Science — CBSE Class 10 board question

(i)(a) Close the switch K to pass current through conductor AB (from B to A). The rod AB gets displaced (moves to the left), showing that a force acts on a current-carrying conductor placed in a magnetic field.

(i)(b) The direction of force can be reversed in two ways:

1. Reverse the current – interchange the battery connections so current flows from A to B; the rod displaces in the opposite direction (right).
2. Reverse the magnetic field – interchange the poles of the horseshoe magnet (N above, S below); the rod again deflects in the opposite direction.

(ii) The magnitude of the force is highest when the direction of current is perpendicular (at right angles) to the direction of the magnetic field.

(iii) Fleming's Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of current, then the thumb points in the direction of the force (motion) on the conductor.

Source: Chapter 12, Section 12.3 – Force on a Current-Carrying Conductor in a Magnetic Field

---

• (i)(a): Simply describe closing the circuit and observing displacement — that is the proof of force.
• (i)(b): Two distinct methods must be stated clearly: reverse current OR reverse magnetic field. Examiners award one mark per method.
• (ii): The keyword phrase is "at right angles" — do not write "parallel" (force is zero then).
• (iii): Name all three fingers and what each represents. Many students lose marks by omitting one finger or swapping forefinger/middle finger roles. Remember: First finger → Field; seCond finger → Current; thuMb → Motion.