Question

Why emf is always greater than terminal potential?

Answer 1

The electromotive force (EMF) is always greater than the terminal potential difference (TPD) because of the internal resistance of the power source (like a battery or a cell). Here's a simple breakdown of why this happens:

1. EMF (Electromotive Force): This is the maximum potential difference that a source (like a battery) can provide when no current is flowing through it (i.e., when the circuit is open). It’s essentially the "ideal" voltage that the power source could deliver.
   

1. Terminal Potential Difference (TPD): This is the potential difference across the terminals of the power source when it is delivering current to a load (like a resistor). When current flows, there is a voltage drop inside the power source itself due to its internal resistance.
   

1. Internal Resistance: Every power source has some internal resistance. When current flows through the source, a voltage drop occurs inside the battery due to this resistance. This reduces the voltage available at the terminals, which results in the terminal potential difference being less than the EMF.
   

Why the EMF is Greater than TPD:

1. When current flows through the power source, some energy is "lost" inside the battery due to its internal resistance.
   

1. The voltage drop across the internal resistance is given by Ohm’s Law: \( V = I \times r_{\text{internal}} \), where \( I \) is the current and \( r_{\text{internal}} \) is the internal resistance.
   

1. This drop reduces the voltage available across the terminals of the battery, so the terminal potential difference (TPD) is less than the EMF.
   

In short, the EMF represents the maximum possible voltage (without any current), while the TPD is the actual voltage you measure across the terminals when the circuit is active and current is flowing, and this is always reduced by the internal resistance.