An electric current passes through a long straight copper wire. At a distance 5cm from the straight wire, the magnetic field is B, The magnetic field at 20cm from the straight wire would be:
A)\[\frac{B}{6}\]
B)\[\frac{B}{4}\]
C)\[\frac{B}{3}\]
D)\[\frac{B}{2}\]
Correct Answer:
B
Solution :
Magnetic field due to straight wire at point A \[{{\Beta }_{\Alpha }}=\Beta =\frac{{{\mu }_{\operatorname{o}}}\operatorname{I}}{2\pi \left( 5\times {{10}^{-2}} \right)}\] ?.(1) \[{{\Beta }_{\operatorname{B}}}=\frac{{{\mu }_{\operatorname{o}}}\operatorname{I}}{2\pi \times \left( 20\times {{10}^{-2}} \right)}\] ?.(2) by (1) & (2) \[{{\Beta }_{\operatorname{B}}}=\frac{B}{4}\]