(a) Fdl = 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