CBSE Class 10 Science 2020 · Paper 31/C/1 · Question 30

Activity: hang a small aluminium rod AB (about $5$ cm long) horizontally from a stand by two connecting wires, so that it lies between the poles of a strong horseshoe magnet, with the north pole vertically below the rod and the south pole vertically above it; the field at the rod then points vertically upwards. Connect the rod in series with a battery, a plug key and a rheostat, as in the labelled diagram. Observation: when the key is inserted and a current flows from B to A, the rod is displaced sideways (to the left as seen from end A), horizontally, at right angles both to its own length and to the vertical field. When the current is reversed, the rod is displaced to the right; when the magnet is turned over, the displacement also reverses. Conclusion: a current-carrying straight conductor placed in a magnetic field experiences a force perpendicular to its length and to the field; its direction, given by Fleming's left-hand rule, depends on the directions of the current and of the field.