Advances in trains
You probably have not had the experience of riding on a steam-hauled train in Hong Kong. When the Kowloon-Canton Railway (now called the East Rail) came into service in 1910, all trains were driven by steam engines. Not until the 1950's were diesel-powered trains introduced. By 1962, all trains in Hong Kong were diesel-powered. In 1982, the first stage of electrification for the Kowloon-Canton Railway was completed, and electric trains are still in use today.
The development from steam-hauled trains to diesel-powered trains, and then to electric trains were advances in transportation made possible by technological development. Performance and energy efficiency were improved, and the operation costs were reduced consequently. The following table gives a comparison between steam, diesel and electric as the mode of power used in trains. Steam locomotives are much less energy efficient than diesel or electric locomotives, while the latter two are comparable. So the change from steam locomotive to diesel or electric locomotive had significant benefits in terms of energy efficiency.
While the cost of building electrified track for electric trains is rather high, electric locomotives are significantly cheaper to run than diesel ones. Therefore, changing from diesel to electrified trains helps reduce the operation costs.
Another advantage of electric trains over diesel-powered trains is that the former are more environmentally friendly. Diesel trains produce exhaust gas from the burning of diesel on board the trains. The exhaust gas produced causes pollution to areas where the trains are travelling. As for electricity-powered trains, exhaust gas produced during the generation of electricity occurs at power stations which are usually far from urban areas. This advantage is especially important for another railway system in Hong Kong, the Mass Transit Railway (MTR). The MTR travels a lot underground and in urban areas. Thus, it is important that the trains do not pollute these areas. When the MTR started service in 1979, all its trains were powered by electricity right from the beginning.
Ordinary trains are propelled forward by the rolling of the train wheels on the railway. Friction between the wheels and the railway is unavoidable. Technological advancements in recent years have made it possible to have trains that "float" above the track without the need of any physical support, eliminating the friction between the train and the track. This kind of train is levitated by magnetic force and is thus known as a magnetic levitated train (MagLev).
It is very difficult to maintain stable levitation of an object by using permanent magnets alone. In some special cases when the object is in motion (e.g. spinning), stable levitation can be achieved. See the following video for a spinning top levitated in air by a magnetic field (See also Fig. 4).
Magnetic levitation of trains is achieved by state of the art technology. Broadly speaking, there are two kinds of magnetic levitation train systems. German engineers have developed an electromagnetic suspension (EMS) system which uses powerful electromagnets to provide an attractive lifting force on the train. Japanese engineers, on the other hand, have developed an electrodynamic suspension (EDS) system which uses superconducting magnets cooled by liquid helium, and track with wound coils to provide a repulsive lifting force.
The EMS attractive levitation system is shown in Fig. 5. Electromagnets (called support magnets) are attached to the train and powered by batteries on the train. There are coils (called stators) built into the lower surface of the track (called the guideway). The coils are comprised of sheets of steel and coil windings. When the electromagnets are switched on, the attractive force between the electromagnets and the coils levitates the train. Guidance electromagnets are located on the side to keep the train in position laterally.
To stabilize the levitation, a feedback system that utilizes gap sensors keeps the gaps between the train and the guideway at prescribed width. The gap sensors attached to the train contain oscillatory circuits that induce eddy currents in the stators under the guideway. When the gap width changes, mutual electromagnetic induction between the gap sensors and the stators will produce characteristic signals in the oscillatory circuit. The signals are analyzed and used to regulate the power delivered to the electromagnets of the train, maintaining the prescribed gap width. In engineering, this kind of control is known as feedback control. One can find toys that achieve stable magnetic levitation using feedback control (Fig. 6). Look at the video clip below:
The power to the train's electromagnets comes from batteries on the train. A charging system is also installed on the train. As the train travels, the stators under the guideway produce a changing magnetic field to charge up the batteries on board the train by electromagnetic induction.
The world's first commercially operated MagLev train system is located in Shanghai and it uses the EMS attractive system for levitation. It operates between the Pudong International Airport and the financial centre in Lujiazui, with a single trip distance of about 30 km. The train travels at a high speed of over 400 kmh-1 and the trip takes only about 7 to 8 minutes .
Look at the animation below to learn more about the MagLev.
Study the motion of this MagLev in the following activity:
By using permanent magnets and a piece of superconductor, you can make a simple model of MagLev in the laboratory and see it glide steadily! Does this sound interesting? See the activity below.
The EDS repulsive levitation system has superconducting electromagnets on board the train. Being cooled to a low temperature by liquid helium and nitrogen, the coils of these electromagnets have extremely low resistance (superconducting). Thus, they allow a large current to flow with little energy dissipation, producing a strong magnetic field.
