It is desired to measure the magnitude of field between the poles of field between the poles of a powerful loud speaker magnet. A small flat search coil of area `2cm^(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^(@)` turns to bring its plane parallel to the field direction. The total charge flown in the coil (measured by a ballistic galvanometer connected to the coil) is 7.5mC. The combined resistance of the coil and the galvanometer is `0.50Omega`. Estimate the field strength of magnet.

Explanation

Charge `Q=(N)/(R)(phi_(i)-phi_(f))`
`B=(phi_(i))/(A),phi_(i)=BA`
`phi_(i)=1.5xx10^(-4),phi_(f)=0,N=25,R=0.5`
`Q=7.5xx10^(-3)C,A=2xx10^(-4)m^(2)`
`therefore B=(RQ)/(NA)=(0.5xx7.5xx10^(-3))/(25xx2xx10^(-4))=(5xx7.5)/(50)`
i.e., `B=0.75T`