20.5 Battery Internal Resistance.
A real battery contains aninternal resistanceto the flow of electricity that causes it to have a lower voltage
when delivering current to a circuit than when it isn’t. The potential difference across a battery’s terminals
when it’snotconnected to a circuit and doing work is called itselectromotive force(or “emf”),E. (Note that
despite the name, this isnota force, but a voltage, measured in volts.)
The actual potential difference across a battery’s terminals when itisdoing work on a circuit is called the
terminal voltage. The terminal voltageVmay be modeled as
VDEIr; (20.16)whereIis the current being delivered by the battery, andris the internal resistance of the battery. The more
current is drawn from the battery, the smaller its terminal voltage.