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30. Question: Why cannot a copper wire be used to make elements in

electric heater?

Answer: Copper melts at 108.30C and orms a black powder on reactin! with

atmospheric o"y!en. #or heater elements a metal should ha$e more

resistance to produce heat.

31. Question: Why are water or mercury droplets always round when

dropped on a clean !lass?

Answer: %he surace o a li&uid is the seat o a special orce as a result o

which molecules on the surace are bound to!ether to orm somethin! like a

stretched membrane. %hey tend to compress the molecules below to the

smallest possible $olume' which causes the drop to take a round shape as

or a !i$en mass he sphere has minimum $olume.