Magnetism & Electromagnetic Induction
Biot-Savart law, Ampere's law, Faraday's law, AC circuits, and transformers
Key Concepts
- A moving charge in a magnetic field feels F = qv × B (perpendicular force)
- Biot–Savart and Ampere's laws give the field due to currents
- Faraday's law: a changing flux induces an emf (Lenz's law sets its direction)
- Inductance opposes changes in current; self and mutual inductance
- AC circuits: reactance, impedance and resonance
Important Formulae
| Force on a charge | F = qvB sinθ |
| Field of a long wire | B = μ₀I/(2πr) |
| Field at loop centre | B = μ₀I/(2R) |
| Induced / motional emf | emf = −dΦ/dt; emf = BLv |
| Reactances | X_L = ωL; X_C = 1/(ωC) |
| Resonance frequency | ω = 1/√(LC) |
Quick Tips
- Lenz's law is a statement of energy conservation — induced current opposes the change.
- At resonance X_L = X_C, impedance is minimum and current is maximum.
- In an ideal transformer V ∝ N (turns ratio).
Sample Practice Questions
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At resonance in a series LCR circuit, the impedance is:
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Answer: Minimum
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The SI unit of magnetic flux is the:
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Answer: weber
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The magnetic force on a charge moving parallel to a magnetic field is:
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Answer: Zero
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Faraday's law of electromagnetic induction relates the induced emf to the:
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Answer: Rate of change of magnetic flux
Practice Questions
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What are the key concepts in Magnetism & Electromagnetic Induction for JEE?
A moving charge in a magnetic field feels F = qv × B (perpendicular force); Biot–Savart and Ampere's laws give the field due to currents; Faraday's law: a changing flux induces an emf (Lenz's law sets its direction); Inductance opposes changes in current; self and mutual inductance; AC circuits: reactance, impedance and resonance.
Which formulae are important for Magnetism & Electromagnetic Induction in JEE?
Force on a charge: F = qvB sinθ; Field of a long wire: B = μ₀I/(2πr); Field at loop centre: B = μ₀I/(2R); Induced / motional emf: emf = −dΦ/dt; emf = BLv; Reactances: X_L = ωL; X_C = 1/(ωC); Resonance frequency: ω = 1/√(LC).
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