JEE MAIN - Physics (2014 (Offline) - No. 12)
The coercivity of a small magnet where the ferromagnet gets demagnetized is $$3 \times {10^3}\,A{m^{ - 1}}.$$ The current required to be passed in a solenoid of length $$10$$ $$cm$$ and number of turns $$100,$$ so that the magnet gets demagnetized when inside the solenoid, is :
$$30$$ $$mA$$
$$60$$ $$mA$$
$$3$$ $$A$$
$$6A$$
Explanation
Magnetic field in solenoid $$B = {\mu _0}ni$$
$$ \Rightarrow {B \over {{\mu _0}}} = ni$$
(Where $$n=$$ number of turns per unit length)
$$ \Rightarrow {B \over {{\mu _0}}} = {{Ni} \over L}$$
$$ \Rightarrow 3 \times {10^3} = {{100i} \over {10 \times {{10}^{ - 2}}}}$$
$$ \Rightarrow i = 3A$$
$$ \Rightarrow {B \over {{\mu _0}}} = ni$$
(Where $$n=$$ number of turns per unit length)
$$ \Rightarrow {B \over {{\mu _0}}} = {{Ni} \over L}$$
$$ \Rightarrow 3 \times {10^3} = {{100i} \over {10 \times {{10}^{ - 2}}}}$$
$$ \Rightarrow i = 3A$$
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