Acoil has n turns enclosing an area of a. in a physics laboratory experiment, the coil is rotated during the time interval δt from a position in which the plane of each turn is perpendicular to earth's magnetic field to one in which the plane of each turn is parallel to the field. the magnitude of earth's magnetic field at the lab location is b. part a what is the magnitude φinitial of the magnetic flux through one turn of the coil before it is rotated?

Explanation:

magnetic flux is the count of magnetic field lines passing through a given loop or area

As we know that magnetic flux is given by the formula

here we also know that magnetic field B and plane of the coil is perpendicular in initial position

So the area vector is always perpendicular to the plane of the coil

so the angle between magnetic field and area vector is parallel to each other and this angle would be zero

so magnetic flux of the coil initially we have

diagram is missing, but i will try to explain your doubt.

answer:

amplitude is the maximum displacement or distance moved on a point of a vibrating body or wave measured from its equilibrium position.it is equal to half the length of vibration path.