EMF

• Stands for electromotive force… but it's not an actual force
• Refers to the energy per unit charge OR work done per unit charge when moving charges between battery terminals, measured in V
• Different from potential difference because it refers specifically to the battery, not components in the external circuit

Internal Resistance

• Resistance (r) inside the battery itself
• In an ideal battery the emf equals the potential difference.
• In a real battery you have to account for the internal resistance:

 or

Primary vs Secondary Cells

• Primary: a cell that can only be used once
• Secondary: a cell that can be recharged and used again.
• When power is positive that denotes power being dissipated, while a negative indicates power being stored.

Discharging a Cell

• The bigger the current, the faster a cell will discharge.
• Voltage drops quickly initially, then remains fairly constant for a long period of time, then drops off quickly again at the end.
• Discharging a Cell        

V vs. t graph