Question

A bulb or a machine becomes hot after working for sometime. Why?

Answer 1

A bulb or a machine becomes hot after working for sometime becase work will be done
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in case of machines

for a machines to work they need to overcome the effect of friction and some part of the energy which is used for useful work is converted into heat energy

in case of bulb 

there will be resistance which the filament gives to the flow of electric charges and it is used to create light and part of it s also converted into heat

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bothe cases obey law of conservation of energy