Consider a current carrying wire (current I) in the shape of a circle. Note that as the current progresses along the wire, the direction of `vecJ` (current density) changes in an exact manner, while the current I remains unaffected. The agent that is essentially responsible for is

Options

A. source of emf
B. electric field produced by charges accumulated on the surface of wire
C. the charges just behind a given segment of wire which push them just the right way by repulsion
D. the charges ahead

Answer

To solve the question, we need to analyze the situation of a current-carrying wire shaped in a circle and understand the factors affecting the current density \( \vec{J} \) as it flows through the wire.

### Step-by-Step Solution:

1. **Understanding the Setup**:
- We have a wire shaped like a circle carrying a current \( I \).
- The current density \( \vec{J} \) changes direction as the current moves along the wire.

2. **Current Density Relation**:
- The current density \( \vec{J} \) is defined as the current per unit area, given by the formula:
\[ \vec{J} = \frac{I}{A} \]
- Here, \( I \) is the total current flowing through the wire, and \( A \) is the cross-sectional area of the wire.

3. **Current Remains Constant**:
- The problem states that the current \( I \) remains unaffected, meaning it is constant throughout the wire.

4. **Effect on Current Density**:
- Since \( I \) is constant and the cross-sectional area \( A \) of the wire is also constant, the magnitude of \( \vec{J} \) will remain constant. However, the direction of \( \vec{J} \) changes as the wire is circular.

5. **Charge Movement**:
- The current \( I \) is due to the flow of charge carriers (like electrons) in the wire. The direction of current \( I \) is conventionally taken as the direction of positive charge flow, which is opposite to the flow of electrons.

6. **Electric Field Influence**:
- The change in direction of \( \vec{J} \) as the current flows around the circle is influenced by the electric field produced by charges accumulated on the surface of the wire. This electric field causes the charge carriers to move in a circular path.

7. **Identifying the Responsible Agent**:
- The question asks for the agent responsible for the change in direction of \( \vec{J} \). Given the options:
- The correct answer is that the electric field produced by charge accumulated on the surface of the wire is responsible for the change in direction of \( \vec{J} \).

### Conclusion:
The agent that is essentially responsible for the change in direction of \( \vec{J} \) while the current \( I \) remains unaffected is the **electric field produced by charges accumulated on the surface of the wire**.