Question

What is the difference between emf and terminal potential difference?

Answer 1

The difference between electromotive force (EMF) and terminal potential difference (TPD) lies primarily in the conditions under which they are measured and their definitions:

1. **Electromotive Force (EMF):**
   - **Definition:** EMF is the maximum potential difference that a source can provide when no current is flowing through it. It represents the work done per unit charge by the source (like a battery or generator) to move charges from one terminal to the other.
   - **Measurement Condition:** EMF is measured when the circuit is open, i.e., no current is flowing.

2. **Terminal Potential Difference (TPD):**
   - **Definition:** TPD is the potential difference across the terminals of the source when current is flowing through the circuit. It represents the actual voltage available to the external circuit under load.
   - **Measurement Condition:** TPD is measured when the circuit is closed and current is flowing.

**Key Points:**
- When no current flows, EMF and TPD are equal.
- When current flows, TPD is typically less than EMF due to the internal resistance of the source. This difference is known as the voltage drop across the internal resistance.

The relationship can be summarized with the equation:
\[ \text{TPD} = \text{EMF} - I \cdot r \]
where \( I \) is the current and \( r \) is the internal resistance of the source.

Answer 2

The terms "emf" (electromotive force) and "terminal potential difference" are related but distinct concepts in electrical circuits:

1. **Emf (Electromotive Force):**
   - **Definition:** Emf is the maximum potential difference provided by a source like a battery or generator when no current is flowing through the circuit. It represents the work done by the source to move a unit charge from one terminal to the other.
   - **Source:** Emf is associated with the source of electrical energy, such as a battery, cell, or generator.
   - **Measurement:** Emf is measured in volts and can be thought of as the "ideal" voltage of the source.

2. **Terminal Potential Difference:**
   - **Definition:** Terminal potential difference is the actual voltage measured across the terminals of the source when a current is flowing. It is the potential difference available to the external circuit.
   - **Effect of Internal Resistance:** Terminal potential difference is less than the emf due to the internal resistance of the source. The formula for terminal potential difference \( V \) is given by \( V = \text{emf} - I \times r \), where \( I \) is the current and \( r \) is the internal resistance of the source.
   - **Measurement:** It reflects the real voltage available for the external circuit's use.

In summary, the emf is the intrinsic voltage of the source without any current, while the terminal potential difference is the voltage that is actually available when the circuit is active and accounts for the internal resistance of the source.