If a long hollow copper pipe carriers a direct current, the magnetic field associated with the current will be:

Options

A. Inside the pipe only
B. Outside the pipe only
C. Both inside and outside the pipe
D. Neither inside nor outside the pipe

Answer

To solve the question regarding the magnetic field associated with a long hollow copper pipe carrying a direct current, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Setup**: We have a long hollow copper pipe, and a direct current (DC) is flowing along the surface of this pipe.

2. **Apply Ampere's Circuital Law**: According to Ampere's Circuital Law, the line integral of the magnetic field \( \mathbf{B} \) around a closed loop is proportional to the current enclosed by that loop. Mathematically, it is given by:
\[ \oint \mathbf{B} \cdot d\mathbf{l} = \mu_0 I_{\text{enclosed}} \]
where \( \mu_0 \) is the permeability of free space.

3. **Consider a Loop Inside the Pipe**: If we draw an imaginary loop inside the hollow part of the copper pipe, there is no current enclosed by this loop since the current flows only on the surface. Therefore, we have:
\[ I_{\text{enclosed}} = 0 \]
This implies that:
\[ \oint \mathbf{B} \cdot d\mathbf{l} = 0 \]
Hence, the magnetic field \( \mathbf{B} \) inside the hollow pipe is zero.

4. **Consider a Loop Outside the Pipe**: Now, if we draw a loop outside the hollow pipe, this loop encloses the current flowing on the surface of the pipe. In this case, the enclosed current \( I_{\text{enclosed}} \) is not zero. Therefore, we can say:
\[ I_{\text{enclosed}} \neq 0 \]
This means that there will be a magnetic field present outside the pipe.

5. **Conclusion**: From the analysis, we conclude that:
- The magnetic field inside the hollow copper pipe is zero.
- The magnetic field exists outside the hollow copper pipe.

Thus, the correct answer to the question is that the magnetic field associated with the current will be **only outside the pipe**.

### Final Answer:
The magnetic field associated with the current will be **only outside the pipe**.