Electromagnetic induction (Class 12): Faraday, Lenz and the AC generator
A changing magnetic flux creates an emf. That one idea runs every power station, transformer and induction cooker. Move a magnet through a coil, slide a rod on rails and watch the induced current and its direction.
NCERT Class 12 Physics, Chapter 6 (Electromagnetic Induction). Every lab below runs in 3D in your browser or on a phone, in English and Hindi.
Try the labs
Faraday's experiments and Lenz's law · plan · Chapter 6
To demonstrate electromagnetic induction (Faraday's experiments), verify Lenz's law and study how induced emf depends on the rate of change of flux.
Lenz's law: a magnet falling through a coil · plan · Chapter 6
To verify Faraday law and Lenz law with a magnet falling through a coil and to see magnetic braking.
Motional emf on a rod in magnetic field · plan · Chapter 6
To study motional emf, Lenz's law and the energy balance for a rod moving in a magnetic field.
AC generator · plan · Chapter 6
To understand the principle of the AC generator: a coil rotating in a uniform magnetic field produces a sinusoidal emf.
Eddy currents and magnetic braking · plan · Chapter 6
To demonstrate eddy currents and magnetic braking (Lenz's law) using a magnet falling through a conducting tube.
LR circuit: current growth, decay and back emf · plan · Chapter 6
To study the growth and decay of current in an LR circuit and understand back emf and the inductive time constant τ = L/R.
RC circuit: charging, discharging and time constant · plan · Chapter 6
To study the charging and discharging of a capacitor through a resistor and understand the time constant τ = RC.
EMF of a rotating rod and a Faraday disc · plan · Chapter 6
To find the emf induced in a rod rotating in a magnetic field (and in a Faraday disc) using motional emf.
Self and mutual inductance, L-R growth · plan · Chapter 6
To study self-inductance, mutual inductance, the growth of current in an L-R circuit and the energy stored in an inductor.
Faraday's and Lenz's laws
The induced emf equals the rate of change of magnetic flux through the circuit, ε = −dΦ/dt. The minus sign is Lenz's law: the induced current opposes the change that produces it, which is conservation of energy.
Φ = B A cos θε = −N dΦ/dt
Motional emf and eddy currents
A rod of length l moving at speed v across a field B has an emf Blv across its ends. Pulling it needs a force B²l²v/R, and the work done appears as heat in the circuit.
In a solid conductor the changing flux drives circulating eddy currents. They slow a magnet falling in a copper tube and heat the pan of an induction cooker.
ε = B l vF = B² l² v / Rε = ½ B ω l² (rotating rod)
Inductance and the generator
A coil opposes a change of its own current with an emf L dI/dt. In an LR circuit the current grows as I = I₀(1 − e^(−t/τ)) with τ = L/R. The energy stored is ½LI².
A coil rotating in a field gives e = NBAω sin ωt, the alternating emf of the AC generator.
ε = −L dI/dtτ = L / RU = ½ L I²e = NBAω sin ωt
Frequently asked questions
Why is there a minus sign in Faraday's law?
It stands for Lenz's law. If the induced current helped the change, the flux would grow without limit and energy would be created from nothing.
Why does a magnet fall slowly through a copper tube?
Eddy currents in the tube oppose the motion of the magnet (Lenz's law), so the magnet reaches a small terminal speed.
What is the difference between self and mutual inductance?
Self inductance links the coil's own flux with its own current; mutual inductance links the flux of one coil with the current of a neighbouring coil, as in a transformer.
How can eddy currents be reduced?
By laminating the core, that is, using thin insulated sheets, which cuts the area of the loops.