Most dc circuits can be boiled down to three major components: a voltage source, a set of conductors, and a resistance. This is shown in Following Figure. The voltage or EMF source is E; the current in the conductor is I; the resistance is R.
The basic elements of a dc circuit. The voltage is E, the current is I, and the resistance is R.
You already know that there is a relationship among these three quantities. If one of them changes, then one or both of the others will change. If you make the resistance smaller, the current will get larger. If you reduce the applied voltage, the current will also decrease. If the current in the circuit increases, the voltage across the resistor will increase. There is a simple arithmetic relationship among these three quantities.