Find the magnitude and direction of the magnetic field at a point located 0.2 m from an infinitely long straight wire carrying a current of 5 A flowing through it. Use the right-hand rule to determine the direction.

easy 2 marks Class 12 · Physics · Magnetism and Matter

Answer

4.19 \times 1 0^{- 6}, T

, directed tangentially around the wire (into the page at the point considered).

The correct answer is $4.19 \times 1 0^{- 6}, T$ , directed tangentially around the wire (into the page at the point considered). — Apply the right‑hand rule: point thumb along the direction of current, fingers curl around the wire; the magnetic field at the point 0.2 m to the right of the wire points into the page.

Step-by-step solution

Use the Biot‑Savart law for an infinite straight wire: $B = \frac{μ _{0} I}{2 π r}$ , because the field is uniform and circles the wire.
Insert $μ_{0} = 4 π \times 1 0^{- 7}, H/m$ , $I = 5, A$ and $r = 0.2, m$ to get $B = \frac{4 π \times 1 0 ^{- 7} \times 5}{2 π \times 0.2} = 4.19 \times 1 0^{- 6}, T$ .
Apply the right‑hand rule: point thumb along the direction of current, fingers curl around the wire; the magnetic field at the point 0.2 m to the right of the wire points into the page.

Common mistake: Using

\frac{μ _{0} I}{π r}

instead of

\frac{μ _{0} I}{2 π r}

Concept tested: Biot‑Savart law for infinite wire

Published 5 September 2026 · Last updated 16 September 2026 · How this solution was prepared and checked

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