Potential Differences in a Circuit with an EMF Source

ℰ

0.66

r

0.4

The graphic shows the potential rises and falls in a circuit with a battery providing a source of electromotive force (EMF) of magnitude

ℰ

. The circuit contains a resistance

R

, as well as an internal resistance

r

. The associated changes in the potential add to

ℰ-ir

-iR

, when a current

i

passes through the points

b

(before the EMF source) and

a

(before the resistance) and back to

b

, with corresponding voltages

V

a

and

V

b

. These changes in the potential, when summed algebraically, must be equal to zero, by Kirchhoff's second rule.