# How is Lenz's Law Used in Drop Tower Rides?

How do magnetic fields help you get down safely from the top of a drop tower ride? Learn how electromagnetic induction works at the amusement park.

Imagine you’re sitting at the top of a drop tower ride at an amusement park. You anxiously wait for the free fall to begin. But before it does, the power goes out. Panicked thoughts begin to run through your head. How am I going to get down safely? How will the braking system work without electricity?

Did you know?

The tallest and fastest drop tower ride is Zumanjaro: Drop of Doom. It is over 125 m tall and drops at a speed of 140 km/h.

Luckily for you, the braking system at the bottom of the ride doesn’t need to be plugged in. It also doesn't rely on a backup battery or generator during a power outage. Instead, drop tower rides use magnetic brakes in conjunction with hydraulic cylinders or air-pressurized brakes. Magnetic brakes work based on electromagnetic induction, a system that can be explained using Lenz’s law.

## What is electromagnetic induction?

In 1831, Michael Faraday discovered that he could create an electric current in a loop of wire by placing a magnet through it. He also found that he could produce an electric current by moving the coil of wire over the magnet when it was stationary.

This was the principle of electromagnetic induction at work. This principle states that a change in a magnetic field produces an electric current. Specifically, when a conductor is in the presence of a dynamic (changing) magnetic field, an electric current is produced. In Faraday's experiment, the wire was the conductor. The magnet he moved through and over the wire created the dynamic magnetic field.

Electromagnetic induction occurs when an alternating current flowing through a circuit generates a current in another circuit simply by being placed near it. Do you or anyone you know have an induction cooktop in your home? If so, you have seen an example of electromagnetic induction. Beneath the heating element there are coils that generate an alternating current. When you cook with induction cookware such as iron, it acts as a second conductor. A current is induced on your cookware without the actual coils beneath the element touching it. Inside your cookware, the induced current converts to heat. The heat is used to cook your food.

Today, many everyday items rely on the principle of electromagnetic induction - even your cell phone batteries!

## What is Lenz’s Law?

Lenz’s Law states that an induced electric current flows in such a direction that its own magnetic field opposes the change that produced it. It was formulated by Russian physicist Heinrich Lenz in 1834, just a few years after Faraday’s discovery.

You can observe Lenz's law in the video below. If you can't watch it, here is what happens. The presenter drops a magnet into a copper tube. The magnet is not touching the sides of the tube. And yet, the magnet moves through the tube very slowly.

Why does this happen? Well, copper is a conductor. We know from Faraday's law that as the magnet passes through the tube, it creates an electric current in the copper. And as Lenz's Law explains, that current will generate a magnetic field that will oppose the magnet. The magnet is moving downwards, so the generated magnetic field will move upwards. As a result, the magnet moves slowly.

The presenter then drops a non-magnetic object through the same copper tube. Because there is no magnet, no current and no opposing magnetic field, the object falls through the tube at normal speed.

Did you know?

The Shanghai Maglev Train uses electromagnetic induction to levitate above its track and reach speeds up to 430 km/h.

Why does a changing magnetic field create an electric current in the first place? To understand, remember the Law of Conservation of Energy. Energy cannot be created or destroyed, but one type of energy can be transformed into another. Lenz's Law and electromagnetic induction demonstrate how kinetic energy is transformed into electrical energy.

Let’s think back to the eddy current tube example shown in the video. When you move the magnet into a coil (in this case, a tube wrapped in aluminum), kinetic energy exists in the movement of the magnet. The kinetic energy is transformed into electrical energy, which creates an induced current in the coil. The electric current in the coil also produces a magnetic field. The magnetic field opposes the direction of the magnet's motion. The magnet is going downward, so the magnetic field is going upward.

