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

2 B

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0.25 B

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0.5 B

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B

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Solution

The correct option is B $0.25 B$
$B = \frac{μ_{0} i}{2 A r}$
$B \propto \frac{1}{r}$ = $\frac{B_{1}}{B_{2}} = \frac{r_{2}}{r_{1}}$
$\frac{B}{B_{2}} = \frac{20}{5} = B_{2} = \frac{B}{4}$

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

The correct option is B 0.25BB=μ0i2Ar B∝1r = B1B2=r2r1BB2=205=B2=B4

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

The correct option is B $0.25 B$
$B = \frac{μ_{0} i}{2 A r}$
$B \propto \frac{1}{r}$ = $\frac{B_{1}}{B_{2}} = \frac{r_{2}}{r_{1}}$
$\frac{B}{B_{2}} = \frac{20}{5} = B_{2} = \frac{B}{4}$