An air core coil and an electric bulb are connected in series with an AC source. If an iron rod is put in the coil, then the intensity of light of the bulb will

Options

A. remains same
B. increases
C. decrease
D. first decrease then increase

Explanation

To solve the problem, we need to analyze the effect of inserting an iron rod into an air core coil that is connected in series with an electric bulb and an AC source.

### Step-by-Step Solution:

1. **Understanding the Circuit**:
- We have an air core coil and an electric bulb connected in series with an AC source.
- The current flowing through the circuit will determine the brightness of the bulb.

2. **Effect of Inserting an Iron Rod**:
- When an iron rod is inserted into the air core coil, it increases the inductance of the coil.
- Inductance is a property of the coil that opposes changes in current.

3. **Inductive Reactance**:
- The inductive reactance (X_L) of the coil is given by the formula:
\[ X_L = 2\pi f L \]
where \( f \) is the frequency of the AC source and \( L \) is the inductance.
- As the inductance increases due to the iron rod, the inductive reactance also increases.

4. **Total Impedance**:
- The total impedance (Z) of the circuit will increase because it is the combination of resistance (R) and inductive reactance (X_L):
\[ Z = \sqrt{R^2 + X_L^2} \]
- With increased impedance, the overall current (I) in the circuit will decrease according to Ohm's law:
\[ I = \frac{V}{Z} \]
where \( V \) is the voltage of the AC source.

5. **Effect on Bulb Intensity**:
- Since the intensity of the light bulb is directly proportional to the current flowing through it, a decrease in current will lead to a decrease in the intensity of the light.
- Therefore, when the iron rod is inserted into the coil, the intensity of the light in the bulb will decrease.

6. **Conclusion**:
- The final conclusion is that the intensity of the light of the bulb will decrease when the iron rod is put in the coil.

### Final Answer:
The intensity of the light of the bulb will decrease.