**Potential Difference and Internal Resistance**

The potential difference, also known as voltage, is a measure of the electric potential energy difference between two points in an electrical circuit. In the case of a cell, it is the difference in electric potential energy per unit charge between the positive and negative terminals. In other words, it is the driving force that causes charges to flow through the circuit.

The internal resistance of a cell represents the resistance offered by the cell itself to the flow of current. It is caused by factors such as the resistance of the electrolyte and the resistance of the cell's internal components. This internal resistance affects the actual potential difference that can be measured across the terminals of the cell when it is connected to an external load.

**Terminal Potential Difference and Internal Resistance Relationship**

When a cell is connected to a resistance equal to its internal resistance, the potential difference across its terminals is reduced. This can be explained by Ohm's Law, which states that the potential difference across a resistor is directly proportional to the current flowing through it and inversely proportional to its resistance.

When the internal resistance of a cell is equal to the resistance of the external load, the total resistance in the circuit is doubled. According to Ohm's Law, if the resistance increases, the potential difference across the terminals will decrease. Therefore, the terminal potential difference will be less than the emf (electromotive force) of the cell.

**Calculating the EMF of the Cell**

To calculate the emf of the cell, we can use the equation:

emf = terminal potential difference + (current × internal resistance)

Since the terminal potential difference is given as 3V when connected to a resistance equal to its internal resistance, we can substitute these values into the equation:

emf = 3V + (current × internal resistance)

Since the internal resistance is equal to the resistance of the external load, the total resistance in the circuit is doubled. Therefore, we can rewrite the equation as:

emf = 3V + (current × 2 × internal resistance)

To determine the emf of the cell, you would need additional information such as the current flowing through the circuit or the internal resistance of the cell. Without this information, it is not possible to calculate the exact value of the emf. However, by understanding the relationship between potential difference, internal resistance, and external load resistance, you can see how the terminal potential difference is affected when the internal resistance is equal to the resistance of the load.