CBSE Class 10 Science 2019 · Paper 31/2/1 · Question 21

Pattern: when a current passes through the straight conductor held vertically through the horizontal cardboard, iron filings on the cardboard arrange themselves in concentric circles centred on the conductor. The field lines are these concentric circles, in the plane of the cardboard; they are crowded near the conductor and farther apart away from it (see the diagram). Right-hand thumb rule: imagine holding the current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current; the fingers wrapped around the conductor then give the direction of the magnetic field lines. Applying it: for a current flowing vertically upward through the cardboard, the fingers curl anticlockwise as seen from above, so the circles are marked anticlockwise (seen from above). For a downward current they would be clockwise. Change with distance: the magnetic field becomes weaker as the point is moved away from the conductor. Reason: the field due to a straight conductor is inversely proportional to the distance from it, $B\propto \frac{1}{r}$ for a given current, so doubling the distance halves the field; accordingly the field lines are farther apart away from the wire.