The magnetic field inside a solenoid is

Options

A. directly prportional to its length
B. inversely prportional to the total numbers of turn
C. inversely proportional to the current
D. directly proportional to the current

Explanation

To determine the nature of the magnetic field inside a solenoid, we can analyze the relationship between the magnetic field (B), the current (I), and the number of turns per unit length (n) of the solenoid.

### Step-by-Step Solution:

1. **Understanding the Solenoid**: A solenoid is a long coil of wire that generates a magnetic field when an electric current passes through it. The magnetic field inside a long solenoid is uniform and directed along the axis of the solenoid.

2. **Formula for Magnetic Field**: The magnetic field (B) inside a solenoid can be expressed using the formula:
\[ B = \mu_0 \cdot n \cdot I \]
where:
- \(B\) is the magnetic field inside the solenoid,
- \(\mu_0\) is the permeability of free space (a constant),
- \(n\) is the number of turns per unit length of the solenoid,
- \(I\) is the current flowing through the solenoid.

3. **Analyzing the Formula**:
- From the formula, we see that the magnetic field \(B\) is directly proportional to the current \(I\). This means that as the current increases, the magnetic field strength also increases.
- The magnetic field \(B\) is directly proportional to the number of turns per unit length \(n\). More turns per unit length result in a stronger magnetic field.
- The magnetic field \(B\) is not dependent on the total length of the solenoid or inversely proportional to the number of turns or current.

4. **Conclusion**: Based on the analysis, we can conclude that the magnetic field inside a solenoid is directly proportional to the current flowing through it.

### Answer:
The magnetic field inside a solenoid is directly proportional to the current.