Faraday’s Law · Magnet Drop

Explore the magnet drop

Your goal: explain how changing magnetic flux creates a current and why that current slows a falling magnet.

  1. Predict which magnet will arrive first. Select an intact copper, slotted copper, or plastic tube.
  2. Press Release magnets. An identical magnet drops freely beside the tube for comparison.
  3. Choose Explain to reveal eddy currents, induced fields and a fixed ring in the tube wall. Watch what changes as the magnet approaches and leaves that ring.
  4. Drag Inspect the drop to pause at any instant, or use slow motion to inspect the current loops above and below the moving magnet. Reverse the poles and repeat: the currents reverse, but the braking still points upward.

Copper is not ferromagnetic. Changing magnetic flux drives circulating currents; their magnetic fields oppose the change (Lenz’s law). A lengthwise slot interrupts the large current loops.

In a real lab, secure the tube and use a soft catch tray. Strong neodymium magnets can pinch fingers and damage electronics.

Magnet drop

LENZ’S LAW IN ACTION
NEODYMIUM MAGNET / 1 m TUBEVIRTUAL LAB BENCH
READY TO RELEASEREAL TIME

Release first. Then drag to pause at any instant; Resume continues from there.

OBSERVE & EXPLAIN

Two identical magnets, one release height. Release them together and compare their motion.

TRY ONE CHANGE

Predict first: will the magnet in copper fall faster, slower, or at the same rate as the free magnet?

How the model works

Faraday’s law: ε = −dΦB/dt. A falling magnet changes magnetic flux through conducting loops in the tube. Above the magnet, the induced field tends to maintain the decreasing flux; below it, the induced field opposes the increasing flux. Both effects produce an upward force on the falling magnet. Gravitational potential energy becomes thermal energy in the copper.

This qualitative model integrates gravity and a speed-dependent magnetic drag, F = −kv, with k proportional to magnet strength squared. Inside the tube, drag can balance weight and produce terminal speed. Slotted copper has much weaker residual braking; plastic and the free-fall comparison have none. Entry and exit effects are smoothed. Air resistance is omitted. The replay slider reconstructs the same drop at the selected time. Pausing freezes simulated time; visible currents in a paused frame represent that instant of motion, not a physically held magnet. Current loops, fields, poles, force arrows, tube wall thickness and magnet size are enlarged or simplified; the cutaway is a visualization and does not physically cut the tube. Strength changes magnetic moment only, with mass fixed. Values illustrate behavior rather than predict a particular commercial magnet.