---
title: "Explain emf, potential difference and current"
subject: "Basic Electrical Engineering"
chapter: 1
key: "semester_1/basic_electrical_engineering:emf-potential-difference-current"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/emf-potential-difference-current
site: BCT Engineering
---

# Explain emf, potential difference and current

Basic Electrical Engineering · Chapter 1: General Electric System

- Asked in: 2068 Chaitra, 4 marks
- Video: [EMF & Internal Resistance - A-level Physics](https://www.youtube.com/watch?v=9r3Xgd79MFw) — Science Shorts
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/emf-potential-difference-current
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-1

## Question

Explain emf, potential difference and current with a circuit diagram.

## Short answer

EMF (E=W/Q) is energy per coulomb supplied by a source. Potential difference (V=W/Q, V=IR) is work per coulomb between two points. Current (I=Q/t) is rate of charge flow. In a circuit I=E/(R+r) and terminal voltage V=E−Ir=IR.

## Answer

#### EMF
**Electromotive force (E)** is the energy given to each coulomb of charge by a source while moving it round the complete circuit (volt). It is the cause of current. E = W/Q.

#### Potential difference
**Potential difference (V)** between two points is the work done in moving one coulomb of charge from one point to the other (volt). V = W/Q. Across a resistor, V = I R.

#### Current
**Current (I)** is the rate of flow of charge through a conductor: I = Q/t (ampere). 1 A = 1 C/s. In the external circuit, conventional current flows from the positive terminal of the source to its negative terminal (electrons move the opposite way).

```text
         I ->
   +-------------+
   |             |
 (E,r)          [R]  V = I R
   |             |
   +-------------+
```

In the circuit: I = E/(R + r), and the terminal voltage V = E − I r = I R. Example: E = 12 V, r = 1 Ω, R = 5 Ω gives I = 2 A, V = 10 V across R, and 2 V is lost inside the source.

## Check yourself

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

1. EMF is best described as:
   A. Energy per coulomb given by a source while moving charge round the complete circuit
   B. Energy used by a resistor
   C. Rate of flow of charge
   D. Opposition to current
   Answer: A. E = W/Q for the source and it is the cause of current. (Key point: EMF)

2. Current is:
   A. Energy per charge
   B. Rate of flow of charge, I = Q/t, with 1 A = 1 C/s
   C. Charge times time
   D. Voltage divided by charge
   Answer: B. One ampere is one coulomb per second. (Key point: Current)

3. In a circuit with EMF E, internal resistance r and load R, the current is:
   A. E/R
   B. E/r
   C. E/(R + r)
   D. E x (R + r)
   Answer: C. Both R and r are in series, so I = E/(R + r). (Key point: Circuit relation)

4. E = 12 V, r = 1 ohm, R = 5 ohm. What is the voltage across R?
   A. 12 V
   B. 2 V
   C. 6 V
   D. 10 V
   Answer: D. I = 12/6 = 2 A, so V = I R = 10 V; 2 V is lost inside the source. (Key point: Example)

## Related in Basic Electrical Engineering

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