- 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.