# NCERT : Moving Charges & Magnetism

Q.1. A circular coil of wire consisting of 100 turns, each of radius 8.0 cm
carries a current of 0.40 A. What is the magnitude of the magnetic
field B at the centre of the coil ?

Sol. Given , N = 100 , r= 0.08m , I= 0.4A

$\displaystyle B =\frac{\mu_0}{4\pi}\frac{2\pi N I}{r}$

B = 10−7×2×3.14×0.4×100/0.08

Q2. A long straight wire carries a current of 35 A. What is the magnitude
of the field B at a point 20 cm from the wire ?

Sol. By formula ,

$\displaystyle B =\frac{\mu_0}{4\pi}\frac{2I}{r}$

B = 10−7×2×35/0.20

= 3.5 × 10−5 T

Q3. A long straight wire in the horizontal plane carries a current of 50 A
in north to south direction. Give the magnitude and direction of B
at a point 2.5 m east of the wire.

Sol. By formula ,

$\displaystyle B =\frac{\mu_0}{4\pi}\frac{2I}{r}$

B = 10−7×2×50/2.5

B = 4 × 10−6 T

Q4. A horizontal overhead power line carries a current of 90 A in east to
west direction. What is the magnitude and direction of the magnetic
field due to the current 1.5 m below the line ?

Sol. By formula ,

$\displaystyle B =\frac{\mu_0}{4\pi}\frac{2I}{r}$

B = 10−7×2×90/1.5

B = 1.2×10−5 T

Q5. What is the magnitude of magnetic force per unit length on a wire
carrying a current of 8 A and making an angle of 30º with the
direction of a uniform magnetic field of 0.15 T ?

Sol. By formula , F = IlBsinθ

F = 8×1×0.15×Sin30º

F = 0.6 N/m

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