Physics
Lightning is produced when

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

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Physics
In a bipolar junction transistor

The collector has the least concentration of impurity
The base has the least concentration of impurity
The emitter has the least concentration of impurity
All the three regions (the emitter, the base and the collector) have equal concentrations of impurity

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Physics
Skating on ice is possible because

The pressure on the ice due to skate melts the ice by lowering its melting point forming a thin film of water
The pressure on the ice due to skate raises its melting point
The surface of ice is smooth
Ice is cold

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