An experiment on magnetism and momentum?

• enroger
In summary: Namely, that electric and magnetic fields always exert forces on each other, even when they're separated by large distances. If the fields were just waves, without any physical reality behind them, then you would expect the fields to cancel each other out as they traveled away from each other. But this isn't what happens at all! The fields always exert a force, no matter how far they are apart. This force is called the "Lorenz Force." And it's quite large, depending on the strength of the fields.
enroger
Say we have two electromagnet A and B a light year apart from each other. Someone turn on A and a magnetic field is created and propagate at c toward B.

It takes a year for the mag field from A to arrive B, B is turned on just before A's field arrive. B's field made to be opposite from A and is then repelled by the A's field.

But since information can not travel faster than c, when B start accelerating A would not know. It takes a year for A to notice B's field.

Ordinarily, if A's magnetic field remains unchanged when B's field arrive, A would experience a repulsion force and accelerate to the other direction in comply with conservation of momentum.

But what if A change it's magnetic field just before B's field arrive? Then when B's field arrive A would feel an attraction force! This makes both A and B feel a force to the same direction resulting in a serious violation to Newton's third law.

This could be used to build a reactionless drive:
step 1: A turn on, mag field out
step 2: just before A's mag field arrive, B turn on with a opposite field direction from A so B gets repelled. And B's mag field travel to A too.
step 3: just before B's mag field reach A, A reverse current direction to reverse field direction=> A is attracted to B when the field arrive.
... get the picture?

as long as A & B switch field direction exactly out of phase, all the force acting on them are of the same direction.

i don't know... and what about the intermediate states?? the two fields will interact in some spacetime point in the middle... and the strenghts vary in function of the relative distances...

ummmhhh..

I don't know the detailed explanation why this system does not violate conservation of momentum. Probably it has something to do with the fact, that electromagnetic field has it's own momentum, so the magnets+field momentum is conserved.
A similar "reactionless drive" can be achieved simply by turning on a lamp on the "vehicle": each foton with energy E also carries a small momentum E/c, so any source of electro-magnetic waves can be used for propulsion.

Sorry, double post.

I don't think this kind of momentum can be compared to the momentum of photons, since the force is actually lorenz force here which can be many magnitude larger than photon's force depends of field strength and current.

Also, the distance can be 3 meter with magnetic fields of the two solenoids oscillate at 100Mhz.

enroger said:
I don't think this kind of momentum can be compared to the momentum of photons, since the force is actually lorenz force here which can be many magnitude larger than photon's force depends of field strength and current.

My dear friend, photons are the smallest quantity of electromagnetic field in excistence, so the momentum of photons *is* very significant here. It is simply a matter of having enough of them.

The concept fails because this momentum depends on the *square* of the field - hence, it is independent of the polarity, and is determined by the propagation vector of the electromagnetic wave. Changeing the current in the loop does not change the propagation of the wave, it only puts it out of phase by a half wave.

no one has actually said why this wouldn't work? At least, not a plausible complete explanation? I've heard something related to this concerning electro hydrodynamics? I don't know maybe.

So, why would the original question NOT violate conservation of momentum??

enroger said:
This could be used to build a reactionless drive:
step 1: A turn on, mag field out
step 2: just before A's mag field arrive, B turn on with a opposite field direction from A so B gets repelled. And B's mag field travel to A too.
step 3: just before B's mag field reach A, A reverse current direction to reverse field direction=> A is attracted to B when the field arrive.
... get the picture?

as long as A & B switch field direction exactly out of phase, all the force acting on them are of the same direction.

In Step 1 the magnetic field of A varies in time, and is therefore accompanied by an electric field. The time-varying electric field in turn induces a magnetic field further out in space. (This is how A's magnetic field propagates out into space.) The momentum density of the electromagnetic field is proportional to E X B. Thus as the fields of A propagate out into space they engender momentum. Etc. I am confident that momentum is conserved throughout a cycle of your system when the momentum of ALL things is considered.

Lojzek said:
electromagnetic field has it's own momentum

In fact, Maxwell deduced the existence of electromagnetic momentum because it was necessary to explain this very problem.
The magnetic field behaves exactly as does an ordinary object with energy and momentum.

You can find his original treatise here if you're interested.
http://www.archive.org/stream/treatiseonelectr01maxwrich#page/n5/mode/2up"

Last edited by a moderator:

1. What is the purpose of conducting an experiment on magnetism and momentum?

The purpose of conducting an experiment on magnetism and momentum is to better understand the relationship between these two fundamental concepts in physics. By observing and measuring the effects of magnetism on an object's momentum, we can gain insights into the behavior of particles and their interactions with magnetic fields.

2. How do you design an experiment to investigate magnetism and momentum?

To design an experiment on magnetism and momentum, you will first need to identify the variables you want to measure and control, such as the strength of the magnetic field, the mass and velocity of the object, and the distance between the object and the magnet. You will also need to choose the appropriate equipment and methods for measuring these variables and ensuring accurate and repeatable results.

3. What materials and equipment are needed for an experiment on magnetism and momentum?

The materials and equipment needed for an experiment on magnetism and momentum may vary depending on the specific design of the experiment, but some common items include magnets, objects with varying mass and velocity, a stopwatch or timer, a ruler or measuring tape, and a device for measuring magnetic fields, such as a magnetometer or Hall effect sensor.

4. What are some potential sources of error in an experiment on magnetism and momentum?

Some potential sources of error in an experiment on magnetism and momentum include variations in the strength and direction of the magnetic field, inaccuracies in measuring the mass and velocity of the object, and external factors such as air resistance or friction. It is important to carefully control these variables and repeat the experiment multiple times to minimize these sources of error.

5. How can the results of an experiment on magnetism and momentum be applied in real-world situations?

The results of an experiment on magnetism and momentum can be applied in a variety of real-world situations, such as in the design of magnetic levitation systems, the development of more efficient motors and generators, and in understanding the behavior of charged particles in electromagnetic fields. This research also has implications for other fields such as engineering, materials science, and astronomy.

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