Why doesn't the magnet just get stuck or fall extremely slowly? I thought eddy currents always oppose motion.

Eddy currents do oppose motion, creating an upward drag force. However, gravity provides a constant downward force. The magnet accelerates until the upward magnetic drag force grows to equal the downward gravitational force. At this point, the net force is zero and the magnet falls at a constant terminal velocity. It doesn't stop because the opposing force only exists while the magnet is moving.

Is the linear drag model (dv/dt = g - k v) realistic for a real copper pipe?

It is a simplified, first-order model. In reality, the drag force on a magnet in a pipe is more complex and not perfectly linear with velocity, especially at higher speeds. The linear model is an excellent teaching tool that captures the essential physics—velocity-dependent damping leading to terminal velocity—without overly complicated math, making the core concept clear.

Where do eddy currents get the energy to create the opposing magnetic field?

The energy comes directly from the kinetic energy of the falling magnet. The work done by the drag force against the magnet's motion converts its kinetic energy into electrical energy in the eddy currents. This electrical energy is then dissipated as thermal energy (heat) in the resistance of the conductive pipe. This is a direct illustration of energy conservation.

Could this principle be used for anything practical?

Absolutely. Electromagnetic braking, used in some trains and roller coasters, operates on this exact principle. Applying a strong magnetic field to a conducting rail induces eddy currents that create a drag force without physical contact, providing smooth, wear-free braking. Conversely, eddy currents are minimized in transformer cores by using laminated sheets to prevent energy loss.

Eddy Current Tube

The Eddy Current Tube simulator visualizes the dramatic difference in how a magnet falls through a non-conductive pipe versus a conductive one, such as copper. At its core, this is a demonstration of Faraday's Law of Induction and Lenz's Law. When a magnet moves near a conductor, its changing magnetic field induces circulating electric currents—eddy currents—within the conductor. Lenz's Law dictates that these currents flow in a direction to create their own magnetic field that opposes the change that produced them. In the case of a falling magnet, this induced field exerts an upward magnetic drag force, slowing the magnet's descent. The simulator models this motion with two distinct force equations. In air (or a non-conductive pipe), the only force is gravity, leading to constant acceleration: dv/dt = g. Inside the conductive pipe, the model adds a velocity-dependent drag force: dv/dt = g − k v. This 'toy drag model' is a simplification, assuming the drag is linearly proportional to velocity, which is a reasonable approximation for relatively low speeds. Students can manipulate parameters like the magnet's strength or the pipe's conductivity (affecting the constant 'k') and observe the resulting position-time and velocity-time graphs. By interacting, they learn to connect the macroscopic motion to the underlying electromagnetic principles, analyze the transition from acceleration to terminal velocity, and understand how energy is conserved—the magnet's lost kinetic energy is dissipated as heat in the pipe via electrical resistance.

Who it's for: High school and introductory undergraduate physics students studying electromagnetism, specifically electromagnetic induction and damping forces. It is also valuable for educators seeking a dynamic demonstration of Lenz's Law.

Key terms

Eddy Currents
Lenz's Law
Faraday's Law of Induction
Magnetic Drag
Terminal Velocity
Damping Force
Electromagnetic Induction
Velocity-Dependent Drag

How it works

Compare free fall with strong velocity-dependent drag that mimics eddy-current braking in a thick conducting pipe.

Frequently asked questions

Why doesn't the magnet just get stuck or fall extremely slowly? I thought eddy currents always oppose motion.: Eddy currents do oppose motion, creating an upward drag force. However, gravity provides a constant downward force. The magnet accelerates until the upward magnetic drag force grows to equal the downward gravitational force. At this point, the net force is zero and the magnet falls at a constant terminal velocity. It doesn't stop because the opposing force only exists while the magnet is moving.
Is the linear drag model (dv/dt = g - k v) realistic for a real copper pipe?: It is a simplified, first-order model. In reality, the drag force on a magnet in a pipe is more complex and not perfectly linear with velocity, especially at higher speeds. The linear model is an excellent teaching tool that captures the essential physics—velocity-dependent damping leading to terminal velocity—without overly complicated math, making the core concept clear.
Where do eddy currents get the energy to create the opposing magnetic field?: The energy comes directly from the kinetic energy of the falling magnet. The work done by the drag force against the magnet's motion converts its kinetic energy into electrical energy in the eddy currents. This electrical energy is then dissipated as thermal energy (heat) in the resistance of the conductive pipe. This is a direct illustration of energy conservation.
Could this principle be used for anything practical?: Absolutely. Electromagnetic braking, used in some trains and roller coasters, operates on this exact principle. Applying a strong magnetic field to a conducting rail induces eddy currents that create a drag force without physical contact, providing smooth, wear-free braking. Conversely, eddy currents are minimized in transformer cores by using laminated sheets to prevent energy loss.

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