Answer:
[tex]B_2 = 1.71 mT[/tex]
Explanation:
As we know that the magnetic field near the center of solenoid is given as
[tex]B = \frac{\mu_0 N i}{L}[/tex]
now we know that initially the length of the solenoid is L = 18 cm and N number of turns are wounded on it
So the magnetic field at the center of the solenoid is 2 mT
now we pulled the coils apart and the length of solenoid is increased as L = 21 cm
so we have
[tex]\frac{B_1}{B_2} = \frac{L_2}{L_1}[/tex]
now plug in all values in it
[tex]\frac{2.0 mT}{B_2} = \frac{21}{18}[/tex]
[tex]B_2 = 1.71 mT[/tex]