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| Q. |
After long periods of use, a grey spot develops on the inside of a bulb. This is because – |
| A. | the tungsten filament evaporates and collects there |
| B. | the heat of the bulb scorches the glass at the top |
| C. | dust inside the bulb condenses on the top |
| D. | glass undergoes a change due to the heat |
| Answer» C. dust inside the bulb condenses on the top | |
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Explanation: At high operating temperatures, tungsten evaporates from the filament and gradually forms a gray film on the inside of the bulb. The inert gas atoms collide with the evaporating tungsten, causing some of the tungsten atoms to return to the filament. Eventually, however, a thin spot develops in the filament. When this happens, the filament will break and the bulb fails. |
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