Science — CBSE Class 10 board question

(i) Force on a Current-Carrying Conductor in a Magnetic Field:

Diagram:
A horizontal aluminium rod AB is suspended between the poles of a horseshoe magnet (N below, S above), so the magnetic field B acts vertically upward. Current flows from B to A (direction of current shown by arrow). The rod experiences a force directed towards the left, perpendicular to both the current and field.

Fleming's Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of the left hand mutually perpendicular. If the forefinger → direction of magnetic field, middle finger → direction of current, then the thumb → direction of force on the conductor.

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(ii)

(I) The electron moves in a specific direction in the magnetic field. Since current is taken opposite to the direction of electron motion, applying Fleming's Left-Hand Rule, the force acting on the electron is directed into the page.

(II) When both the direction of the magnetic field and the direction of motion of the electron are reversed, the two reversals cancel each other out. The direction of force on the electron remains the same (into the page) — unchanged.

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

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• (i): Examiners expect a labelled diagram (rod AB, magnet poles, arrows for current and field) plus a clear statement of Fleming's Left-Hand Rule. Both together fetch full marks.
• (ii)(I): The key trick CBSE tests here is that electron motion is opposite to conventional current direction. Applying FLH rule with reversed current gives force into the page.
• (ii)(II): Reversing both field and current simultaneously means the force direction is unchanged — this is the standard paired-reversal result examiners look for. Don't make the mistake of saying it reverses.
• Keep answers tight; a diagram counts as content for a 5-mark question like this.