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NCERT Class XII Chapter
Moving Charges and Magnetism
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Question : 26
Total: 28
A solenoid 60 cm long and of radius 4.0 cm has 3 layers of windings of 300 turns each. A 2.0 cm long wire of mass 2.5 g lies inside solenoid (near its centre) normal to its axis; both the wire and axis of the solenoid are in the horizontal plane. The wire is connected through two leads parallel to the axis of the solenoid to an external battery which supplies a current of 6.0 A in the wire. What value of current [with appropriate sense of circulation] in the windings of the solenoid can support the weight of the wire?
g = 9.8 m s–2.
Solution:  
Magnetic in the solenoid B = µ0nI
where n =
300×3
60×10−2
 = 1500 turns/m
B = 4π × 10–7 × 1500 × I = 18.84 I × 10–4 T
A current carrying wire is suspended inside solenoid and a current I = 6.0 A is flowing in it.
To balance the weight of the wire by the magnetic force IBl = mg
6 × [18.84 I × 10–4] 2 × 10–2 = 2.5 × 10–3 × 9.8
I = 108.4 A
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