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