(a) Magnetic field at the center due to the straight parts is
zero.
Magnetic fild due to the arcs of radius `r_1:`
`=4(mu_0i//2r_1)(1//8)=(mu_0i//4r_1)`
Similarly, the magnetic field due to the arcs of radius `r_2`:
`=(mu_0i//4r_2)`
`implies "Net magnetic field"`
`=(mu_0i//4){(1//r_1+1//r_2}=6.54xx10^-5T`
(b) (i) As the current in the wire at the centre is antiparallel
to the direction of magnetic field, the force on the wire
everywhere will be zero.
(ii) Further due to the current at the centre, the magnetic
field at AC will be tangential and hence the force on
AC will be zero.
(iii). Force on `CD=int_(r_1)^(r_2) (mu_0i)/(2pix) idx=(mu_0i^2)/(2pi) In ((r_2)/(r_1))`
`=8.11xx10^-6N ("Vertically downward")`