# How is it possible electromagnets keep a heavy maglev train levitating without forcing anything metallic to be flinged away?

I always wondered how it is possible to have electromagnets so strong that they keep a heavy maglev-train levitating without forcing anything metallic to be flinged away or to be pressed against the ceiling of the vehicle-cabin? And how come a person inside that train which has a pacemaker implanted is not affected? With a pacemaker someone cannot have a MRI scan but still ride a maglev train?

• Shaped pole pieces. Look at a horseshoe magnet - the field is especially strong right between the poles. You can design a magnet system to concentrate the field where you want it, and make sure there's very little elsewhere, like the passenger cabin. Commented Feb 3, 2017 at 15:51

## 1 Answer

First off there are many different types of maglev prototypes out there each with their pros and cons. In fact some designs have the drawback of needing shielded compartments for people with pacemakers! However, instead of going into much detail about all the different kinds of magnets that can be used on maglevs, I will try to explain how magnetic fields can be manipulated.

For example, some maglevs use an array of magnets called a Halbach array.

On the left you can see that the magnetic field lines are somewhat uniform and on the right the maganetic field is not only on the top but also much more condensed than the image on the left. This is just one of the ways that enginerers can mitigate the problems caused by strong magnetic fields.

Also it is important to note that magnets will pull or repel each other with much more strength that when they are close together than when they are far apart. For example dipole magnetic fields decay at a rate of $1/d^3$ which means that as the distance $d$ increases magnetic field gets weak very fast.

Maglev trains exploit these properties of magnetic fields by fitting powerful magnets on the ground and on the bottom of the train. As the magnets approach each other the fields repel with a force strong enough to keep the train afloat while keeping the passengers from experiencing a strong magnetic field.

Hopefully this has shown you that magnetic fields can be shaped and that magnetic fields tend to decay quickly as you move away from them. As a final note consider how little magnetic material humans tend to carry around! Most metals in common use are not strongly attracted to magnetic fields like aluminum. So unless you are moving you weight set or you cast iron skillet you can ride the maglev in safety without prying your stuff from the ceiling!