D.4
Electromagnetic Induction
Magnetic flux, Faraday's law, Lenz's law, induced EMF in moving conductors and rotating coils, and AC generators. HL only.
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D.4 Electromagnetic Induction
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Key Concepts, Electromagnetic Induction
Magnetic Flux and Flux Linkage
Magnetic flux Φ is a measure of the total magnetic field passing through a given area: Φ = BA cosθ, where B is the magnetic flux density in Tesla (T), A is the area in m² and θ is the angle between the field and the normal to the surface. When θ = 0° (field perpendicular to the surface), flux is maximum; when θ = 90° (field parallel to the surface), flux is zero. Magnetic flux is measured in Webers (Wb), where 1 Wb = 1 T m². For a coil of N turns, the magnetic flux linkage is NΦ. It is flux linkage (not flux alone) that determines the induced EMF.
Faraday's Law
Faraday's law states that the magnitude of the induced EMF in a circuit equals the rate of change of magnetic flux linkage: ε = NΔΦ/Δt. The negative sign (Lenz's law) indicates that the induced EMF opposes the change that caused it. Flux can change because B changes, A changes, or the angle θ between them changes. A larger rate of change of flux produces a larger EMF. All three scenarios are assessed in IB.
Lenz's Law and Conservation of Energy
Lenz's law states that the direction of any induced current is such that its magnetic effect opposes the change in flux that caused it. This is a consequence of conservation of energy: if the induced current reinforced the change rather than opposing it, we would get an ever-increasing current for free, violating energy conservation. In practice, you must do work against the opposing force to move a conductor in a magnetic field, and that work becomes electrical energy in the induced current.
EMF in a Moving Conductor
When a straight conductor of length L moves with velocity v perpendicular to a uniform magnetic field B, free charges in the conductor experience a force F = qvB. This separates positive and negative charges, creating a potential difference between the ends. The induced EMF is ε = BvL. This is a direct application of Faraday's law: the conductor sweeps out an area Lvt in time t, so the rate of change of flux is BLv. The direction of the induced current is found using the right-hand rule or by considering the force on positive charges.
The AC Generator
An AC generator converts kinetic energy into electrical energy by rotating a coil in a magnetic field. As the coil rotates, the magnetic flux linkage changes sinusoidally: NΦ = NBA cosωt. By Faraday's law, the induced EMF is ε = NBAω sinωt, with peak EMF ε₀ = NBAω. The EMF is maximum when the coil is parallel to the field (rate of change of flux is greatest) and zero when perpendicular (flux is maximum but momentarily not changing). Increasing rotation frequency, number of turns, coil area or field strength all increase the peak EMF.
Fixed Coils in Changing Magnetic Fields
An EMF is also induced in a stationary coil if the magnetic field through it changes over time. This is the principle behind transformers. In a transformer, an alternating current in the primary coil creates a changing magnetic field, inducing an alternating EMF in the secondary coil. The ratio of voltages equals the ratio of turns: V_s/V_p = N_s/N_p. In an ideal transformer, power is conserved: V_pI_p = V_sI_s. The key insight is that it is the rate of change of flux linkage, not the magnitude of the field, that drives the induced EMF.
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Frequently Asked Questions, IB Physics Electromagnetic Induction
What is Electromagnetic Induction in IB Physics? ↓
Magnetic flux, Faraday's law, Lenz's law, induced EMF in moving conductors and rotating coils, and AC generators. HL only. This topic is part of Theme D (Fields) in the current IB Physics syllabus.
Is Electromagnetic Induction SL or HL in IB Physics? ↓
Electromagnetic Induction is an HL-only topic. It is not assessed in the SL IB Physics exam.
What equations do I need for IB Physics Electromagnetic Induction? ↓
The key equations for Electromagnetic Induction are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Electromagnetic Induction? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Electromagnetic Induction for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.