A conducting square loop having edges of length 2.0 cm is rotated through 180° about a diagonal in 0.20 s. A magnetic field B exists in the region which is perpendicular to the loop in its initial position. If the average induced emf during the rotation is 20 mV, find the magnitude of the magnetic field
Given:
Average induced emf in the loop
Time taken to rotate the loo 
Edge length of square loop 
Area of square loop 
We know that,
Average induced emf in time interval Δt is given by
…(i)
Where
are flux across the cross section at time intervals 
respectively.
Magnetic flux(ϕ) through the loop is given by the formula
Where B=magnetic field intensity
A=area of cross section
θ =angle between area vector and magnetic field
Initially, angle between area vector and magnetic field is 0°
Therefore, initial flux through the coil is
When it is rotated by 180° flux passing through the coil is given by
Putting this values in eqn.(i) we get,
Putting the values of ϵ, B and Δt in the above eqn.
Therefore, magnitude of magnetic field intensity is 5T