Electrical sources and internal resistance
An electrical cell is made from materials (metal or chemicals, for example). All materials have some resistance. Therefore, a cell must have The opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms.. This resistance is called the internal resistance of the cell.
A cell can be thought of as a source of electromotive force (EMF) with a An electrical component that restricts the flow of electrical charge. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance. connected in A circuit where one component follows directly from another, eg three bulbs in a row with no junctions are said to be connected in series..
When a load resistance is connected, current flows through the cell and a voltage develops across the internal resistance. This The potential difference across a cell, electrical supply or electrical component. It is measured in volts (V). is not available to the A closed loop through which current moves - from a power source, through a series of components, and back into the power source. so it is called the lost volts, \(V_{L}\).
\(V_{L}\) can also be calculated as \(I r\) using The rule that states that the current (I) flowing through a resistor (R) is directly proportional to the voltage (V) across the resistor, provided the temperature remains constant..
The voltage across the ends of the cell is called the terminal potential difference, \(V_{tpd}\).
\(V_{tpd}\) can also be calculated as \(I R\) where \(R\) is the load resistance.
Voltage is a measure of energy, and energy is always conserved. So the EMF \(E\) of a cell is equal to the sum of its terminal potential difference, \(V_{tpd}\), and the lost volts, \({V_L}\).
This gives rise to the equation \(E= {V_{tpd}} + {V_L}\)
This equation can be written in different forms, eg \(E= I(R + r)\). To solve problems on internal resistance it should be remembered that such circuits involve using a series circuit with the internal resistance and the load.