An electric fan operates on the principle of which of the following?

Assertion : An electric motor will have maximum efficiency when back $emf$ becomes equal to half of applied $emf$.
Reason : Efficiency of electric motor depends only on magnitude of back $emf$.

$A$ rectangular coil of a simple $A.C.$ generator rotates about one of its diameters with a constant angular frequency in the presence of a uniform magnetic field. The direction of the induced current reverses for the third time after how many rotations of the coil?

When the speed of a $dc$ motor increases, the armature current:

The output of a dynamo using a split-ring commutator is

If a coil made of conducting wires is rotated between the pole pieces of a permanent magnet,the motion will generate a current. This device is called: