A long straight wire carries a current of 35 A. What is the magnitude of the field B at a point 35 cm from the wire ?

Explanation

(a) As shown in diagram all five wires A, B, C, D, E are perpendicular to plane of papers.
Magnetic field at O point due to this pentagon be zero.
(b) As per discussion in option (a), magnetic field at O point is zero due to five wires,
`vecB_(A)+vecB_(B)+vecB_(C)+vecB_(D)+vecB_(E)=O`
`thereforevecB_(B)+vecB_(C)+vecB_(D)+vecB_(E)=-vecB_(A)`
Resultant magnetic field due to four wires = `(mu_(0)I)/(2piR)`, AO left side and perpendicular to OA.
( c) If reversing the direction of electric current of A, magnetic field due to it is given by `vecB_(A)`.
`therefore` Resultant magnetic field,
`|vecB_(A)|=|vecB_(B)|+|vecB_(C)|+|vecB_(D)|+|vecB_(E)|=B`
`thereforevecB_(R)=-vecB+vecB+vecB+vecB+vecB`
= `3vecB`
`therefore|vecB_(R)|=3(mu_(0)I)/(2piR)`
left side and perpendicular to AO.