How will you induce an emf by changing the area enclosed by the coil? Induction of emf by changing the area of the coil:

Explanation

Let us consider a conducting rod of length l moving with a velocity v towards left on a rectangular metallic framework.
The whole arrangement is placed in a uniform magnetic field `vecB` whose magnetic lines are perpendicularly directed in to the plane of the paper. As the rod moves from AB to DC in a time dt, the area enclosed by the loop and hence the magnetic flux through the loop decreases.

The change in magnetic flux in time dt is
`dphi_(B)=Btimes"change in area"`
`=Btimes"Area ABCD"`
`=Blvdt`
`[therefore"AreaABCD"=l(vdt)]`
(or) `(dphi_(B))/(dt)=Blv`
On account of change in flux, an emf is generated in the loop. The magnitude of the induced emf is
`e=(dphi_(B))/(dt)`
`e-Blv`
This emf is called motional emf.