Simply CBSE

Magnetic Effects of Electric Current — Class 10 Science

Revision notes for magnetic effects.

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📌 Key Points

  • Current-carrying wire creates concentric circular magnetic field. B = μ₀I/(2πr), inversely proportional to distance
  • Right-hand rule: Thumb = current direction, fingers curl = field direction around wire
  • Solenoid = coil on cylinder. Inside: Strong uniform field B = μ₀nI. Outside: Nearly zero. n = turns/unit length
  • Electromagnet: Solenoid with soft iron core. Temporary magnet, controllable strength, reversible polarity
  • Force on conductor: F = BIL sinθ. Maximum (F = BIL) when perpendicular to field (θ = 90°)
  • Fleming's left-hand rule: Thumb = force, Index = field (N→S), Middle = current. Used for motors
  • DC motor: Coil in magnetic field. Split-ring commutator reverses current every half rotation for continuous rotation
  • Split-ring commutator essential: Without it, coil oscillates; with it, continuous rotation because force stays in same direction
  • Faraday's law: ε = -N(dΦ/dt). Induced EMF proportional to turns and rate of flux change
  • Lenz's law: Induced current opposes flux change. Negative sign in Faraday's law represents this opposition
  • Magnetic flux: Φ = BA cosθ. Flux change maximum when θ changes fastest (perpendicular to field)
  • AC generator: Rotating coil in field produces sinusoidal EMF ε = ε₀ sin(ωt). Output AC voltage
  • Transformer: Vs/Vp = Ns/Np (ideal). Power conservation: Vp × Ip = Vs × Is
  • Step-up transformer: Ns > Np increases voltage, decreases current. Step-down: Ns < Np vice versa
  • Transformer works only with AC (changing flux). DC produces constant field, no induction
  • Ampere's circuital law: B = μ₀I/(2πr) for straight wire. Μ₀ = 4π × 10⁻⁷ (permeability of free space)
  • Solenoid field strength B = μ₀nI increases with: more turns (higher n), higher current (higher I)
  • Motor components: Coil (armature), permanent/electromagnet (field), commutator (switches current), brushes (deliver current), axle (rotation)
  • Generator EMF maximum ε₀ = NABω increases with more turns (N), larger area (A), stronger field (B), faster rotation (ω)
  • Energy conversion: Motor (electrical → mechanical). Generator (mechanical → electrical). Both use magnetic force/field interaction

📘 Important Definitions

Ampere's Circuital Law
Magnetic field around current-carrying wire B = μ₀I/(2πr). Field strength inversely proportional to distance.
Solenoid
Coil of wire wound tightly on cylinder. Creates uniform field inside (B = μ₀nI), nearly zero outside.
Electromagnet
Solenoid with soft iron core. Temporary magnet magnetizing when current flows. Strength/polarity controllable.
Force on Conductor
F = BIL sinθ where B = field, I = current, L = length, θ = angle. Maximum when perpendicular.
Fleming's Left-Hand Rule
For motor force: Thumb = force direction, Index = field (N to S), Middle = current.
Split-Ring Commutator
Two half-rings reversing current direction every half rotation, enabling continuous motor rotation.
Faraday's Law
ε = -N(dΦ/dt). Induced EMF equals negative of rate of flux change times number of turns.
Lenz's Law
Induced current direction always opposes change in magnetic flux. Ensures energy conservation.
AC Generator
Rotating coil in magnetic field produces sinusoidal alternating voltage ε = ε₀ sin(ωt).
Transformer
Device transferring AC power between circuits with voltage ratio Vs/Vp = Ns/Np. Works only with AC.

⚠️ Common Mistakes

✗ Wrong: Magnetic field increases with distance from current-carrying wire.

✓ Correct: Field decreases with distance. B ∝ 1/r (inversely proportional).

✗ Wrong: Solenoid has same strong field outside as inside.

✓ Correct: Solenoid field mostly confined inside. Field nearly zero outside.

✗ Wrong: DC motor works fine without split-ring commutator.

✓ Correct: Without commutator, coil oscillates back-and-forth, not rotates. Commutator reverses current every half turn.

✗ Wrong: Force on conductor always F = BIL.

✓ Correct: F = BIL sinθ. Formula correct only when conductor perpendicular to field (θ = 90°). General formula needed.

✗ Wrong: Lenz's law and Faraday's law are same thing.

✓ Correct: Faraday gives magnitude ε = N(dΦ/dt). Lenz gives direction (negative sign means opposition).

✗ Wrong: Transformer increases power (step-up transformer).

✓ Correct: Transformer transfers power, doesn't create it. Step-up increases voltage, decreases current (power same).

📝 Exam Focus

These questions are frequently asked in CBSE exams:

Derive B = μ₀I/(2πr) for straight wire. Explain right-hand rule.
5m
Draw DC motor diagram. Explain role of split-ring commutator in continuous rotation.
5m
Explain Faraday's law and Lenz's law with applications.
5m
Derive transformer equation Vs/Vp = Ns/Np. Explain step-up and step-down.
5m
Compare AC generator and transformer working principles.
5m
Calculate force on conductor using F = BIL sinθ with given values.
3m
Explain why transformer doesn't work with DC current.
3m
Describe electromagnet. How does it differ from permanent magnet?
3m

🎯 Last-Minute Recall

Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!