
- When the iron core is being inserted, the inductance increases, which causes the current in the circuit to decrease momentarily.
- A decrease in current will lead to a reduction in the brightness of the bulb since the power consumed by the bulb is proportional to the current flowing through it.

**Hint**: Consider how changes in current affect the brightness of the bulb.

5. **Conclusion**:
- During the time interval when the iron core is being inserted, the bulb will dim or may even go out temporarily due to the decrease in current caused by the increase in inductance.

**Hint**: Think about the transient response of inductors in a circuit when changes occur.

### Final Answer:
During the time interval when the iron core is being inserted, the bulb will dim or may go out temporarily due to the decrease in current in the circuit.