The energy needed to move a unit positive charge around a complete electric circuit is called the

Analysis

The question asks for the specific term used to describe the total energy required to move a unit positive charge through an entire electric circuit (including the internal resistance of the source).

Concept Explanation

In physics, we distinguish between two similar terms:

1. Potential Difference (p.d.): This is the work done in moving a unit charge between two specific points in a circuit.
2. Electromotive Force (e.m.f.): This is the total energy supplied by a source (like a battery or cell) to drive a unit charge around the complete circuit.

Think of the e.m.f. as the "push" the battery gives to the charge to complete the full loop, whereas potential difference is the "drop" in energy as the charge passes through a specific component (like a resistor or bulb).

Step-by-Step Solution

1. Identify the definition: The key phrase here is "around a complete electric circuit."
2. Evaluate the options: • Electric potential difference refers to the energy change between two points, not the whole circuit. • Electric energy is a general term for the capacity to do work, not the specific definition of the source's driving force. • Kinetic energy relates to the motion of particles, not the electrical potential energy provided by a source. • Electromotive force (e.m.f.) is defined precisely as the energy per unit charge for the entire circuit.
3. Conclusion: Since the question specifies the energy to move a charge around the complete circuit, it refers to the e.m.f.

Exam Tip

A simple way to remember this is: e.m.f. is for the Entire circuit, while p.d. is for a Part of the circuit.

The correct option is A.