$(a)$ Describe an activity to determine the direction of magnetic field produced by a current-carrying straight conductor. Also,show that the direction of the magnetic field is reversed on reversing the direction of current.
$(b)$ An $\alpha$-particle (which is a positively charged particle) enters a uniform magnetic field at right angles to it as shown below. Stating the relevant principle,explain in which direction will this $\alpha$-particle move?

(N/A) Take a battery $(12\, V)$,a variable resistance (or a rheostat),an ammeter $(0-5\, A)$,a plug key,and a long straight thick copper wire. Insert the thick wire through the center,normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide.
Connect the copper wire vertically in series with the battery,a plug key,and a rheostat. Sprinkle some iron filings uniformly on the cardboard. Keep the rheostat at a fixed position and note the current through the ammeter. Close the key so that a current flows through the wire. Gently tap the cardboard a few times. You would find that the iron filings align themselves in a pattern of concentric circles around the copper wire. This represents the magnetic field around the current-carrying conductor.
If the direction of current through the straight conductor is reversed (by reversing the battery terminals),the direction of the magnetic field lines also gets reversed.
$(b)$ Here,the motion of the $\alpha$-particle ($+$ve charge) in the given direction represents the direction of conventional current. By using Fleming's left-hand rule,we find that the force acting on the $\alpha$-particle is directed perpendicular to the plane of the paper,pointing inward. Hence,the $\alpha$-particle will be deflected into the plane of the paper.