Physics
Rectifiers are used to convert:

Low voltage to high voltage
High voltage to low voltage
Alternating current to Direct current
Direct current to Alternating current

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Physics
It is easier to roll a barrel full of coal tar than to pull it because

the full weight of the barrel comes into play when it is pulled
more surface area of the barrel is in contact with the road when it is pulled
the rolling friction is much less than the sliding friction
coal tar is a liquid and it flows in the barrel when it is rolled

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