Summarizing Electromotive Force

Summary

• All voltage sources have two fundamental parts—a source of electrical energy that has a characteristic electromotive force (emf), and an internal resistance \(r\).
• The emf is the potential difference of a source when no current is flowing.
• The numerical value of the emf depends on the source of potential difference.
• The internal resistance \(r\) of a voltage source affects the output voltage when a current flows.
• The voltage output of a device is called its terminal voltage \(V\) and is given by \(V=\text{emf}-\text{Ir}\), where \(I\) is the electric current and is positive when flowing away from the positive terminal of the voltage source.
• When multiple voltage sources are in series, their internal resistances add and their emfs add algebraically.
• Solar cells can be wired in series or parallel to provide increased voltage or current, respectively.

Glossary

electromotive force (emf)

the potential difference of a source of electricity when no current is flowing; measured in volts

internal resistance

the amount of resistance within the voltage source

potential difference

the difference in electric potential between two points in an electric circuit, measured in volts

terminal voltage

the voltage measured across the terminals of a source of potential difference