Physics
After long periods of use, a grey spot develops on the inside of a bulb. This is because

The tungsten filament evaporates and collects there
Glass undergoes a change due to the heat
The heat of the bulb scorches the glass at the top
Dust inside the bulb condenses on the top

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Physics
Lightning is produced when

Water vapour produces electricity in the clouds
Strong opposite charges in different clouds break down the resistance offered by the intervening air
Similar charges of electricity rush towards each other and then get repelled
Clouds strike against impurities in air and the friction burns up these impurities

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Physics
Railway tracks are banked on curves so that

Necessary centripetal force may be obtained from the horizontal component of the weight of the train
The train may not fall down inwards
Necessary centrifugal force may be obtained from the horizontal component of the weight of the train
No frictional force may be produced between the tracks and the wheels of the train

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