Physics · Electromagnetic Induction · NEET
They proved that a changing magnetic flux through a coil produces (induces) an emf and hence a current. Oersted had shown current makes a magnetic field; Faraday and Henry showed the reverse: a changing magnetic field can make a current. The key word is CHANGING — the flux must vary with time. This is the whole foundation of Faraday's law that comes next.
A bar magnet is pushed toward coil C1 connected to a galvanometer. The needle deflects only WHILE the magnet moves, because only then is the flux through C1 changing. Hold the magnet still and the flux is constant, so the needle reads zero. Pull the magnet away and the needle deflects the OTHER way (flux now decreasing). Faster motion gives a bigger deflection — more flux change per second.
Coil C2 carries a steady current (so it acts like a magnet) and is moved toward or away from coil C1. C1's galvanometer deflects only while C2 is moving. It also works if C1 moves and C2 is fixed. This proved it is the RELATIVE MOTION between the two coils — not which one moves — that changes the flux and drives the induced current.
Here both coils C1 and C2 are held still. C2 has a battery and a tapping key K. Pressing K (current rises) gives a momentary deflection; holding K (steady current) gives nothing; releasing K (current falls) gives a deflection the opposite way. This proved relative motion is NOT required — a changing current in C2 changes the flux through C1, and the change alone induces the emf.
Soft iron is a strong magnetic material. Placing it inside the coils concentrates and greatly increases the magnetic field, so the flux through C1 is larger. A larger flux change per second means a larger induced emf and a bigger galvanometer deflection. This is why real transformers and inductors use iron cores.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Michael Faraday (England) and Joseph Henry (USA) independently discovered electromagnetic induction around 1831. Faraday published first and also found the mathematical law; Henry discovered self-induction. The SI unit of inductance, the henry (H), is named after Joseph Henry.
The experiments themselves are usually tested as concept or assertion-reason questions (for example, 'a stationary magnet induces no current'). The numericals come from the law that follows — Faraday's law, emf = N dphi/dt — so understanding these experiments makes those calculations obvious.
An emf is induced in a coil whenever the magnetic flux through it changes with time — whether the change comes from a moving magnet, a moving coil, or a switching current.
A steady current makes a steady magnetic field, so the flux through C1 stays constant. With no rate of change of flux, the induced emf is zero. Only pressing or releasing the key (rising or falling current) changes the flux.
