Science — CBSE Class 10 board question

(i) 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 force (motion) on the conductor.

(ii) A fuse is a short piece of wire made of a material with low melting point, connected in series in a circuit. It melts and breaks the circuit when current exceeds the safe limit, preventing damage to appliances and wiring. Hence it is called a safety device.

(iii) Electrical appliances with metallic bodies (e.g., iron, refrigerator) use a three-pin plug because the third pin connects the metal body to the earth wire. If a live wire accidentally touches the body, current flows safely to the earth, preventing electric shock. An electric bulb has no metallic body exposed to touch, so earthing is unnecessary; a two-pin plug suffices.

(iv) Applying Fleming's Left-Hand Rule, the direction of force on conductor AB is downward (or as determined by the field and current directions shown in the diagram).
Law applied: Fleming's Left-Hand Rule.

Source: Chapter 12, Sections 12.3 and 12.5 (Domestic Electric Circuits)

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• (i) Always state all three fingers and what each represents — examiners award marks for each element.
• (ii) Define fuse first, then explain the safety function — both parts needed for full marks.
• (iii) The key idea is that earthing protects from electric shock in metal-body appliances; bulbs have no exposed metal, so no earth is needed.
• (iv) Without seeing the exact diagram, state the rule clearly and apply it logically. In board exams, method (naming the rule + applying it) fetches marks even if the diagram differs slightly.