Question

A current of $ I = 20 \, \mu\text{A} $ flows in a long straight wire. What is the magnetic field $ B $ at a distance $ r = 1 \, \text{cm} $ from the wire?

• A. \( 20 \times 10^{-11} \, \text{T} \)
• B. \( 30 \times 10^{-11} \, \text{T} \)
• C. \( 40 \times 10^{-11} \, \text{T} \)
• D. \( 50 \times 10^{-11} \, \text{T} \)

Hint:

Use \( B = \frac{\mu_0 I}{2\pi r} \) for magnetic field around a straight conductor — always check units!

Answer 1

The Correct Option is C

Magnetic field due to a long straight current-carrying wire at a distance \( r \) is given by: \[ B = \frac{\mu_0 I}{2\pi r} \] Substitute: \[ \mu_0 = 4\pi \times 10^{-7} \, \text{T}\cdot\text{m/A}, \quad I = 20 \times 10^{-6} \, \text{A}, \quad r = 1 \, \text{cm} = 1 \times 10^{-2} \, \text{m} \] \[ B = \frac{4\pi \times 10^{-7} \times 20 \times 10^{-6}}{2\pi \times 10^{-2}} = \frac{80 \times 10^{-13}}{10^{-2}} = 40 \times 10^{-11} \, \text{T} \]