It is desired to measure the magnitude of field between the poles of a powerful loud speaker magnet. A small flat search coil of area `2 "cm"^2` with 25 closely wound turns, is positioned normal to the field direction, and then quickly snatched out of the field region. Equivalently, one can give it a quick `90^@` turn to bring its plane parallel to the field direction). The total charge flown in the coil (measured by a ballistic galvanometer connected to coil) is 7.5 mC. The combined resistance of the coil and the galvanometer is 0.50 `Omega`. Estimate the field strength of magnet.

Explanation

Here initial magnetic flux linked with given coil is,
`phi_1=ABcos0^@ " " (because vecA||vecB)`
=AB
Final magnetic flux `phi_2=AB cos 90^@`
=0
OR
`phi_2=AB cos phi`
=A(0)cos`theta`
=0
Induced emf,
`epsilon=-N(dphi)/(dt)`
`therefore epsilon=-N((phi_2-phi_1)/t)`
`=-N((0-AB)/t)`
`therefore epsilon="NAB"/t`
`therefore IR=(NAB)/t`
`therefore (Q/t)R=((NAB)/t)` (Where Q=charge induced in the coil in t time )
`therefore B=(QR)/(NA)`
`therefore B=((7.5xx10^(-3))(0.5))/((25)(2xx10^(-4))`
`therefore` B=0.75 T