Question

Which of the following graph represent the variation of magnetic flux density $B$ with distance $r$ for a straight long wire carrying an electric current?

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The correct option is D

The explanation of the correct option:

In the case of option D,

1. Magnetic flux due to a long current-carrying wire at a distance $r$ from the center is $B=\frac{{\mu }_{0}I}{2\pi r}$. Here, $I$ is the current in the wire $r$ is the distance, and $B$ is inversely proportional to the distance $r$
2. Hence when $r$ increases $B$ decrease and when $r$ decreases $B$ increases
3. $B$ tends to zero when $r$ tends to infinity and vice versa
   

The explanation for the incorrect options:

1. In the case of option A, $B$ tends to infinity, not a constant value when $r$ tends to zero.
2. In the case of option B, as $r$ increases $B$ decreases but here the value of $B$ is increasing.
3. In the case of option C, as $r$ increases $B$ decrease but here the value of $B$ is increasing.

Hence, option (D) is the correct answer.