A Damaged Lamp, a Water Spill and Identical Results
During a Grade 12 practical on electrical resistance, 28 learners use 12 V dc power supplies (maximum output 5 A), 6 V 0.3 A filament lamps in holders, ammeters, voltmeters and loose copper connecting wires. One group connects a lamp directly across the 12 V supply; the lamp flashes brightly and the glass envelope cracks. A learner then reaches for a 240 V 60 W mains lamp from a nearby cupboard, saying, 'This one is safer because it is rated for a higher voltage.' At the same time, a water bottle is knocked over near a power supply, and two other groups submit identical tables of results. The teacher has one spare 6 V lamp, a roll of insulating tape, and 20 minutes before the lesson ends.
1. Identify the immediate hazards and the correct order of actions.
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Immediate hazards are a hot, cracked lamp with exposed filament and glass; loose copper wires possibly touching; water close to a power supply; and students reaching into the apparatus. First isolate the 12 V supply at the switch and unplug it, keeping the group back. Do not touch the broken lamp until it has cooled; use insulated tongs or heat-resistant gloves and place it in a designated bin. Move the water bottle away and dry the bench and floor before re-energising anything. Check the supply and leads for damage. Only then replace the lamp with the spare 6 V lamp. The 240 V lamp should not be brought into the 12 V circuit at all; its holder and insulation are not a substitute for a low-voltage lamp holder, and the lesson should not continue with improvised mains equipment.
2. Explain, using current and resistance, why the 240 V lamp is not a safe or suitable replacement on the 12 V supply.
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For the 240 V 60 W lamp, normal resistance R = V^2/P = 240^2/60 = 960 ohms. On 12 V it would draw only 12/960 = 0.0125 A and dissipate 0.15 W, far below its operating temperature, so it would barely glow. That sounds harmless, but the lamp is physically designed for a mains holder; its cap and any exposed metal are not rated for student handling in this set-up, and a 12 V supply can still deliver 5 A into a fault. The 6 V 0.3 A lamp has a hot resistance of about 20 ohms but a much lower cold resistance, so directly across 12 V it can draw well above 0.3 A and overheat, which is what destroyed the first lamp. The correct fix is to use the 6 V supply setting or a series resistor, not a higher-voltage lamp.
3. How should the teacher handle the identical results as an assessment issue?
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Identical tables are not proof of copying, but they require checking. Separately ask each learner to explain how a reading was taken, to predict the effect of changing the supply voltage, and to calculate one resistance from their own data. If they cannot, the teacher should treat the work as invalid for assessment and follow the school's malpractice procedure. The immediate teaching response is to issue different lamp or resistor values to each group, or require individual data sheets signed at the bench. The assessment should reward the method, uncertainty treatment and explanation, not just a correct table, so that collaboration does not automatically produce identical credit.