Georg Simon Ohm (1787-1854) formulated the relationships among voltage, current, and resistance as follows:

The circuit below demonstrates these relationships.

• Click on the resistance slider to change resistors.
• Click on the voltage slider to adjust voltage of the variable power supply.

Mathematically Ohm's law can be written as:

where I is current in amperes, E is the applied voltage in volts, and R is resistance in ohms.

This circuit contains an ammeter measuring current flow in milliamperes. Notice that as you increase voltage, current flow increases. As you increase resistance, current flow decreases.

It is important to note that resistance cannot be changed by changing voltage or current. Resistance in a circuit is a physical constant. Resistance in a circuit can only be changed by changing components or resistors rated at more or fewer ohms. The changing of resistance in this circuit simulates the physical changing of resistors with different ratings.

Discover how resistors are color coded in our Interactive Java Resistors Tutorial.