Ohm's Law: The current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.
where V is the potential difference measured across the resistance in units of volts; I is the current through the resistance in units of amperes and R is the resistance of the conductor in units of ohms (?).
The following applet is a simulation for AC and DC circuit.
You can click the resistor to add it to the circuit or remove it from the circuit.
The same is true for batterys for DC circuit.