The magnetic field due to a straight conductor of uniform cross-section of radius $a$ and carrying a steady current is represented by

• Correct answer
  A

  
  
• B

  
  
• C

  
  
• D

  
  

Solution

Magnetic field at a point outside the straight conductor is $$ B=\frac{\mu_0 I}{2 \pi r} $$ i.e. $B \propto(1 / r) \quad(r>a)$. Magnetic field at a point lying inside the conductor is $$ B \propto r \quad(r < a) . $$

Hence, the correct answer is option: 'A'.