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Question

A small flat search coils of area 4 cm2 with 20 closely wound turns is positioned normal to the field direction and then quickly snatched out of the field region. The total charge flowing in the coil (measured by a ballastic galvanometer connected to the coil) is 7.5 mC. The resistance of teh coil and galvanometer is 0.8 Ω. The field strength of the magnet is

A
1.25 T
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B
0.50 T
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C
0.75 T
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D
2.10 T
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Solution

The correct option is A 1.25 T
Given: Area of coil, A=4cm2
Number of turns =20
Total charge =7.5mC
Resistance of the rod =0.8Ω
Solution:
The total charge flowing through the coil,

q=t2t1|dt|=1Rt2t1Edt

where, induced EMF, |E|=Ndϕdt Edt=Ndϕ
Hence,

q=NRϕ2ϕ1dϕ=NR(ϕ1ϕ2) .........(i)

Given,
N=20,q=7.5×103C,ϕ2=0,R=0.8Ω
Putting these values in equations (i), we get
ϕ1=5×104Wb,A=4.0×104m2
Q=qRN( From i)
B=ϕ1ϕ2A=1.25T
So,the correct option:A

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