**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

B = (μ_{o}/4π)2NI/r

B = 10^{−7}×2×0.4×100/0.08

= 10^{−4} T

**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 , B = (μ_{o}/4π)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 , B = (μ_{o}/4π)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 , B = (μ_{o}/4π)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