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

From Eq. (4.4), 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:
`m g = I lB`
`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 ×x 10^(–5) T` and we have ignored it.