(a) The magnetic flux through the coil is given by
where
N is the number of turns in the coil
B is the intensity of the Earth's magnetic field
A is the area enclosed by one coil
is the angle between the direction of B and the perpendicular to A.
Initially, the coil is perpendicular to the Earth's magnetic field, therefore
and 
, so the magnetic flux through the coil is given by:
(b) After the coil is rotated, its plane is parallel to the magnetic field. This means that
and 
, so the magnetic flux through the coil is simply zero:
(c) The variation of magnetic flux through the coil is
we can ignore the negative sign since we are interested only in the magnitude of the average emf, not in its direction.
For Faraday-Neumann-Lenz law, the average emf induced in the coil is given by
where
N is the number of turns in the coil
B is the intensity of the Earth's magnetic field
A is the area enclosed by one coil
Initially, the coil is perpendicular to the Earth's magnetic field, therefore
(b) After the coil is rotated, its plane is parallel to the magnetic field. This means that
(c) The variation of magnetic flux through the coil is
we can ignore the negative sign since we are interested only in the magnitude of the average emf, not in its direction.
For Faraday-Neumann-Lenz law, the average emf induced in the coil is given by