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Q & A: electromagnetic drive

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Most recent answer: 10/22/2007
Q:
ay you had a device that can create an electromagnetic field of positive polarity in the shape of a hollow tube. You have a space ship that has that device in it and can use it to create the hollow tube and a electromagnetic field "skin" around the ship, the skin and the tube are seperated. The "skin" has two parts, the front, and the back, the front has a negative polarity, and the back has a positive. Remember that the tube has a positive polarity. The front part of the "skin" attracts the tube because the tube has positive polarity(pole) and the "skin" has a - pole. The back also has a + pole., just like the tube, so, the space ship gets pushed and pulled through the tube much like food traveling through the esophogus. Keep in mind that the "skin" is attached to the space ship so the skin helps the space ship move through the tube. The tube is kind of like the bottom of the tank tread, it fades at the back and grows foward at the front: ++++++++++++++++++++++++++++ ----Skin---- |Space Ship| ----Skin---- ++++++++++++++++++++++++++++ Since the device that creates the + tube has a certain distance limit to where it can no longer create a magnetic field strong enough to pull or push the ship along. When the ship enters the tube, the ship enters the tube under its own power(in the form of rockets,etc.) so the +, back part of the ship can enter and push it along just like + and + repel each other, - and - repel each other, and - and + attract each other and vice-versa. Would that work? What is the device, if it exsists, that can create an elec. mag. field in such a way? And another question, I know that to enter an electromagnetic field, you have to enter at a certain degree of angle, and the object entering the elec. mag. field has to go at a slighty faster velocity than the elec. mag. field. If you took an elec. mag. field just like Earth’s and spun it on an axis at the velocity of 99.9999...% light whereas only light can enter, whould that effectively stop objects from entering, such as a bullet? What is the name of the device, if it exsists, that creates an elec. mag. field and can spin it that way to make a sheild? Many Thanks :)
- Clayton Richardson (age 13)
O.P. Norman Jr. High, Kaufman,Texas,U.S.A
A:
Hi Clayton,

That is an interesting (and long!) question.

It's not quite clear from your statement whether the "device" is then to be ejected, or whether it is to be carried with the ship, so that it could be repeatedly used. If you try to keep the whole device on board without ejecting anything, then you can't accelerate the ship, because that would violate Newton's third law. The total momentum of the ship and device can't change. If you do eject some particles, then you're describing a type of rocket drive which has been considered.

Anyway, the setup you describe in your first question is similar to a setup actually used in particle accelerators at various laboratories around the world, although it doesn't work in exactly the way you describe. Instead of space ships, the objects that are accelerated are electrons and protons and atoms that have extra electrons or are missing some so that they are charged and can feel the forces of an electric field.

A negatively-charged object will be attracted to a positively-charged object, but curiously enough, not if it is inside. The reason for this is that the negative charges feel the combined forces of all the positive charge on all sides of it, and they all cancel each other out. The electric field inside of a long, uniformly-charged tube is zero(!).

But don't lose hope just yet -- the field *outside* of the positively-charged tube (and for a little bit inside near the edges, but it disappears quickly) does in fact want to drag a negatively charged object into the tube. These two features can be taken advantage of to accelerate particles to very high energies and surmount one of the main difficulties of charged-particle acceleration. The problem is that if you want to give, say, an electron 100 million electron-volts of energy, you have to drop it across the terminals of a 100-million volt battery, of which there are none easily available.

The neat trick is to arrange a sequence of tubes and gaps between the tubes in a straight line. Charge up the first tube to be positive and allow an electron to accelerate into it (say, from a hot wire or an electron gun like the one in a TV set). While the electron is traveling through the tube, quickly reverse the charge on the tube so that it becomes negative. The electron in the tube won't notice this because the field inside is zero. Meanwhile, charge up the next tube in line to be positive, so that the electron gets a push from the tube it leaves and a pull towards the tube it approaches. When the electron is inside the next tube, flip its polarity too. Then, to get the electron going as fast as you like, just arrange as many tubes as you can afford. It is easiest to connect adjacent tubes with a low-resistance wire and make them oscillate in polarity with microwaves. That way, the maximum voltage around is just the voltage between two adjacent tubes; nowhere do 100 million volts need to be applied. In practice, these are sometimes made out of superconducting rings with the tubes on the ends ("split-ring resonators"). See

for a description.


On the second question, some of the things you say don't sound familiar. What is true is that if you have an intense magnetic field, any electrical conductor entering it will form eddy currents. Those make a magnetic field which tends to repel the object from the magnetic field which was already there. So in some cases magnetic fields can be shields against quickly moving conducting objects, like bullets. Unfortunately, the field needed to do this for a bullet is much larger than can easily be produced. Furthermore, such a field will turn any loose iron object nearby into a deadly bullet. Of course if anyone wanted to circumvent such a shield, they would only need to make bullets that were poor conductors.

If the field oscillates as you say (this is easier to do electrically than to mechanically spin the field-producing object on its axis), then a large amount of power will be radiated at the frequency of oscillation. The effect will be to produce a very powerful radio station or microwave or visible light source, depending on the frequency chosen. Some astrophysical objects (quasars, neutron stars, black holes) may produce radiation powerful enough to stop a bullet, but it would be far more likely just to melt the incoming bullet (or vaporize it). Not only impractical, this object would probably melt or vaporize everything else nearby, defeating the original purpose of reducing damage from incoming bullets.


Tom and Mike

(published on 10/22/2007)

Follow-Up #1: magnetic shields

Q:
I was in science class juts a few weeks ago and we were reviewing the possibilities of magnetism and the effects on various metals. As I was sitting in class I suddenly had the realization that anything that is composed of matter has some sort of electric current coursing through it. So I composed a theory in relation to this process and it states that

"If a produced magnetic field were strong enough, in theory it should be able to repel objects such as bullets or anything else considered dangerous."

Was it right scientifically that I made this assumption?
- Robert (age 17)
WV, USA
A:
Nope. Many objects are attracted to magnetic fields, not repelled. Some, good conductors, are temporarilly repelled if they’re moving quickly into a magnetic field.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.