Above: Drop tower ride at the Los Angeles County Fair (istockphoto.com/elkor)
Imagine sitting at the top of a drop tower ride at an amusement park, as you anxiously wait for the free fall to begin. But before it does, the power goes out and 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 and it doesn't rely on a backup battery or generator during a power outage. Instead, drop tower rides use an ingenious system based on electromagnetic induction, a system that can be explained using Lenz’s law.
In 1831, Michael Faraday discovered that an electric current is produced in a conductor in the presence of a dynamic magnetic field. That is to say that a magnet has to be in motion in the vicinity of the conductor in order to form a magnetic field. Today, many everyday items rely on this principle called electromagnetic induction, including induction cooktops and chargers for electric toothbrushes and cell phone batteries.
Eddy Currents, Magnetic Braking and Lenz’s Law
Electromagnetic induction occurs when an alternating current flowing through a circuit generates current in another circuit simply by being placed nearby. Take induction cooktops: beneath the heating element there are coils that generate an alternating current. When you cook with iron (conductive) cookware, it acts as a second conductor and a current is induced on it without the actual coils beneath the element touching it. The induced current is converted to heat inside the cookware, which is used to cook food.
In 1834, a few years after Faraday’s discovery, Russian physicist Henrich Lenz formulated a law to describe and predict the direction of the induced current and the surrounding magnetic field. Lenz’s law states that if a changing magnetic field induces a current in a coil, the electric current is in such a direction that its own magnetic field opposes the change that produced it.
You can observe Lenz’s law in action by building an eddy current tube. Simply take a roll of aluminum foil and a rare earth magnet. If you drop the magnet through the centre of the roll, it will induce an electric current in the aluminum and create a magnetic field that acts upward against the magnet. As a result, the magnet will drop noticeably slower than it would if the tube was not a conductive material (or if you were dropping something other than a magnet).
Did you know? The Shanghai Maglev Train uses electromagnetic induction to levitate above its track and reach speeds up to 430 km/h.To understand Lenz’s law, you need to remember the law of conservation of energy: Energy cannot be created or destroyed, but one type of energy can be transformed into another. When you moved 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 (downward).
Getting back to the drop tower ride, where you’ve been stranded all this time, each cart has permanent magnets under its seat. Along the bottom third of the tower, copper strips are mounted vertically. When the cart falls, there is kinetic energy in the movement of the magnets below the seats. When the magnets move past the copper conductor, the kinetic energy is transformed into electrical energy, inducing an electric current. The induced current in the copper strips also creates a magnetic field that opposes the motion of the magnets. As a result, the magnetic field pushes up against the seat, creating a reliable, no-friction braking system.
So relax: even if the power goes out, you’ll get back to the bottom safely!