Mr. Scandrett's ClassroomOS

Physics · Lesson 09

Falling Magnet in a Conductive Tube

Electromagnetism Induction Lenz's Law Eddy Currents

This lesson borrows from the Easy Java/JavaScript Simulation source you shared for a magnet falling through different tube materials. Motion changes magnetic flux, flux change induces current, and that induced current pushes back on the magnet.

The Big Idea

As a magnet falls through a conducting tube, the magnetic field threading each ring of the tube changes over time. By Faraday's Law, that changing flux induces an emf. In materials like copper and aluminum, the induced emf drives circulating currents in the tube walls.

Lenz's Law: The induced current opposes the change that caused it. Since the magnet is moving downward, the induced magnetic field acts upward on the magnet, producing a braking force.

What Happens Step by Step

  1. The magnet begins to fall and the flux through nearby loops of the tube changes.
  2. That changing flux induces a voltage in each conducting loop.
  3. The tube material determines how much current can actually flow.
  4. The induced currents create their own magnetic field that opposes the magnet's motion.
  5. The magnet falls slowly in good conductors and nearly free-falls in nonconductors.
Good conductors Copper and aluminum support stronger induced currents, so the braking effect is larger.
Poor conductors Plastic produces almost no induced current, so the magnet behaves much more like ordinary free fall.

Simulator

Press Drop to release the magnet. Compare copper, aluminum, plastic, iron, or no tube at all to see how material properties change the braking force.

Live graph: purple shows velocity and orange shows induced current. Each drop traces how the tube material changes the magnet's motion over time.

Magnet Velocity0.00 m/s
Induced Current0.00 A

Adapted from the EJS model features: selectable tube material, magnet strength, and a multi-loop tube representation.

Real-World Applications

💡
Electric GeneratorsChanging magnetic flux in rotating machines induces current and converts mechanical motion into electrical energy.
🚂
Magnetic BrakesTrains and roller coasters can use eddy-current braking to slow motion without physical contact between moving parts.
📱
Wireless ChargingA changing magnetic field in the charger pad induces current in a receiver coil inside the device.
🧲
Material TestingEngineers use induction-based methods to compare conductivity, detect flaws, and inspect metal components.

Practice Problems

Think about conductivity, flux change, and how induced currents affect the magnet's motion.

Easy1. A magnet falls into a copper tube and speeds up downward. What direction is the magnetic force from the induced currents on the magnet?

Hint: Lenz's Law says the induced effect opposes the change. The magnet is moving downward, so the braking force must be upward.

Easy2. Which tube should produce the weakest induced-current braking effect?

Hint: A poor conductor cannot support strong eddy currents.

Medium3. If the magnet falls faster through the same tube, what happens to the induced emf?

Hint: Faster motion means flux changes more quickly, so the induced emf grows.

Challenge4. A 21-loop tube model experiences a flux change of 0.04 Wb per loop in 0.10 s. Estimate the induced emf magnitude using EMF = N × ΔΦ/Δt.

Hint: EMF = 21 × 0.04 / 0.10 = 8.4 V

Innovator Profile · See Also