Theory — Electromagnetic Induction
Faraday's Law of Induction
A changing magnetic flux through a coil induces an electromotive force (EMF). The faster the flux changes, the larger the induced EMF and current. This is the fundamental principle behind generators, transformers, and electric motors.
Where:
N = number of turns in the coil
ΔΦ = change in magnetic flux (Weber, Wb)
Δt = time over which flux changes
Lenz's Law — Direction of Induced Current
The induced current flows in a direction such that its own magnetic field opposes the change in flux that caused it. This is why reversing the magnet reverses the current direction.
North pole withdrawing → induced current creates South pole facing magnet (attracts)
Experiment 1 — Bar Magnet + Coil
Moving the bar magnet into or out of the coil changes the magnetic flux through the coil. This induces a current pulse detected by the galvanometer. Faster motion = larger pulse. Passing the magnet through the center produces two pulses — one as it enters (positive) and one as it exits (negative).
Experiment 2 — AC Electromagnet + Rings
A 60 Hz AC electromagnet reverses its field 60 times per second. This induces eddy currents in a conducting aluminum ring. The solid ring experiences strong eddy currents that heat it up and create a repulsive force. The split ring breaks the current path — no significant eddy current, no heating, no force.
Eddy Currents and the Split Ring
Eddy currents are circular currents induced in a solid conductor by a changing magnetic field. They flow in closed loops within the material and dissipate energy as heat. A split in the ring breaks the circular path, preventing eddy currents from forming. This is why power transformer cores are made of thin laminated sheets — each sheet is insulated from the next, limiting eddy current paths and reducing energy loss.
Instructions — Running the Virtual Experiments
Experiment 1 — Bar Magnet and Coil
Experiment 2 — AC Electromagnet and Aluminum Rings
Simulation — Magnetic Induction Experiments
Magnet Controls
Motions
Coil Properties
More turns → larger induced EMF for same flux change (Faraday's Law: EMF = N×ΔΦ/Δt)
Electromagnet
Ring Selection
Observations
Observations Table
Record your observations for each motion and ring condition below.
| Action | Galvanometer Response | Your Observation / Explanation |
|---|---|---|
| Magnet moving into coil (fast) | Large positive pulse | |
| Magnet moving out of coil | Negative pulse | |
| Magnet held still inside coil | Zero current | |
| Magnet passed through center | Two pulses — opposite sign | |
| Magnet reversed, moved in | Negative pulse (reversed) | |
| Solid ring in AC field | Repelled + heats up | |
| Split ring in AC field | No repulsion, no heat |
Team Questions
Example Lab Report
Magnetic Induction
Physics SC-172 | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Objective
To explore electromagnetic induction using a bar magnet and coil, and to investigate the behavior of solid and split aluminum rings in an alternating magnetic field. No calculations are required — all conclusions are based on observation and theoretical explanation.
Theory Summary
Faraday's Law states that a changing magnetic flux through a coil induces an EMF proportional to the rate of flux change. Lenz's Law states the induced current opposes the change causing it. Eddy currents are induced in solid conductors by changing fields; a gap in the conductor breaks the current path.
Observations — Experiment 1: Bar Magnet and Coil
| Motion | Galvanometer Response | Explanation |
|---|---|---|
| Magnet moving in (fast) | Large positive deflection | Rapidly increasing flux → large EMF (Faraday). Fast = large ΔΦ/Δt. |
| Magnet moving out | Negative deflection | Decreasing flux → EMF reverses. Lenz's Law: current opposes the withdrawal. |
| Magnet held still | Zero — no deflection | No change in flux → no induced EMF. ΔΦ/Δt = 0. |
| Magnet passed through | Positive then negative pulse | Entry increases flux (positive pulse); exit decreases flux (negative pulse). |
| Magnet reversed, moved in | Negative deflection (reversed) | N/S reversed → flux change in opposite direction → current reverses. |
| Coil moved (not magnet) | Same as magnet moving | Only relative motion matters. Physics is the same regardless of which moves. |
Observations — Experiment 2: AC Electromagnet and Rings
Solid ring: When the AC electromagnet was activated, the solid aluminum ring was visibly repelled upward and became warm to the touch. The ring is not magnetic — the force is entirely due to electromagnetically induced eddy currents creating an opposing magnetic field (Lenz's Law).
Split ring: The split ring showed no repulsion and no heating in the same field. The split breaks the circular path for eddy currents — without a complete circuit, no significant current flows, no opposing field is created, and no energy is dissipated as heat.
Discussion — Key Question
The force on the solid ring arises through three connected principles: (1) the 60 Hz AC field changes direction 60 times per second, creating a continuously changing magnetic flux through the ring; (2) by Faraday's Law, this changing flux induces an EMF and therefore eddy currents in the conducting ring; (3) by Lenz's Law, these currents create a magnetic field that opposes the inducing field — the ring's field and the electromagnet's field always point in opposite directions at the ring's location, producing repulsion. The heating is a consequence of I²R energy dissipation as the eddy currents flow through the aluminum's resistance.
Conclusion
All observations were fully consistent with Faraday's and Lenz's Laws. The largest current pulse is produced by the fastest relative motion between magnet and coil. Reversing the magnet reverses the current. Moving the coil produces the same effect as moving the magnet. The solid ring is repelled and heated by eddy currents; the split ring is not, because the gap prevents current flow. Aluminum is not ferromagnetic — the observed forces are entirely electromagnetic in origin.