Voltage/Current/Resistance Circuits

Most direct current (dc) circuits can be boiled down ultimately to three major components: a voltage source, a set of conductors, and a resistance. This is shown in the schematic diagram of Fig. 1. The voltage of the emf source is called E (or sometimes V); the current in the conductor is called I; the resistance is called R. The standard units for these components are the volt (V), the ampere (A), and the ohm (Ω), respectively. Note which characters here are italicized and which are not. Italicized characters represent mathematical variables; nonitalicized characters represent symbols for units.

You already know that there is a relationship among these three quantities. If one of them changes, then one or both of the others also will change. If you make the resistance smaller, the current will get larger. If you make

Fig. 1. A simple dc circuit. The voltage is E, the current is I, and the resistance is R.

the emf source smaller, the current will decrease. If the current in the circuit increases, the voltage across the resistor will increase. There is a simple arithmetic relationship between these three quantities.