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 F =Il xx B` we find that there is an upward force F, of magnitude Ilb,. For mid-air suspension, this mustbe balanced by the force due to gravity.
m g = IlB
`B=(mg)/(Il)`
`=(0.2xx9.8)/(2xx1.5)=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 `4xx10^(-5)T` and we have ignored it.