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

Hint (a) A permanent magnet is a type of magnet which displays its magnetic behavior all the time and always has its magnetic field. But for a commercial motor, we need a type of magnet whose magnetic behavior can be manually controlled.
(b) In any lens, focal length is the distance of its focus from the optical center. That’s why it is expressed in metre. Power of a lens is defined as the reciprocal of the focal length of the lens and Dioptre is its SI unit.

Complete step by step answer
In these types of questions we have to relate the second term in the same way the first term is related.
(a) A motor is an electrical device which is able to convert electrical energy into mechanical energy. It uses magnetism and magnets to create motion and convert electrical energy into mechanical energy.
A permanent magnet is a type of magnet which displays its magnetic behavior all the time and always has its magnetic field. It can be used for the regular motors. But for a commercial motor, we need a type of magnet whose magnetic behavior can be manually controlled.
Electromagnets are the type of magnets which use electrical energy to control its magnetism and hence used for commercial motors.

Hence, correct answer of part (a) is Electromagnet.

(b) In a lens, focus is the point where light rays coming from infinity converges and forms an image and Focal length is the distance of its focus from the optical center (the central part of a lens). That’s why focal length is expressed in metre and this is also its SI unit.
Power of a lens is defined as the reciprocal of the focal length of the lens i.e $P = \dfrac{1}{f}$ and as metre is the SI unit of focal length, $\dfrac{1}{m}$ will be the SI unit of Power of the lens which is known as Dioptre.

Hence, correct answer for part (b) is Dioptre.

Note: (a) Although electromagnets have much more powerful magnetism than normal permanent magnets and can also be manually controlled. But they require so much energy that we cannot use it as for normal uses.
(b) As we know that convex lenses are called converging lenses and concave lenses are called diverging lenses. So, converging ability is the power of a convex lens whereas diverging ability is the power of a concave lens.