Well, lets see if this makes any sense.
A difference in Potential Energy is needed from a power supply [or similar], in order to have a current flow. EMF, or Voltage, is a resulting effect of the Potential Energy available from the power supply.
The EMF becomes a pressure when applied to a Resistance - an opposition to current flow.
If there is absolutely no opposition to current flow, there will be no need to push current, or any way to build up a pressure in a conductor / circuit, because the charges will migrate and equalize by simple attraction / repulsion. If the source which supplies currents has a continuous amount of available charges [such as an alternator or a battery at full charge], it will allow as many charges to flow as can possibly flow, due to the regeneration of a potential difference.
When the potential difference no longer exists [like the battery completely discharges], the charges are equalized throughout the entire circuit and supply, which results in no voltage or moving currents [although this never will exist completely].
An opposite polarity is not the only thing which will compose a potential diference which causes a current flow - any lower / higher energy level will allow charges to flow, like points in a circuit having the same polarity, but a slightly different level of voltage. This in it's self would fill a 1000 word document to explain!!
The fact that the EMF is a component required to push charges through a resistance is why electrical circuits can be used to transmit Kinetic Energy from one place to another - or simply, electricity can do some work.
Applying the level of current flowing through a given load device [like an incandescent lamp] and at what pressure, is what composes the energy level "dissipated" into the load.
These two factors, figured over a certain time frame, results in the Work being done.
Voltage, or pressure, is built up at the load, so it's a component of resistance.
Going more in-depth from here would require a lot of typing.
I would suggest to you and others, which are having difficulty understanding my examples, obtaining some books in physics and study them for at least a few months, in order to help grasp the terms and details involved.
I do, and will continue to, enjoy discussing this stuff, but I do not want to make it so unreal, or complex, there is a difficulty in understanding stuff - resulting in persons becoming overly confused [not trying to baffle people with bulls--t
One great area which would be extremely helpful here would be Electronic component studies and Semiconductor theories.
Not only are they more closely related to the Electrical fields than their physics counter parts, but they make more sense to persons with electrical backgrounds, due to the use of familiar terms and examples.
This would be the most effective way to advance one's theory knowledge base.
Remember, this stuff takes time to understand, so try not to get discouraged if it takes you months, even years, to fully comprehend. It's a field which requires a constant updating of knowledge [plan to never stop studying
Hope this made a little more sense.