Physics
The propagation of sound waves in a gas involves

Isothermal compression and rarefaction
Adiabatic compression and rarefaction
Isobaric compression and rarefaction
Isochoric compression and rarefaction

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Physics
In a railway track, two rails are joined end to end with a gap in between them because

Steel can be saved
Accidents due to contraction in winter can be avoided
Air-gaps are necessary for bearing the weight of running train
Accidents due to expansion in summer can be avoided

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Physics
A transformer

Converts DC to AC
Converts AC voltage to D.C. voltage
Is used to decrease or increase AC voltage
Converts electrical energy into mechanical energy

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