The terminal voltage of a source is lower than the EMF by the voltage drop across the internal resistance.
Symbols
U
terminal voltage
[V]
ε
electromotive force (EMF)
[V]
I
current
[A]
r
internal resistance of the source
[Ω]
Where it comes from
A real source has internal resistance — part of the voltage is “lost” inside the battery. That is why the terminal voltage drops as the current drawn increases.
When to use it
Circuits with a real source (battery, accumulator). With no load (I=0) the voltage equals the EMF.
Example
A cell's EMF is 4.5 V, its internal resistance 0.5 Ω, the current 1 A. Calculate the terminal voltage.