Cells, EMF, Internal Resistance — Definition
Definition
Imagine a cell, like the AA battery in your remote control, as a tiny power station. Its job is to push electric charges around a circuit, making things like light bulbs glow or motors spin. This 'push' is what we call the electromotive force, or EMF for short.
Think of EMF as the maximum potential difference a cell can provide when it's not working, meaning no current is flowing through it. It's like the full potential energy stored in a spring before you release it.
The unit of EMF is Volts (V).
Now, no real-world cell is perfect. Inside every cell, there are chemicals (electrolyte) and metal plates (electrodes) that aren't perfectly conductive. They offer some resistance to the flow of charges *within* the cell itself.
This inherent opposition to current flow inside the cell is known as its internal resistance, denoted by 'r'. It's like a small, hidden resistor built right into the battery. When the cell starts pushing current through an external circuit, some of its 'push' (EMF) is used up just to overcome this internal resistance.
This 'lost' push manifests as a voltage drop across the internal resistance, given by , where is the current flowing through the cell.
Because of this internal resistance, the actual voltage available at the terminals of the cell (the points where you connect it to an external circuit) when current is flowing is less than its EMF. This actual voltage is called the terminal potential difference, or terminal voltage, denoted by 'V'.
The relationship is quite simple: the terminal voltage is the EMF minus the voltage drop across the internal resistance. Mathematically, this is expressed as . So, if a cell has an EMF of 1.
5 V but has an internal resistance and is supplying current, the voltage you measure across its terminals will be slightly less than 1.5 V. This difference becomes more significant as more current is drawn from the cell.
Understanding these three concepts – EMF, internal resistance, and terminal voltage – is crucial for analyzing how real-world batteries and power sources behave in electrical circuits.