Electric Circuits: Internal Resistance

Internal Resistance

Every electrical device, including a battery or a cell, has some internal resistance. It’s the resistance caused by the components inside the battery or cell itself.
When current flows through a power source, some of the electrical energy is wasted in overcoming the internal resistance. This reduces the available energy to the external circuit and is why a power source’s operational voltage (terminal pd) is less than its emf.
Electromotive force (E), or emf, relates to the energy provided by a battery or cell. On a basic level, you can consider it as the ‘voltage when no current is drawn from the source’.
The terminal potential difference (or terminal voltage, V) is the voltage output of a battery or cell when current is drawn from it.
Internal resistance (r) and emf can be determined from knowing the terminal potential difference at different levels of current using the formula E = V + Ir.

EMF, Internal Resistance, and Terminal PD

When a current is flowing (I ≠ 0), the terminal voltage is less than the emf due to the energy loss in overcoming the internal resistance.
Kirchhoff’s Second Law (the Principle of Conservation of Energy) states the total emf around a series circuit is equal to the sum of potential differences across the components.
The sum of the emf is expended as the terminal pd (V) plus the ‘lost volts’ in overcoming the internal resistance, or Ir.
Recognising the similarities and differences between emf and terminal pd is crucial, particularly the impact of internal resistance on energy transfer.

Understanding & Drawing Circuit Diagrams

Properly understanding and drawing out circuit diagrams is a critical skill, this includes recognising symbols like emf (ε).
When drawing circuit diagrams for problems including internal resistance, remember that it’s usually represented by a resistor in series with the cell or battery.
Familiarity with circuit diagrams including internal resistance will aid in understanding how current, voltage, resistance, and emf interact in the circuit.

Thoroughly understanding internal resistance, how it interacts with emf and terminal potential difference, as well as how to apply these to circuit diagrams, is crucial for grasping more complex circuit behaviour and potential energy loss situations.