CBSE Class 10 Science 2019 · Paper 31/1/3 · Question 17

(a) The field lines are concentric circles centred on the wire, lying in the plane of the cardboard (see the diagram in step 1). Rule: the right-hand thumb rule. Imagine holding the current-carrying straight conductor in your right hand so that the thumb points in the direction of the current; then the fingers wrapped around the conductor give the direction of the magnetic field lines. Applying it: for a current flowing up through the cardboard, the field lines run anticlockwise when seen from above; if the current flows down, they run clockwise. (b) The strength of the magnetic field decreases as the point is moved away from the wire. Justification: the concentric circles representing the field become larger and larger, and more widely spaced, as the distance from the wire increases, so the field there is weaker; a compass needle moved away from the wire is deflected less.