Magnetic flux and Faraday–Lenz induction
Induction responds to flux change, not simply to the presence of a magnetic field.
Key knowledge
Magnetic flux through an oriented surface is Φ_B=∫B·n dA. For a fixed conducting loop, Faraday’s law gives induced electric emf ∮E·dl=−dΦ_B/dt with surface and boundary orientations related by the right-hand rule. N identical fixed turns linking the same flux give ε=−N dΦ_B/dt. The minus sign sets the circulation; any resulting current depends on the circuit response.
Values and equations
Φ_B=∫_S B·n dA; ε=∮_∂S E·dl=−dΦ_B/dt; ε_N=−N dΦ_B/dt
- Φ_B
- Oriented magnetic flux through S Wb
- B
- Magnetic flux density T
- n
- Oriented surface unit normal 1
- A
- Area m²
- E
- Electric field V m⁻¹
- dl
- Oriented path displacement m
- ε,ε_N
- One-loop and N-turn induced emf V
- N
- Number of identical turns 1
- t
- Time s
- S,∂S
- Fixed spanning surface and its right-hand-oriented boundary
Conditions and scope
- Classical smooth fields and fixed loop geometry for the displayed electric-loop integral; use a spanning surface with compatible orientation.
- Moving circuits additionally require the motional term in the emf model. Flux is a surface integral, not a literal count of field lines. Lenz’s opposition concerns flux change; it does not say an induced field always opposes the existing field.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Samuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)2 sources
- University Physics Volume 2, §13.1: faradays lawSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
- University Physics Volume 2, §13.2: lenzs lawSamuel J. Ling, Jeff Sanny and William Moebs — OpenStax, Rice University, University Physics (2016)Textbook chapter
Evidence and review
Read §13.1 flux/induction13.1–13.2, common N-turn flux13.3 and §13.2 Lenz flux-change direction. Fixed-loop integral is a scoped specialization of the existing Maxwell equation; no assertion that loop emf is an electrostatic endpoint voltage.
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