How does emf relate to current, elec potential, & resistance

[PhysicsBoyMan]

Homework Statement

How does emf relate to current, electrical potential, and resistance? Specifically, in the context of the equations I have posted below.

Homework Equations

V = IR [/B]where V is electric potential, I is current, and R is resistance.

V = emf - IR

How can V = emf - IR if V = IR ?
Here is another similar example:

R = V/I

R = (emf - V) / I

The Attempt at a Solution

[/B]
emf seems to be some kind of change to the original electric potential of a circuit. emf - IR seems to calculate ΔV. Am I right about this? As far as I can tell, Ohm's law uses V, not ΔV. When is ΔV useful and when is V useful? Or do I have this all wrong?Thanks.

 

[CWatters]

I think some context is missing.

Can you draw the circuit for the examples you mention. Are you talking about a battery with internal resistance or a DC motor or ?