Answer:
The Magnetic field is 59.13 mT.
Step-by-step explanation:
Given that,
Number of turns = 400
Radius = 1.0 cm
Frequency = 90 Hz
Emf = 4.2 V
We need to calculate the angular velocity
Using formula of angular velocity



We need to calculate the magnetic flux
Relation between magnetic flux and induced emf


Put the value into the formula



Hence, The Magnetic field is 59.13 mT.