Internal Resistance in Physics 

Understanding energy loss inside a real power source 

Real energy sources are not perfect. When energy flows faster through a system, more energy is lost inside it. This means that not all of the energy produced inside a source reaches the external circuit.

Internal resistance describes this internal energy loss — energy that is turned into heat inside the cell instead of being delivered to the device.

Consider a factory 

Consider a factory producing chocolate bars and loading them onto delivery lorries.

If lorries arrive slowly, workers are not rushed and machines are not overloaded. Each lorry can be filled with perfectly stacked, undamaged chocolate bars before it leaves the factory.

This represents the best possible output the system can achieve if nothing is wasted.

As more lorries arrive and the factory is forced to run faster, workers rush, machines overheat, and handling becomes less careful. Chocolate bars are dropped, damaged, or wasted inside the factory.

Lorries now leave the factory more frequently, but each lorry is less full, because more bars are being lost inside the system. 

Connecting the factory model to an electrical circuit: internal resistance 

In an electrical circuit, the same process occurs inside a cell.

When current is low, little energy is lost inside the cell and most of the energy produced is delivered to the external circuit.

As current increases, resistance inside the cell causes more energy to be turned into heat. This energy is lost inside the source and never reaches the external circuit.

The EMF of a cell represents the best possible output — all of the energy produced by the cell is delivered to the charge.

The terminal potential difference is the energy per unit charge that actually reaches the external circuit. It is like the lorries that are not full because the factory is operating too quickly.

The difference between these two values is caused by internal resistance. Internal resistance turns useful energy into wasted heat inside the cell instead of delivering it to the charge. This is like workers wasting chocolate bars when they are hurried.

As current increases, more energy is lost inside the cell.

This means the energy delivered to the external circuit gets smaller.

Chocolate bars per lorry = best possible output − chocolate bars wasted inside the factory 

This has the same structure as the circuit relationship: 

terminal p.d. = EMF − (current × internal resistance)

V = ε − Ir