In a wire of radius 1 mm a steady current of 2 A uniformly distributed across the cross-section of the wire is flowing. Then the magnetic field at a point 0.25 mm from the centre of the wire is

The magnetic field at the centre of a current carrying circular coil of radius $R$ is $B_c$ and the magnetic field at a point on its axis at a distance $R$ from its centre is $B_a$. The value of $\frac{B_c}{B_a}$ is

The force per unit length on a straight wire carrying current of 8 A making an angle of $30^{\circ}$ with a uniform magnetic field of 0.15 T is

An alpha particle moves along a circular path of radius 0.5 mm in a magnetic field of $2 \times 10^{-2} \mathrm{~T}$. The de-Broglie wavelength associated with the alpha particle is nearly (Planck's constant $=6.63 \times 10^{-34} \mathrm{~J} \mathrm{~s}$ )