Question

AlNiCo is used for making permanent magnets because it has
A. high coercivity and high retentivity
B. high coercivity and low retentivity
C. low coercivity and low retentivity
D. low coercivity and high retentivity

Answer 1

Hint: As a first step, you could give a brief note on the characteristics of alnico. You could also mention the uses of alnico. Then you could define both the terms that are given in the options. You could then find out whether these characteristics of alnico are high or low to make it apt to be used as permanent magnets.

Complete solution:
In the question, we are asked to find which among the given options gives the characteristics of AlNiCo that makes it apt for being used as a permanent magnet.

Before going into the solution, let us understand what AlNiCo is. Alnico is that class of iron alloys that contains aluminum, Nickel and Cobalt in addition to iron. They may also contain copper and titanium. Being ferromagnetic they’re used as permanent magnets. They were the strongest permanent magnet type before the era of rare earth metals.

Now let us understand the terms retentivity and coercivity. The property by virtue of which a magnetic material gains the ability to retain magnetism even when the magnetizing field is absent is known as retentivity. Coercivity is the measure of resistance offered against demagnetization.

Both coercivity as well as retentivity of Alnico is known to be high and thereby making it ideal for being used in making permanent magnets. Hence, option A is found to be the correct answer.

Note:
The magnetic field strength at the poles of Alnico magnets are found to be as high as 1500gauss which is approximately 3000 times earth’s magnetic field strength. Isotropic brands of alnico can be magnetized in any direction and for anisotropic ones there will be a preferred direction for magnetization.