Look ma, no rails!
|The Shanghai Meglev Train in China has a speed of 430 kilometers per hour, and will travel the 33-kilometer distance from downtown Shanghai to the Pudong International Airport in about eight minutes.|
|Passengers ride in a Meglev train on September 25, 2006, in Shanghai, China, which has the world's only commercially operating Meglev.|
Anyone who has ever played with two magnets will know that the magnets can do one of two things - either attract or repel each other. This simple experience confirms the most basic law of magnetism - that opposite 'poles' attract and like 'poles' repel.
A magnet attracts only one of four magnetic substances: iron, steel, nickel, or cobalt. The magnet will not be attracted to, for example, copper or aluminum.
There is a magnetic field around a magnet. When the magnetic substance enters the magnetic field it becomes magnetized. This is called `induced' magnetism, and it causes attraction between the substance and the magnet. When the substance is removed from the magnetic field, it loses its magnetism and returns to being just an ordinary piece of metal.
The Earth has its own magnetic field, which is aligned in a north-south direction. Sailors used this to navigate for over a thousand years. A small magnet, if allowed to spin freely, always comes to rest pointing along this north-south line. The Chinese were the first to discover this, not using actual magnets, but a type of rock called lodestone, which is rich in iron. The first compass was made from a pointed piece of lodestone hung by a fine thread.
The type of magnet you might have played with, stuck to your fridge, or have in your compass is a permanent magnet. There is, however, another type of magnet, called an electromagnet, which can be turned on and off by a switch.
It was the Danish scientist Hans Christian Oersted (1777-1851) who discovered this, by accident. He noticed that when he connected a wire to a battery to make a complete electric circuit, a compass near the wire changed the direction it was pointing in. When he disconnected the battery, the compass went back to pointing north-south once more. Oersted quickly realized that the electric current in the wire was producing its own magnetic field.
An electromagnet is very simple to make. Wrapping an insulated (plastic covered) wire around an iron nail and connecting it to a battery is all that is required. This should be enough to deflect a compass needle or pick up a few steel paperclips.
Electromagnets are particularly useful for a number of reasons. First, unlike permanent magnets, they can be switched on and off. Secondly, their strength can be increased by increasing the amount of electric current. By having a large number of coils of wire around the iron `core' of an electromagnet and a large electric current flowing through these coils, a very powerful magnetic field can be produced.
It is also possible to reverse the poles of the electromagnet simply by reversing the direction of the current. If we swap around the + and - of the battery, then the north and south poles of the electromagnet are also reversed.
What is maglev and how does it work?
Maglev stands for magnetic levitation. This means making something `float' in the air using magnetic fields. Remarkably, this has been applied successfully to an entire train.
The Shanghai Maglev Train carries passengers from Pudong International Airport to Longyan Road station. It travels a distance of 33 km in less than eight minutes and has a top speed of 430 km per hour. It took approximately three years to build and opened on New Year's Day 2004.
The difference between a maglev train and an ordinary train is that a maglev train doesn't have wheels that run along metal tracks - it floats or levitates above the track! In this way it can reduce friction and noise enabling it to go much faster and more smoothly than regular trains.
The levitation is achieved by having large electromagnets in the track and train, which repel each other. The electromagnetic force is used to counteract the effects of the gravitational force. The train is propelled or pushed forwards using a 'linear' motor. One half of the linear motor is the electromagnets on the train. The other half of the motor is made of electromagnets inside the track, on which the maglev floats.
To make the train move forward, the electromagnets in the track behind the train are set to repel it, while those in the track in front of the train attract it. The electric currents are carefully controlled so that as the train passes an electromagnet its polarity changes so that the train keeps moving. (See diagram on the right.) To stop the train the electromagnets in front are made to repel it and those behind are made to attract it.
I was extremely impressed by the speed and smoothness of the Shanghai Maglev. However, as one of my traveling companions commented, ``What was the point in getting to the airport in seven minutes, when our flight was delayed by two hours?''
Readers wishing to find out more abosut maglev should visit http://users.bigpond.net.au/com/maglevvideogallery .
Next time: The laser - a solution without a problem?
Corrin Funnell is a laser physicist with a specialty in laser spectroscopy. He has taught in the UK, Egypt, at Thailand's own Harrow International School, where he became head of the physics department. Currently, he is head of Physics at Island School, Hong Kong.