NEET-XII-Physics
37: Magnetic Properties of Matter
- #4A long, straight wire carries a current i. The magnetising field intensity H is measured at a point P close to the wire. A long, cylindrical iron rod is brought close to the wire, so that the point P is at the centre of the rod. The value of H at P will
(a) increase many times
(b) decrease many times
(c) remain almost constant
(d) become zerodigAnsr: cAns : (c) remain almost constant
From the Biot-Savart law, magnetic field `` \left(B\right)`` at a point P close to the wire carrying current i is given by,
`` \stackrel{\to }{B}=\frac{{\mu }_{0}}{4\,\mathrm{\,\pi \,}}\frac{i\stackrel{\to }{dl}\times \stackrel{\to }{r}}{{r}^{3}}``
Magnetising field intensity (H) will be,
`` H=\frac{B}{{\mu }_{0}}=\frac{1}{4\,\mathrm{\,\pi \,}}\frac{i\stackrel{\to }{dl}\times \stackrel{\to }{r}}{{r}^{3}}``
Now, as the cylindrical rod is brought close the wire such that centre of the rod is at P, then distance of point P from the wire(r) will remain same. Hence, magnetic field intensity will remain almost constant. Also even when the rod is carrying any current then B will be zero at the centre of the rod so the value of Magnetising field intensity will remain the same at point P.
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