It seems to me that voltage, or Electromotive force, would be measured in regular force units, not Joules/Coulomb. I'm confused about this, so I've been thinking about it a lot and studying a lot and here are my conclusions. Please let me know if you agree or disagree, what you think, or how I am wrong or right.
If there is a separation of positive and negative charge between two points, such as the terminals of a battery, there is a force attracting them.
This is the force that causes charge flow or current in a circuit.
Since the charges would naturally mesh together, work must have been done on the negative charge to separate them (since only electrons can be moved). This increases the potential energy of the negative charge.
As charge flows through the circuit, it loses its PE as it collides with atoms of the metal in the circuit and provides energy to the load.
We can measure this change in PE of the charge.
This change in PE of the charge is a way to indirectly measure the forces acting on the charge or how likely it is to flow. This is because a higher PE loss would mean that more work had to have been done on the neg. charge to initially seperate the two plus and minus charges. If more work was done, this means there was a stronger electric field force acting on the charge.
I imagine it would be more difficult to measure the forces acting on the charge flowing through a circuit, than it is with static electricity, so this is why we measure Electromotive force in terms of change in PE of charge between two points.