On the track (guideway) there are guidance-levitation coils that are wound in the shape of an "8" (Fig. 9). When the train travels along the guideway, the superconducting magnets pass by the guidance-levitation coils and create a changing magnetic flux in the coils. By electromagnetic induction, currents are induced in the coils, and these currents produce a magnetic force to levitate the train. When the centre of the superconducting magnets is at the same vertical level as the mid-point of the "8"-shaped guidance-levitation coils, stable levitation of the train is achieved (Fig. 10a). If the train falls below this levitated equilibrium position, the magnetic flux through the upper and lower part of the "8"-shaped coils will change, and a current with direction as shown in Fig. 10b will be induced in the guidance-levitation coils, producing a net upward magnetic force (Fig. 10c) to restore the train to its levitated equilibrium position.
As the induced current in the coil increases with the rate of change of magnetic field, the upward magnetic force will only be strong enough to levitate the train when the train is travelling at a high enough speed. Thus for the Japanese system, the train travels on wheels at first and levitation only begins when the train has reached a high enough speed.
MagLev trains can travel much faster than ordinary trains because friction between the train and the track is minimized. Energy loss due to friction is much reduced and hence MagLev trains are generally more energy efficient than ordinary trains. Without friction, MagLev trains produce much less noise and need fewer mechanical repairs. However, the main disadvantage of MagLev is its very high construction costs. Building the guideway for a MagLev is expensive and this is why the MagLev is not popular in most countries.
It was only at the turn of the 20th century that man first successfully flew an airplane. Now we have jumbo jets to take us all over the world. Rockets that allow us travel to space were also developed during the past few decades. Have you ever wondered what makes an aeroplane fly? And do you want to know how a rocket works?
To learn how an aeroplane flies, see Fig. 13 for the names of the forces acting on a plane during flying. Thrust is what moves the plane forward, generated by the engines of the plane. Weight is the force of gravity on the plane. Lift is the upward force that raises the plane, generated by the plane moving through the air. Drag is the force opposing the motion of the plane through air.
An aeroplane can fly because the upward force lift is generated when the plane is moving in the air. Lift is generated mainly by the wings when they move through the air. When the lift balances the weight of the plane, the plane can stay in the air and this is flying.
How do the wings of a plane generate the lift? Fig. 14 shows the cross-section of a typical wing. The upper and lower parts of the wing are not symmetrical, and the wings are tilted at a small angle to the horizontal. As such, when the plane is moving forward, the velocity of the air flowing past the wing has different values at different points on the wing. Using the principles in fluid mechanics, one can relate the pressure to the velocity of the air at different points on the wing. Adding up (integrating) the pressure variation times the area around the entire wing gives the total (aerodynamic) force on the wing. Hence, the special shape and the tilt of the wing are important factors affecting the aerodynamic force acting perpendicular to the flow direction of the air. This is lift.
Drag is essentially the force due to air resistance. According to Newton's first law, the thrust generated by the plane engines must counterbalance the drag so that the plane can maintain its uniform forward motion. During a long journey, the energy generated by the plane's engines is mainly dissipated in counteracting the drag.
You probably have seen rockets launched on television. When a rocket launches, burning gas is expelled from the rocket and the rocket moves upwards. In fact, the upward movement of a rocket has to do with a basic law of physics, the Newton's third law of motion, which says,
"To every action there is an equal and opposite reaction."
A rocket works by burning fuel to produce hot gas. The hot gas rushes out from the nozzles at the bottom of the rocket at high speed. There is a downward force that expels the hot gas out of the rocket. According to Newton's third law, an equal and opposite force must exist. This force propels the rocket upwards. The force exerted on the hot gas and the force exerted on the rocket are a pair of action and reaction forces.
Another way of looking at the launch involves another fundamental physics principle, the conversation of momentum. Initially, the rocket (with the fuel) is at rest and has zero momentum. When the fuel is burnt, the hot gas rushes out of the rocket due to the great heat and pressure produced by the release of chemical energy in burning. The hot gas moves downwards at a high speed, and thus carries a downward momentum. By conservation of momentum, the rocket will acquire an upward momentum of equal magnitude, so that the total momentum of the gas and the rocket remains zero. Hence the rocket moves upward with an increasing velocity (acceleration) as more hot gas is expelled.
Have you noticed that rockets, aeroplanes and MagLev trains are all long and thin in shape, and have small and pointed or sloping frontal areas? The shapes of these modes of transportation are specially designed to reduce the air resistance when they are travelling at high speeds. The effect of air resistance becomes more and more important as speed increases. Scientists have found that for a certain range of speeds, the air resistance (force) experienced by an object is given by the equation:
where Cd is called the coefficient of drag which depends on the shape of the object only, r is the density of air, A is the reference area related to the frontal area of the object and v is the speed of the object. Shapes that are long with a sloping or pointed front have a lower coefficient of drag. This partly explains the shapes of high-speed trains and aeroplanes. A lower drag means lower energy loss in moving the object forwards, and therefore better energy efficiency can be achieved.