Question

The coercivity of a bar magnet is $ 100A{m^{ - 1}} $ . It is to be demagnetized by placing it inside a solenoid of length $ 100cm $ and number of turns $ 50 $ . The current flowing through the solenoid will be:
(A) $ 4A $
(B) $ 2A $
(C) $ 1A $
(D) $ zero $

Answer 1

Hint :Coercivity is the minimum magnetic intensity which is used to bring the material to its original state. Here, we have been given the solenoid and we know how to calculate the magnetic field in it. With the help of magnetic fields we can be able to use formulas for coercivity.

Complete Step By Step Answer:
According to the given data we have,
 $ H = 100A{m^{ - 1}} $ …..(coercivity of bar magnet)
Length of solenoid $ = 100cm $
No. of turns. $ n = 50 $ (turns per $ 100cm $ ) $ = 50 $ (turns per meter)
Now by using the formula of coercivity of the substance we know that,
 $ H = \dfrac{B}{{{\mu _0}}} $
 $ \Rightarrow H = nI $…..( $ \because B = {\mu _0}nI $ )
Now place all the given values we get,
 $ \Rightarrow 100 = 50 \times I $
 $ \Rightarrow I = 2A $
Therefore, the current flowing through the solenoid is $ 2A $ .
The correct answer is option B.

Note :
Coercivity is the minimum magnetic intensity which is used to bring the material to its original state. When the magnetic field is increased in the opposite direction then the material loses its magnetic property, the point at which magnetization is lost by the substance is known as coercivity. We used a standard unit of all the terms as per our convenience.