A solenoid of lenght 30 cm with 10 turns per centimetre and area of cross-section `40cm^(2)` completely surrounds another co-axial solenoid of same length, area of cross-section `20cm^(2)` with 40 turns per centimetre. The mutual inductance of the system is

Options

A. 10 H
B. 3 mH
C. 30 mH
D. `30 mu H`

Answer

B

Explanation

Using `M=mu_1n_1n_2A_2l`, we get
`M=(4pixx10^-7)(4000)(1000)(20xx10^-4)`
`(30xx10^-2)=3mH`