Magnetics 5 Question 6
6. A bar magnet is demagnetised by inserting it inside a solenoid of length $0.2 m, 100$ turns and carrying a current of 5.2 A. The coercivity of the bar magnet is (Main 2019, 9 Jan I) (a) $1200 A / m$ (b) $285 A / m$ (c) $2600 A / m$ (d) $520 A / m$
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Answer:
Correct Answer: 6. (c)
Solution:
- Coercivity of a bar magnet is the value of magnetic field intensity $(H)$ that is needed to reduce magnetisation to zero. Since, for a solenoid magnetic induction is given as,
$$ B=\mu _0 n I $$
where, $n$ is the number of turns $(N)$ per unit, length $(l)$ and $I$ is the current.
Also,
$$ B=\mu _0 H $$
$\therefore$ From Eqs. (i) and (ii), we get
$$ \mu _0 n I=\mu _0 H \text { or } H=n I=\frac{N}{l} I $$
Substituting the given values, we get
$$ H=\frac{100}{0.2} \times 5.2=2600 A / m $$
Thus, the value of coercivity of the bar magnet is $2600 A / m$.
