Introduction to Magnetism

You and your friends have waited more than an hour for this ride. Everyone has been talking about the new roller coaster, and finally you get to ride it! You sit down in the car, a little apprehensive. The car accelerates out of the station at high speed and climbs straight up. Screams erupt as it reaches the top and then accelerates backwards through the station and up another track. With a strong jolt, the car comes to a quick stop back in the station. As you slowly climb down the stairs, you remember the first roller coaster you rode as a kid, where it seemed like every steep climb of the car was accompanied by the metal-on-metal sound of the chain slowly pulling the car up to the top. Ever wonder what's changed?

Magnets have been around for a very long time. The ancient Greek philosopher Plato described lodestone, also known as magnetite. Lodestone comes from a region of Greece called Magnesia, from which our word magnet derives. Navigators used compasses made with needles magnetized by rubbing them repeatedly across a lodestone. While Chinese literature mentions the properties of lodestone as early as the 4th century BCE, clear evidence for the use of a magnetic compass in navigation comes from 1117 CE. These are examples of permanent magnets.

In contrast, electromagnets are temporary magnets. Electromagnets take advantage of the fact that current in a coil of wire can induce a magnetic field that can be turned on and off. Electromagnets are critical components of electrical motors and telegraphs. Telegraphs took advantage of the fact that a moving magnet can produce an electrical current in a wire. The current traveled to the receiving station and moved the magnet there.

In 1996, the first roller coaster using linear induction motors became operational at Kings Dominion in Doswell, Virginia. Linear induction motors are located on the roller coaster tracks. They produce an electric field that induces a current in a conductor mounted on the coaster car. One example of a conductor is the small metal fins protruding from the car. The opposing magnetic fields of the motor and the fins repel each other, thus pushing the car forward. A large number of induction motors are mounted inside the station so cars can accelerate to about 100 km/hr in three to four seconds. A smaller number of induction motors are mounted before each element in the track, such as a loop. The roller coaster described in the first paragraph relies on computer controls to switch the polarity of the induction motors to propel the cars backwards.

The roller coaster car also has to be brought to a stop. Modern coasters use a combination of permanent magnets and mechanical brakes to stop the car. Rapid deceleration relies on magnets. The brake magnet is attached to the car. It creates a moving magnetic field in the head of the rail. This generates eddy currents, disturbing the magnetic field and creating a horizontal force. Eddy currents are much like eddies in rivers: movement in eddies is disordered and often in a different direction from the main direction. The horizontal force works against the movement of the magnet and therefore the car. Eddy current brakes don't work at low speeds. Permanent magnets don't require power; therefore, power outages don't affect the braking. Additional mechanical braking using friction is also required to bring the car to a complete stop.

So, magnets have played a critical role in making roller coasters more fun. Safety for riders has been enhanced through computer modeling of the track, resulting in smoother curves. Biomedical engineering models the forces on the riders and also improves safety. Next time you shoot out of the station in a roller coaster, remember: magnets make it possible!

Multiple Select

Naturally occurring magnetic rock is called (may be more than one answer)

• magnetite

• lodestone

• iron ore

• Plato's rock

Multiple Choice

Which of the following items does NOT use a magnet?

• Compass

• Telegraphs

• Linear induction motor

• Ballpoint pen

Multiple Choice

What is one of the advantages of electromagnets?

• They can be turned on and off.

• They are usually very small.

• They can have more than two poles.

• They work during an electrical outage.

Multiple Choice

Which part of the roller coaster track has the most linear induction motors close together?

• The top of a loop

• The station

• The bottom of a loop

• A long straightaway

Multiple Choice

To make a roller coaster go both forward and backwards along the same track —

• permanent magnets are mounted on both the car and the track.

• the polarity of an electromagnet is reversed.

• the car has fins on both sides.

• the track is heated.

Multiple Choice

Which type of magnet do roller coasters use for braking?

• An electromagnet

• An eddy magnet

• A permanent magnet

• An energized lodestone magnet