Further physics - MagLev
Ma Sau-ying and Tong Shiu-sing   
Translation by Janny Leung   

A magnet cannot float stably on another.
Fig. 1  A magnet cannot float stably on another.

The mechanism of the Transrapid system
Fig. 2  The mechanism of the Transrapid system.

A moving magnet induces a current in a nearby conductor
Fig. 3  A moving magnet induces a current in a nearby conductor.

The technology of Electrodynamics Suspension
Fig. 4  The technology of Electrodynamics Suspension.

How to accelerate a MagLev?
Fig. 5  How to accelerate a MagLev?

If we want to travel to somewhere several hundred or even a few thousand kilometres away by the fastest means, we will probably choose to take an airplane. However, in the near future, a new means of transport will enable us to shuttle across cities at a high speed.

So far, a common bullet train can travel at a speed of 200 km/h. Since the frictional force between the train and the rail limits the maximum speed of the train, some people have started to study on trains which can float above the rail, and thus the appearance of Magnetically Levitated train.

Magnetically Levitated train is often abbreviated as MagLev. As its name signifies, a Magnetically Levitated train is levitated above the rail by magnetic force. However, ordinary magnets cannot stably levitate a train. If you place the North poles of two pieces of magnets together, you will find that you cannot place one stably on the top of the other (Fig. 1). Hence levitating a train may not be as easy as one might have imagined.

How to levitate a real MagLev? At present, MagLev is still at an experimental stage. German scientists have designed a system called Transrapid which uses the technology of Electromagnetic Suspension (EMS) to levitate a train (Fig. 2). In this system, a guidance rail is embedded at the bottom of the train. The electromagnet at the undercarriage beneath the train is oriented towards the guidance rail, creating a magnetic force which levitates the train to about 1 cm above the guidance rail; so that even when the train is at rest, it is still levitated. Other guidance magnets ensure the stability in motion.

On the other hand, some Japanese scientists use the technology of Electrodynamics Suspension (EDS) to levitate a train. Do you remember what is electromagnetic induction? When a magnet moves beside a conductor (Fig. 3), the magnetic field inside the conductor will change and a current will be induced. The induced current in turn generates a magnetic field which, according to Lenz's Law, tends to resist the change that caused the induction. The method of EDS utilizes the principle of electromagnetic induction. Fig. 4a shows the principle of this MagLev. The train travels in a guideway which has a series of "8"-shaped coils on each side. When the train travels by with a high speed, the superconducting magnets on each side of the train will induce a current on the coils. The trick is that the superconducting magnet passes below the centre of the "8"-shaped coils, thus the magnetic flux change in the lower half of the "8"-shaped coils is greater than that in the upper half, and a current as shown in the Fig. 4b is induced, generating a magnetic force. The magnetic pole in the lower half of the "8"-shaped coils is the same as that of the superconducting magnet, while the upper half has the exact opposite of it, so that both halves of the coils generate an upward component of magnetic force on the superconducting magnet and levitates the train. Since the "8"-shaped coils can induce a current and generate magnetism only when the superconducting magnets are in motion, the train cannot be levitated when it is at rest. Therefore, the train first starts by sliding on wheels. When the magnetic force generated is large enough to overcome the weight of the train, the wheels are hid like those of a taking-off airplane.

What puts the MagLev in motion then? Its basic principle is simple. Take the Japanese MagLev as an example. The train in motion, along with its superconducting magnets, induces a current on the coils on each side of the guideway. Based on these signals, the system will input alternating currents into the propulsion coils on each side of the guideway, producing an alternating series of North and South magnetic poles (Fig. 5) which pull and push the superconducting magnets and accelerates the train.

Levitating a MagLev in motion above the rail eliminates the frictional force between the train and the rail, thus the train can travel at an express speed. It is estimated that a future MagLev may travel at a speed as high as 500 km/h, which doubles the speed of the existing fastest train. Besides, a MagLev is extremely quiet. German farmers working nearby the MagLev railway could hardly notice a train passes by! However, a demerit of the MagLev is the expensive construction fee of the guidance rail. MagLev can only travel on the top of this kind of guidance rail, and this seriously limits its development. It is expected that traditional trains will still play a major role in the future railway development.

A piece of exciting news to the Chinese: the first commercial MagLev railway in the world will begin to operate in China in 2003. This project costs 26 billion RMB. By then the Transrapid MagLev will carry people to and from the Shanghai city centre and the Shanghai Pudong International Airport, taking only 10 minutes for the whole journey!