In hydrogen atom, an electron is revolving in the orbit of radius $0.53 \mathop {\rm{A}}\limits^{\rm{o}} $ with $6.6 \times 10^{15} \mathrm{rps}$. Magnetic field produced at the centre of the orbit is

A solenoid of length 0.4 m and having 500 turns of wire carries a current of 3 A . A thin coil having 10 , turns of wire and of radius 0.01 m carries a current of 0.4 A . The torque required to hold the coil in the middle of the solenoid with its axis perpendicular to the axis of solenoid.

Which of the following graph represents the variation of magnetic flux density $B$ with distance $r$ for a straight long wire carrying an electric current?

A current carrying conductor experiences maximum force in magnetic field when the angle between current element and direction of magnetic field is