Understanding EMF and Potential Difference

The question asks us to identify the statement that is NOT a true difference between Electromotive Force (EMF) and Potential Difference (PD). Let's examine each option to understand the core concepts.

EMF is the maximum potential difference between two terminals of a source (like a battery or generator) when no current is flowing through it. It represents the energy supplied by the source per unit charge to drive the charge around the circuit.

Potential Difference (PD), also known as voltage, is the difference in electric potential between two points in a circuit. It represents the energy converted or dissipated as a charge moves between these two points, typically across a component like a resistor.

Analyzing the Differences between EMF and PD

Let's look at each given statement:

• Statement 1: "EMF refers to source of electrical energy, while PD exists between any points in a circuit."

  This statement is true. EMF is associated with the source that provides energy (e.g., battery's EMF). PD is the potential difference observed across any two points, which could be across a resistor, a wire segment, or even the terminals of the source when current is flowing.

• Statement 2: "EMF is measured in an open circuit, whereas PD is measured in a closed circuit."

  This statement is generally true. EMF is ideally measured when no current is drawn from the source, which happens in an open circuit. PD is measured across components in a closed circuit where current is flowing, resulting in voltage drops.

• Statement 3: "PD is a cause of current flow, while EMF is the effect of current flow."

  This statement is NOT true. In a circuit, the EMF provided by the source is the driving force, the cause, that tends to push charges around the circuit. The flow of current through components with resistance results in a potential difference (PD) across those components (according to Ohm's Law, $\text{PD} = \text{Current} \times \text{Resistance}$). Therefore, EMF is the cause of current flow (and subsequent PDs), and PDs are the effects (voltage drops) resulting from current flow across resistance.

• Statement 4: "PD is less than the EMF in the same circuit, whereas EMF is greater than the PD in the same circuit."

  This statement is generally true for a closed circuit with a real source having some internal resistance. When current flows, there is a voltage drop across the internal resistance of the source. The PD across the external circuit is equal to the EMF minus the voltage drop across the internal resistance ($V_{external} = \text{EMF} - I \times r$, where $r$ is internal resistance). Thus, the PD across the external circuit is typically less than the EMF of the source, and consequently, the EMF is greater than the PD across the external circuit.

Conclusion

Based on the analysis, Statement 3 presents an incorrect relationship between PD and EMF. It incorrectly states that PD causes current flow and EMF is the effect, when the opposite is true: EMF causes current flow, which leads to PDs across resistances.

Therefore, the statement that is NOT a true difference between EMF and potential difference is:

"PD is a cause of current flow, while EMF is the effect of current flow."