CBSE Class 10 Science 2019 · Paper 31/1/3/C · Question 18

(a) Every part of the circular loop acts like a short straight wire, so the magnetic field lines close to the wire are small concentric circles. Moving towards the centre of the loop, these circles become larger, and at the centre their arcs appear as straight lines. By the right-hand thumb rule every section of the loop makes the field lines inside the loop point in the same direction, so the field at the centre is strong, nearly uniform and perpendicular to the plane of the loop, and the lines return round the outside of the loop, as for a short bar magnet. The diagram shows the loop standing at right angles to the page, with the current coming out of the page at one side and going into it at the other, circles round each of these points, and straight, parallel lines through the centre. The field produced at a point by a current-carrying wire is directly proportional to the current. In a coil of $n$ turns the current flows in the same direction in every turn, so the fields of all the turns point the same way and add up: the field is $n$ times that of a single turn. (b) In the right-hand thumb rule the thumb indicates the direction of the current (the curled fingers give the direction of the magnetic field lines). The right-hand thumb rule gives the direction of the magnetic field produced by a current, whereas Fleming's left-hand rule gives the direction of the force (motion) on a current-carrying conductor placed in a magnetic field; in Fleming's rule the thumb indicates the force (motion), the forefinger the field and the middle finger the current.