Question

An electric current passes through a long straight copper wire. At a distance 5cm from the straight wire the magnetic field is B. The magnetic field at 20cm from the straight wire would be:

• A. B/6
• B. B/4
• C. B/3
• D. B/2

Answer 1

Answer: (B)

B/4

Answer 2

For a long straight current carrying wire, the magnetic field $B$ at a distance $r$ is given by

$$B = \frac{\mu_0 I}{2\pi r}$$

Thus, the magnetic field is inversely proportional to the distance $r$. Given that at $r_1 = 5\,\text{cm}$ the field is $B$, at $r_2 = 20\,\text{cm}$ the field will be

$$B_2 = B \times \frac{5}{20} = \frac{B}{4}.$$