current r is flowing in a straight conductor of length l. The magnetic induction at a point distant l /4 from its centre will be(fig)

See fig. We know that magnetic induction B at a distance r from a long straight conductor carrying a current i is given by B=μ0i4πr[sin⁡α+sin⁡β]Here α=α, β=α  and  r=l/4∴ (sin⁡α+sin⁡β)=2sin⁡α=2l/25l/4=45∴ B=μ0i4π(l/4)×45=4μ0i5πl