In a bent wire shown in figure, a current I is passed. Find the value of B at the common centre.

The straight segments make no contribution to the field at the centre. By Biot-Savart's law, the field at the centre due to curved segments isΔB=μ0I4πa 2π−θa2 − b 2π−θb2=μ0I4π  2π−θ  1a −1b  = μ0I2π−θ b−a 4πab