$A$ device which converts electrical energy into mechanical energy is

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 $emf$.

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:

$A$ motor operating on $100 \ V$ draws a current of $1 \ A$. If the efficiency of the motor is $91.6 \%$,then the loss of power in units of $cal/s$ is

If the rotational velocity of a dynamo armature is doubled, then the induced $e.m.f.$ will become

$A$ motor having an armature of resistance $2\,\Omega$ is designed to operate at $220\,V$ mains. At full speed,it develops a back $e.m.f.$ of $210\,V$. When the motor is running at full speed,the current in the armature is: (in $,A$)