Physics
In a conductor

The valence band and the conduction band overlap each other
There is no conduction band
The forbidden energy gap is very narrow
The forbidden energy gap is very wide

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

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

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