Two parallel wires PQ, RS of resistance `10Omega` and `20Omega` are seperated by a distance of 10 cm and connected in parallel across a cell of emf200V and negligible intemal resistance. A wire AB of mass I g and length 1 cm is balanced exactly, between them. What must be the current in it.

Explanation

AB experience a force of attraction due to PQ and RS. ApplyV=iR
`200=i_(1)xx10rArri_(1)=20A`
`200=i_(2)xx20rArr=10A`
AB will be in equilibrium if
`(mu_(0))/(2pia)i_(2)i_(3)I+mg=(mu_(0)i_(1)i_(3))/(2pia)`
`(mu_(0))/(2pia)xx(i_(1)i_(3)-i_(2)i_(3))I=mg`
`(4pixx10^(-7))/(2pixx5xx10^(-2))(20i_(3)-10i_(3))xx1/100=10^(-3)xx9.8`
`(2xx10^(-5)xx10i_(3))/(5xx100)=9.8xx10^(-3)`
`i_(3)=24500A`.