## Maglev Trains

A forum designed mainly for high school physics students in New South Wales, Australia.
braveduck
Posts: 4
Joined: Mon Aug 30, 2004 10:53 am

### Maglev Trains

Hi there,
In regard to the Maglev trains I have a question that puzzles me, but you might
find easy to answer.
I was wondering if it would be possible to design a similar train by putting
the electric field to use (not using a magnetic field at all).I am doing some
research on the benefits and possible alternatives to the more common maglev
train.
Thanks very much.
Ian
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
Good question: if it is easy (on a dry day, anyway) to pick up a small piece of paper with a silk-rubbed rod, why not manipulate larger charges to pick up a train? Like the trick with the paper, it won't work in the rain. But could we use it for a rapid transit on the moon? I'm imagining that the train could be one side of a van der Graaf generator, hanging below the rail, which would be earthed.

Like many problems in physics and engineering, you have to put some numbers in to see whether it's plausible. Let's consider the supporting geometry. Suppose we have two surfaces, with area A = 1 square metre, separated by d = 1 cm, we charge one negatively and one positively and we use their attraction.

What charge q do we need to put on these, and what voltage V will that produce? The ratio q/V is called the capacitance C (a measure of how easy it is to store charge). For the geometry we have,

C = epsilon A/d

where epsilon is the permittivity of space and has a value 9 pF/m, or 9 picoCoulombs/Volt. (Here you can see our problem: small static charges produce big voltages.) In our case, the capacitance is 900 picofarads, which we'll round off to 1 nanofarad. We get a Volt of potential difference for each nanoCoulomb we put on the surfaces.

The electric field E between these two surface E = V/d. And the force on each is F = qE, so

F = qV/d = CV^2/d

Let's suppose we want our square metre to lift one tonne in Earth's gravity, so F = 10 kN.

I leave it to you to do the calculations, but the problem is this: any field much bigger than 1 MV/m in air is very difficult to insulate, even in dry air, and much bigger than this can cause sparking even in vacuum. There's also the problem of insulation elsewhere, and controlling the charge (static charges are notoriously difficult to control actively). With these small charges and large fields, charges move from place to place easily (remember that the silk in the rubbing experiment is an insulator at low fields but allows current movement at high fields).

You should be able to find web pages on sparking and dielectric breakdown. And of course you might want to vary the geometry, or look into repulsion rather than attraction. But the most important thing is to put in reasonable values.

One problem I haven't mentioned is shaping the field for stability. This is only modestly difficult with magnetism because one can control the current in each coil, but more difficult with static charges because they tend to flow from one place to another, even on insulators.
phk
Posts: 4
Joined: Sat May 27, 2006 6:02 pm

### Identifying problems with maglev technology

Hi, I have a question from an assessment task:

"Identify some problems with the development of technology used in maglev trains", and have found it quite difficult to find information on the internet, other than the financial outlay required to build the infastructure - rails, cooling mechanisms, etc.

Any help would be greatly appreciated.

Thanks,

Phoebe
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
Other than those? There is also the problem of energy use.

If there were high T, high current density superconductors, the energy and expense would be less.

joe
phk
Posts: 4
Joined: Sat May 27, 2006 6:02 pm
Thanks.

also - i think that when type 2 superconductors experience the transition from a normal to a superconducting state, the slope is gradual across a region of "mixed state" behaviour.

Therefore, is the critical temperature at the point of zero resistance, or when the steep slope begins?

Ta, Phoebe
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
I think that in this field thry refer to both the onset and the end of the mixed state as critical.
phk
Posts: 4
Joined: Sat May 27, 2006 6:02 pm
sorry, but does the meissner effect come into play with maglev trains?

thanks, phoebe
smiley
Posts: 1
Joined: Sat Feb 07, 2009 3:55 am

### Maglev

I see there has been a post related to Maglev posted before, so I am adding my question here. I would like to know if the same effect as in Maglev trains is used horizontally would be possible to use for vertically accelerating an object?
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
sorry, but does the meissner effect come into play with maglev trains?
If the components are superconducting, yes it does. (And sorry for the very late reply: I guess that the server didn't notify me of this message.)

I see there has been a post related to Maglev posted before, so I am adding my question here. I would like to know if the same effect as in Maglev trains is used horizontally would be possible to use for vertically accelerating an object?
Yes, though there are some advantages in starting off in a horizontal direction. Search for "rail gun" and see if those are what you are looking for.

Joe