---
title: "Potential difference versus emf"
subject: "Basic Electrical Engineering"
chapter: 1
key: "semester_1/basic_electrical_engineering:potential-difference-vs-emf"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/potential-difference-vs-emf
site: BCT Engineering
---

# Potential difference versus emf

Basic Electrical Engineering · Chapter 1: General Electric System

- Asked in: 2071 Shrawan, 4 marks
- Video: [Voltage or Potential difference vs EMF | Easiest Explanation | TheElectricalGuy](https://www.youtube.com/watch?v=QTW5LcEaNkE) — Gaurav J - TheElectricalGuy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/potential-difference-vs-emf
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-1

## Question

What is the difference between the potential difference and electromotive force?

## Short answer

EMF is energy supplied by source per coulomb, exists without current; potential difference is energy used per coulomb between two points, exists when current flows. In a source, V = E − I·r. Unit is volt.

## Answer

| Point | EMF (E) | Potential difference (V) |
|---|---|---|
| Meaning | Energy given by the source to each coulomb of charge | Energy used by each coulomb between two points |
| Where it exists | Source (cell, generator) | Across any element or between any two points |
| Cause | Chemical, magnetic or other energy conversion | Current flowing through a resistance |
| Circuit condition | Exists even when no current flows | Exists only when current flows (or charge is separated) |
| Value | Greater than or equal to the terminal voltage | Less than EMF in a source with load |
| Relation | E = V + I·r (r = internal resistance) | V = E − I·r, or V = I·R |
| Unit | Volt | Volt |

EMF is the cause that drives current; potential difference is its effect across a circuit element. When a source supplies no current, its terminal potential difference equals its EMF.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which statement distinguishes EMF from potential difference?
   A. EMF is energy given by the source per coulomb; PD is energy used per coulomb between two points
   B. EMF is energy used per coulomb; PD is energy given by the source
   C. They are the same in every circuit
   D. EMF is measured in volts and PD in amperes
   Answer: A. EMF is energy supplied by the source per unit charge, PD is energy used between two points per unit charge. (Key point: Meaning)

2. Which is true when no current flows?
   A. EMF is zero
   B. EMF exists, and the terminal PD of the source equals the EMF
   C. PD across a resistor is maximum
   D. EMF equals I*r
   Answer: B. EMF exists independent of current, and with no current there is no internal drop so terminal voltage equals EMF. (Key point: No current condition)

3. For a loaded source with internal resistance r, which is correct?
   A. V = E + I r
   B. E = V - I r
   C. V = E - I r, so V is less than E
   D. V = E / (I r)
   Answer: C. The internal drop I r reduces the terminal PD: V = E - I r. (Key point: Relation)

## Related in Basic Electrical Engineering

- [Potential difference, emf, resistivity](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Aemf-pd-resistance-resistivity) — Basic Electrical Engineering, chapter 1
- [Explain emf, potential difference and current](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Aemf-potential-difference-current) — Basic Electrical Engineering, chapter 1

## Related in other subjects

- [Electrode potential; Cu/Ag cell EMF](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aelectrode-potential-cell-emf) — Engineering Chemistry, chapter 1
- [Non-standard electrode potential; Fe/Ag cell emf](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Anonstandard-emf-fe-ag) — Engineering Chemistry, chapter 1
- [Single electrode potential; Ni/Ag cell EMF](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ani-ag-cell) — Engineering Chemistry, chapter 1
- [Standard electrode potential; Daniell cell EMF](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adaniell-cell-emf) — Engineering Chemistry, chapter 1