(a) F_{dl} = i × dl × B towards centre. (By cross product rule)

(b) Let the length of subtends an small angle of 20 at the centre.

Here 2Tsinθ= i × dl × B

=> 2Tθ = i × a × 2θ × B [As θ → 0, Sinθ ≈ 0]

=> T = i × a × B dl = a × 2θ

Force of compression on the wire = iaB