## How does Lenz’s Law work in drop tower rides?

Let’s get back to the drop tower ride, where you’ve been stranded all this time. Each cart in the ride has permanent magnets under its seat. Copper strips are mounted vertically in the lower third of the tower. When the cart falls, the fall generates kinetic energy. That kinetic energy is transferred to the cart, including the magnets below the seats. When the magnets move past the copper conductor, the kinetic energy is transformed into electrical energy. This induces an electric current. The induced current in the copper strips also creates a magnetic field. And as Lenz’s law states, that magnetic field opposes the motion of the magnets. As a result, the magnetic field pushes up against the seat, causing the cart to slow down. This is how a drop tower ride’s magnetic brakes work. These magnetic brakes are used with hydraulic cylinders to further slow the falling cart down. The result is a reliable, no-friction braking system.

So relax! Even if the power goes out, you’ll get back to the bottom safely!

### Want to experiment with Lenz’s Law yourself?

Try  building an eddy current tube. Simply take a roll of aluminum foil and a rare earth magnet (a magnet made out of a rare earth element). Drop the magnet through the tube and observe it slowing down.

• Have you ever gone on an amusement park ride? What is your favourite ride?
• How safe do you feel on amusement park rides that drop you straight down?
• Have you ever thought about the science and technology that is involved in an amusement park ride? Explain.
• Explain how the development of a frictionless braking system using magnets is an example of the relationship between science and technology.
• Could our knowledge of Lenz's Law help to design an automobile braking system that does not rely on electricity or friction? Explain.
• The technology and principles used to make the braking system for amusement park rides is also used to build railguns. Is it possible to apply new technologies for peaceful purposes only?
• How can the braking system of the ride be explained using Lenz's Law?
• Could Lenz's Law be utilized in car braking systems? Explain.
• What other ways can Lenz's Law and magnetism benefit us?
• Do you think the energy created from magnets in motion could be developed into an energy supply? Explain.
• Neither Faraday nor Lenz could have predicted the types of applications in which their discovery would eventually be used. Does this give you any concerns about the scientific discoveries being made today? Why/why not?
• How are amusement parks portrayed in movies or on TV? Does this make you feel safe attending them? Why/why not?
• How do news stations report on amusement park accidents? Is it fair and balanced? Explain.
• This article and embedded videos can be used for Engineering & Technology and Math & Physics teaching and learning related to electromagnetism, magnetic fields and energy transformations. Concepts introduced include electromagnetic induction, Lenz’s Law, electrical current, dynamic magnetic field, circuit, alternating current, eddy current tube, Law of Conservation of Energy, kinetic energy, transformed and electrical energy.
• To begin this topic, students could read the above article and watch the embedded videos.
• To further develop an understanding of the concepts, students could then watch the video "World's First Electric Generator.”
• Teachers could use the Key Ideas Round Robin learning strategy to help students summarize and consolidate the information they have gained from their reading and viewing of these resources. Ready-to-use reproducibles for this learning strategy are available to download in [Google doc] and [PDF] formats.
• This lesson could be concluded by having the students complete an exit slip which the teacher can review to assess learning. The ready-to-use reproducibles using the Exit Slip learning strategy for this topic can be downloaded in [Google doc] and [PDF] formats.

This video (3:48 min.) from Veritasium talks about how Michael Faraday created the first electric generator in 1831 using a coil of wire and a permanent magnet.

In this video (4:39 min.) from EdisonTechCenter, Electrical Engineer Bill Kornrumpf describes how an induction cooktop works.

In this article from ComputerWorld, Lucas Meiran explains how wireless charging works.

In this article for HowStuffWorks, Kevin Bonsor gives an explanation of maglev train techology.

An article from TheVerge about a startup that is using "passive magnetic levitation" to power its prototype transportation system.

## References

Larsen International Inc. (n.d.). Super shot ride description.

Physics Classroom. (n.d.). Kinetic energy.

Travel China Guide. (n.d.). Shanghai maglev train (SMT).

Tuckerman, M. E. (2011, September 3). Law of conservation of energy. New York University.

Unterman, N. A. (2005, June 21). Free fall ride braking. U.S. Department of Energy, Office of Science.