Assertion (A): The energy of a charged particle moving in a magnetic field does not change.

Reason (R): It is because the work done by the magnetic force on the charge moving in a magnetic field is zero.

Two long parallel wires kept $$2 \mathrm{~m}$$ apart carry $$3 \mathrm{~A}$$ current each, in the same direction. The force per unit length on one wire due to the other is

Assertion (A): The deflecting torque acting on a current carrying loop is zero when its plane is perpendicular to the direction of magnetic field.

Reason (R): The deflecting torque acting on a loop of magnetic moment $$\vec{m}$$ in a magnetic field $$\vec{B}$$ is given by the dot product of $$\vec{m}$$ and $$\vec{B}$$.

A long straight wire of radius '$$a$$' carries a steady current $$I$$. The current is uniformly distributed across its area of cross-section. The ratio of magnitude of magnetic field $$\vec{B}_1$$ at $$\frac{a}{2}$$ and $$\vec{B}_2$$ at distance $$2 a$$ is