Question

Fill in the blanks.
a) For a motor : a permanent magnet, then for a commercial motor : ..................
b) Focal length of a lens : metre, then for power of a lens : ..................

Answer 1

a) In commercial motors electromagnets are used.

Hence, for a commercial motor: an electromagnet. An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off. Electromagnets usually consist of insulated wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the hole in the center of the coil. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful magnet.
b) Unit of power of a lens is dioptre.

Hence, for power of a lens: dioptre. The diopter is the unit of measure for the refractive power of a lens. The power of a lens is defined as the reciprocal of its focal length in metres, or $P=\frac{1}{f}$, where $P$ is the power in diopters and $f$ is the focal length in metres.