Question

# The strength of the magnetic field at a point $$r$$ near a long straight current carrying wire is $$B$$. The field at a distance $$\dfrac r2$$ will be

A
B2
B
B4
C
2 B
D
4 B

Solution

## The correct option is A $$2\ B$$The strength of magnetic field at a distance $$r$$ from a long straight current carrying wire is inversely proportional to the distance of the point$$B \propto \dfrac 1r$$$$\Rightarrow \dfrac{B_1}{B_2}=\dfrac{r_2}{r_1}$$$$\Rightarrow \dfrac{B}{B_2}=\dfrac{r/2}{r}$$$$\Rightarrow B_2=2B$$PhysicsNCERTStandard XII

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