Mammoth Memory

Copper atoms

To help us decide which way electricity flows lets look at the structure of a copper atom which is used universally as an electrical carrier.

Atomic structure of a copper atom

In this neutral state of charge the atom has the same amount of +ve protons as -ve electrons i.e. 29.

But the outside shell only has one electron and is vulnerable to being removed. 

Outer shell of a copper atom

Electrons close to the nucleus are hard to remove while those in the outer ring require relatively little energy to be removed and become free electrons.

More Info