First slide

Magnetic field due to a straight current carrying wire

Question

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

Easy

A

$\frac{B}{6}$
B

$\frac{B}{4}$
C

$\frac{B}{3}$
D

$\frac{B}{2}$

Solution

$B = 10^{- 7} \frac{2 i}{r} \Rightarrow \frac{B}{B^{'}} = \frac{20}{5} \Rightarrow B^{'} = B / 4$

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