# 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 cm2 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 . Estimate the field strength of magnet.

Given: Area of small flat search coil = 2 cm2

Total number of turns in the coil n = 25

Total charge measured by the ballistic galvanometer = 7.5 mC=7.5 ×10-3C

combined resistance of the coil and the galvanometer R= 0.50 The current induced in the coil can be calculated as follows:

i = e/R

also, substituting in the equation we get  …………..(i)

Initial flux through the coil, Ф1 = AB

Final flux through the coil, Ф2= 0

Integrating equation (i) on both sides, we get Total charge Therefore,  Q = (NBA)/R

Therefore, B = (QR)/NA

So, substituting all the values in above equation, We get On solving, we get

B = 0.75 T

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