Refer figure a it forms a closed loop and the current completes the loop. Therefore, net force on the loop in uniform field should be zero.
Refer figure b. In this case although it forms a closed loop, but current does not complete the loop. Hence, net force is not zero.
`F_(ACD)=F_(AD)`
`:.F_(loop)=F_(ACD)+F_(AD)=2F_(AD)`
`:. |F_(loop)|=2|F_(AD)|`
`2ilB sintheta [l=2r=2.0m]`
`=(2)(2)(2)(1)sin90^@=8N`