A straight wire of mass 200 g and length 1.5 m carries a current of 2 A. It is suspended in mid-air by a uniform horizontal magnetic field B . What is the magnitude of the magnetic field?

Explanation

we find that there is an upward force F, of magnitude IlB,. For mid-air suspension, this must be balanced by the force due to gravity:
mg = IlB
`B = (mg)/(Il)`
` = (0.2 xx 9.8)/(2 xx 1.5) = 0.65 T`
Note that it would have been sufficient to specify m/l, the mass per unit length of the wire. The earth’s magnetic field is approximately `4 xx 10^(-5) T` T and we have ignored it.