Electromagnetic Induction
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Magnetic Flux and Faraday's Law
Part of: Flux, Faraday's Law and Lenz's Direction
Flux is field times area times cosine of the tilt, counting field lines through a surface. Whenever this flux changes, an emf equal to turns times the rate of change appears; the minus sign records that nature opposes the change.
Why it matters. Transformers, generators and induction stoves all run on changing flux. Missing the cosine, or thinking steady flux induces emf, breaks every induction calculation.
Formulas
- Flux: Phi = B A cos(theta)
- Faraday: e = -N dPhi/dt
- Unit: 1 Wb = 1 T m^2
Key words
- Magnetic flux
- : Field times area times cosine of tilt, measured in webers.
- Weber
- : Unit of flux equal to one tesla times one square metre.
- Faraday's law
- : Induced emf equals minus turns times rate of flux change.
Try it in two minutes
- Multiply field, area and cosine of tilt for the flux.
- Divide flux change by time and multiply by turns.
- Check that faster change means bigger emf.
Try at home. Tilt a loop in thought and predict when flux peaks and vanishes.
Lenz's Law and Induced Direction
Part of: Flux, Faraday's Law and Lenz's Direction
Lenz's law says induced current flows so its own field fights the change: an approaching north pole meets an induced north pole, while a withdrawing one meets an induced south pole. This opposition is energy conservation at work, since the pushing hand supplies the electrical energy.
Why it matters. Motor back-emf, magnetic braking and transformer dots all follow this rule. Saying induction opposes the field instead of the change flips every direction answer.
Formulas
- Lenz: induced field opposes the CHANGE in flux
- Work by hand = electrical energy produced
Key words
- Lenz's law
- : Induced effects oppose the change in flux that causes them.
- Induced current
- : Current driven by changing flux rather than by a battery.
Try it in two minutes
- Ask whether flux through the loop grows or shrinks.
- Pick the induced field that fights that change.
- Curl fingers to that field to read the current.
Try at home. Predict the needle kick when a magnet enters versus leaves a coil.
Motional EMF
Part of: Motional EMF
Charges in a moving conductor feel a magnetic force that piles plus at one end and minus at the other, giving emf length times speed times field for perpendicular motion. A rod spinning about one end develops half B omega l squared, and even an unconnected cut shows this voltage while it crosses the field.
Why it matters. Generators, maglev trains and Earth's-field effects all use motional emf. Forgetting the perpendicular requirement, or the half in the spinning rod, doubles or zeroes answers wrongly.
Formulas
- Sliding rod: e = B l v
- Pivoted spinning rod: e = B w l^2 / 2
- Direction by Lorentz force on positive charges
Key words
- Motional emf
- : Voltage across a conductor from motion through a magnetic field.
- Lorentz separation
- : Charge piling at conductor ends by magnetic force on moving charges.
Try it in two minutes
- Check that motion is across the field lines.
- Multiply length, speed and field for a sliding rod.
- Halve B omega l squared for a rod pivoted at its end.
Try at home. Predict which wing tip of a turning plane charges positive in Earth's field.
Inductance and AC Generator
Part of: Inductance and AC Generator
Self-inductance measures how strongly a coil fights current change, with emf minus L times rate of change and stored energy half L I squared. Mutual inductance does the same between two coils. A coil spun in a uniform field produces sinusoidal emf with peak NBA omega, sent out through slip rings.
Why it matters. Chokes, transformers and mains power all depend on these. Mixing L with N squared wrongly, or expecting DC from slip rings, breaks designs.
Formulas
- Self emf: e = -L dI/dt
- Mutual emf: e = -M dI/dt
- Stored energy: U = L I^2 / 2
- Generator peak: e0 = N B A w
Key words
- Self-inductance
- : Flux linked per unit current in the same coil, in henries.
- Mutual inductance
- : Flux linked in one coil per unit current in another.
- Slip rings
- : Contacts that carry alternating current out of a spinning coil.
- Back emf
- : Self-induced emf opposing the change driving it.
Try it in two minutes
- Divide induced emf by current rate for L.
- Multiply half L by current squared for stored energy.
- Multiply turns, field, area and spin rate for peak emf.
Try at home. Explain why a tubelight choke delays the current surge at switch-on.
What your child will practise
- Inductance and AC Generator: Desert Quick Fire (Quick Fire)
- Lenz's Law and Induced Direction: Jungle Puzzle (Clue Puzzle)
- Lenz's Law and Induced Direction: Space Quick Fire (Quick Fire)
- Magnetic Flux and Faraday's Law: Ocean Quick Fire (Quick Fire)
- Motional EMF: Desert Puzzle (Clue Puzzle)
- Quiz: Inductance and AC Generator (8 questions)
- Quiz: Lenz's Law and Induced Direction (8 questions)
- Quiz: Magnetic Flux and Faraday's Law (8 questions)
- Quiz: Motional EMF (8 questions)