internal resistance
/ in-TER-nal rih-ZIS-tance /
Internal resistance is the resistance hidden inside a battery or other power source, the friction charge feels while passing through the source itself. No real battery is a perfect pump; its own chemicals and electrodes fight the current a little on the way through. It answers a very practical question: why does a battery seem to 'sag' and deliver less voltage exactly when you ask it to supply a big current?
Precisely, internal resistance r is modelled as a small resistor sitting in series inside the ideal source. When a current I flows, a voltage I times r is dropped across this internal resistance, energy that is lost as heat inside the battery rather than delivered to the circuit. The terminal voltage the source actually provides is therefore V = EMF - I r while discharging. The larger the current, the bigger this internal drop, which is why heavy loads pull the terminal voltage down.
Internal resistance is why batteries get warm in use, why a nearly-dead battery can still show a healthy voltage until you load it and it collapses, and why some batteries can deliver a big surge (low internal resistance) while others cannot. A fresh AA cell might have a few tenths of an ohm; a car battery is built for very low internal resistance so it can pour out hundreds of amperes to crank the engine. One honest note: internal resistance is not perfectly constant, it grows as a battery ages, discharges, or gets cold, which is why old or cold batteries struggle under load.
A battery of EMF 12 V and internal resistance r = 0.5 ohms supplies a current of 4 A. The internal voltage drop is I r = 4 x 0.5 = 2 V, so its terminal voltage falls to V = 12 - 2 = 10 V while delivering that current.
The heavier the current, the more the internal resistance eats into the delivered voltage.
Internal resistance is why a battery's terminal voltage sags under load and why it warms up. It is not fixed: it rises as a cell ages, discharges, or gets cold.