A long straight wire carries a current of $5, A$ . Find the magnetic field magnitude and direction at a point $0.20, m$ from the wire, where the current is directed upwards along the $+ z$ –axis.

medium 2 marks Class 12 · Physics · Magnetism and Matter

Answer

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

, directed along the

+ y

–axis.

The correct answer is $5.0 \times 1 0^{- 6}, T$ , directed along the $+ y$ –axis. — Apply the right–hand rule: thumb along $+ z$ (current direction), fingers at a point $+ x$ from the wire curl in the $+ y$ direction. Thus $v ec B$ is along $+ y$ .

Step-by-step solution

Use the Ampère’s law for a long straight wire: $B = \frac{μ _{0} I}{2 π r}$ , with $μ_{0} = 4 π \times 1 0^{- 7}, N,A^{- 2}$ .
Insert the values: $B = \frac{( 4 π \times 1 0 ^{- 7} ) \times 5}{2 π \times 0.20} = 5.0 \times 1 0^{- 6}, T$ .
Apply the right–hand rule: thumb along $+ z$ (current direction), fingers at a point $+ x$ from the wire curl in the $+ y$ direction. Thus $v ec B$ is along $+ y$ .

Common mistake: Using

2 p i r

instead of

2 p i r

in denominator

Concept tested: Magnetic field of long wire

Published 19 September 2026 · How this solution was prepared and checked

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A long straight wire carries a current of 5,A. Find the magnetic field magnitude and direction at a point 0.20,m from the wire, where the current is directed upwards along the +z–axis.

Step-by-step solution

A long straight wire carries a current of $5, A$ . Find the magnetic field magnitude and direction at a point $0.20, m$ from the wire, where the current is directed upwards along the $+ z$ –axis.