"A conductor carrying an electric current will feel a mechanical force when placed in a magnetic field." Name a device which works on this principle?

Hint: A DC motor is a device which can convert the electrical energy into the mechanical energy. And also any electric motor which has been operated by the use of direct current or DC can be referred to as a DC motor. This will help you in answering this question.

Complete answer:
A DC motor can be explained as a class of electrical motors which can convert direct current electrical energy into mechanical energy. From the above mentioned definition, we can understand that any electric motor which has been operated by the use of direct current or DC can be referred to as a DC motor. Now let us look at the working of this motor. A magnetic field will be present in the air gap if the field coil of the DC motor is energised. This generated magnetic field will be in the direction of the radii of the armature. The magnetic field will be passing the armature from the North Pole side of the field coil and exit the armature from the South Pole side of the field coil. The conductors placed on the other pole will be acted by a force of the similar intensity but in the opposite direction. These two forces which are opposing in nature develop a torque that will result in the motor armature to rotate.

Note:
DC motors are having a wide range of applications which are ranging from electric shavers to automobiles. In order to explain this wide range of applications, they are divided into different kinds on the basis of the field winding connections to the armature. One of the divisions is a self-excited DC motor and the other is a separately excited DC motor.