---
title: "Faraday's laws; Fleming's rules"
subject: "Electrical Machine"
chapter: 1
key: "semester_4/electrical_machine:faraday-fleming-rules"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/faraday-fleming-rules
site: BCT Engineering
---

# Faraday's laws; Fleming's rules

Electrical Machine · Chapter 1: Magnetic Circuits and Induction

- Asked in: 2074 Bhadra, 3 marks
- Video: [Lecture-3 || Faraday's Laws 1&2 || Electrical Machines-I](https://www.youtube.com/watch?v=td31OTCvalo) — Pranni Tech Edu
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/faraday-fleming-rules
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-1

## Question

Explain Faraday's laws of electromagnetic induction.

## Short answer

Faraday's first law: changing flux linkage induces emf. Second law: emf magnitude is -N dΦ/dt. Lenz's law gives direction opposing flux change.

## Answer

Faraday's laws describe how an emf is induced in a circuit when the magnetic flux linked with it changes.

#### First law
Whenever the magnetic flux linked with a circuit changes, an emf is induced in it. The emf lasts only as long as the flux is changing. (Flux can change by moving the conductor or magnet, or by varying the current producing the flux.)

#### Second law
The magnitude of the induced emf is equal to the rate of change of flux linkage.

```text
e = − N dφ/dt     (volts)
```
- *N* = number of turns, φ = flux in webers.
- The negative sign is Lenz's law: the induced emf drives a current whose magnetic effect opposes the change producing it. This is a consequence of conservation of energy.

**Example:** a coil of 100 turns in which the flux changes from 0 to 5 mWb in 0.1 s has an average emf of 100 × 0.005/0.1 = 5 V.

## Check yourself

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

1. What does Faraday's first law state?
   A. Induced emf is always constant
   B. An emf is induced whenever the flux linked with a circuit changes
   C. Emf is induced even with steady flux
   D. Emf is induced only in a straight conductor
   Answer: B. Any change in linked flux induces an emf, which lasts only as long as the flux changes. (Key point: First law)

2. What does the second law give for the magnitude of induced emf?
   A. The number of turns only
   B. The rate of change of flux linkage, N dphi/dt
   C. Flux times resistance
   D. The flux itself
   Answer: B. e = N dphi/dt in magnitude. (Key point: Second law)

3. What does the negative sign in e = -N dphi/dt mean?
   A. The emf is always negative
   B. The induced emf drives a current opposing the change that produced it
   C. Energy is created
   D. The flux is decreasing
   Answer: B. Lenz's law: induced effects oppose the cause, consistent with conservation of energy. (Key point: Lenz's law)

4. A 100-turn coil has flux changing 0 to 5 mWb in 0.1 s. What is the average emf?
   A. 0.5 V
   B. 50 V
   C. 5 V
   D. 500 V
   Answer: C. e = 100 x 0.005/0.1 = 5 V. (Key point: Numerical example)

## Related in Electrical Machine

- [Faraday's laws; static and dynamic emf](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Afaraday-static-dynamic-emf) — Electrical Machine, chapter 1
- [Soft magnetic material, Faraday's laws](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Asoft-magnetic-faraday-laws) — Electrical Machines, chapter 1
- [Faraday laws; static vs dynamic emf](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Afaraday-static-dynamic-emf) — Electrical Machines, chapter 1
- [Fleming's right and left hand rules](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Afleming-hand-rules) — Electrical Machine, chapter 1

## Related in other subjects

- [Faraday's laws; mechanical to heat energy](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Afaraday-law-mechanical-to-heat) — Engineering Physics, chapter 8
- [Electromagnetic induction in neighbouring facilities](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aelectromagnetic-induction-neighbouring) — High Voltage Engineering, chapter 5