The specification for a bulb is given 220V,0.45A. Obtain the filament resistance. An ohmmeter is an instrument to measure the resistance of a device or circuit. The same ohmmeter connected across the bulb terminals measures 29.4Ω. Where is the discrepancy then?
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The ohmmeter measures defectively (measured value of 29.4 ohms as against rating of ~489 ohms.
Explanation:
The defective reading can be on account of any of following reasons:
- internal wiring of ohm-meter is incorrect.
- emf of potential source of ohm-meter might be too low for injection.
- ohm-meter might be un-calibrated.
- reader may be using the incorrect scale for taking reading.
- ohm-meter might have zero error or mechanical defect in case of conventional type of measurement system of instrument.
- ambient conditions might be beyond the prescribed limits for healthy functioning of ohm-meter.
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Answer:
In first case,
V = 220 V
I = 0.45 A
So, R = V/I = 220/0.45 = 488.9 Ω
In the second case, the ohmmeter measures resistance at room temperature - which it shows to be 29.4Ω. In the first case, when it is subjected to 220 V, resistance increases in the circuit due to rise in temperature due to rise in voltage.
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