PS-6.10   Compare alternating current (AC) and direct current (DC) in terms of the production of electricity and the direction of current flow.
Key Concepts
Current:  AC current, DC current
Generator: electromagnetic induction
Battery : cell  
    We know that electric current is the flow of electrons through a conductor. Direct current or "DC" current, is current that flows only in one direction. Direct current is produced by batteries or cells, but can also be produced by some electrical generators. When electrical current flows in one direction it is said to have a single polarity, that is, the electrons only flow from the negative terminal of the cell or battery to the positive terminal.  Direct current is useful when applied to situations where the current does not have to be carried by conductors over long distances. This is because when direct current flows over long distances the electrical resistance of the conductor converts too much of the electrical energy into heat.
    When electrical energy must be carried over long distances, such as from the power generating station to homes, alternating current or "AC" current is used. AC current is produced by special electrical generators called alternators. During the operation of an electrical generator the polarity of the positive and negative terminals switch back and forth 60 times every second. The result is that all of the electrons, throughout the entire length of the conductor, vibrate slightly from side to side. Alternating current is useful when transmitting electrical energy over long distances because less is wasted as heat. The following animation illustrates how electrons of an AC current may behave when moving along a conductor. Note that the polarity of the + and - terminals reverse as the electrons change directions. Also note that each electron only moves a very short distance.
 
Study this animation of an alternator and a generator and you will quickly see the difference between DC and AC current. Read more about alternating and direct current here.