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General Physics:
Electricity and Magnetism

Reflect Gravity?
Electromagnetism and Gravity
The Casimir Force
Electron Volts
Measuring AC and DC Current
Electromagnetic Effects on Water
Compass and Magnetic Poles
Magnetic Fields
Magnetic Field Lines
Magnetic Field Lines vs. Magnetic Force
Reversing a Magnetic Field
Can You Stop a Magnetic Field?
Magnetic Fields and Light
Motion and Magnetic Fields
Two Magnets
Testing Magnets
Magnets and Heat
Catching a Space Ship with an Electromagnet
Magnetic Fields Harmful?
Magnetic Shielding
Propagation of Gravitational and Electromagnetic Fields
Does Charge Affect Gravity?
Magnet-Powered Travel?
Perpetual Magnetic Flywheel
Levitating Superconductor

  1. Reflect Gravity?

    I have heard that it is possible to reflect gravity with rotating superconductor disks. What do you know about this?

    This is not quite correct; you don't actually "reflect" gravity. You can generate a magnetic pressure strong enough to levitate a superconducting disk (it doesn't have to be rotating, but it frequently does due to small disturbances). A superconductor doesn't allow a magnetic field into it, and so placing a superconductor disk over a magnet will "levitate" the disk high enough so that the magnetic field has room to bend and go around the disk. It is the bending of the magnetic field that generates the upward force that holds the disk up.

    Dr. Eric Christian

  2. Electromagnetism and Gravity

    Is it possible that the force we call gravity is actually a magnetic field? Could high energy plasma rotating on an axis explain this? Could this explain the behavior of the surface of the Sun, such as solar flares and sunspots?

    Gravity and electric/magnetic forces are quite distinct in physics, and are not generally thought of as closely related.

    Gravity is related only to mass and not to electrical charge. The gravitational force between two static point masses is proportional to the product of the masses and inversely proportional to the square of the distance between them. This leads to the concept of a gravitational field. The gravitational force is not directly related to the electric or magnetic forces.

    The electrical force between two static charges is proportional to the product of their electrical charges and also is also inversely proportional to the square of the distance between them. This leads to the concept of an electric field.

    The magnetic force is very closely related to the electric force, but no magnetic charges (magnetic monopoles) or monopolar magnetic fields have been observed, in spite of many careful searches. There are currently only (very, very small) upper limits on their abundances. There is no accepted proof that magnetic monopoles cannot exist, so magnetic monopoles may be found some day. The sources of magnetic fields are electrical currents, caused by moving electric charges or small dipoles associated with molecular and atomic structure.

    The relationship between electricity and magnetism was written down in the 19th century by Maxwell, and his equations show elegantly the intimate relationship between electricity and magnetism.

    To go back to your question: all magnetic forces on matter are also proportional to the electric charge on that matter, but also depend on the velocity of the matter. Hence, like electric forces, they are not proportional to the mass. Hence, any attribution of gravitation to electromagnetism is not possible in the current paradigms of physics.

    Dr. Randy Jokipii
    (February 2005)

  3. The Casimir Force

    Where can I find detailed information about the Casimir force?

    You can check out the AIP web site for more information.

    Dr. Louis Barbier
    (October 2001)

  4. Electron Volts

    I'm finding it hard to understand the relationship of an electron volt (eV) to the standard RMS voltage measurement. Is there a quantity of electron volts that make up a volt? What instruments are used to measure eV?

    Volts and electron volts are measurements of two different quantities: volts are a measure of electric potential, and eV are a measure of energy. One eV is the energy that a particle with a charge the same as an electron picks up in one volt of electric potential. Any instrument that measures the energy of a particle (scintillator, silicon solid-state detector, etc.) can be said to measure eV (with some conversion).

    Dr. Eric Christian

  5. Measuring AC and DC Current

    I have a nickel-cadmium battery charger. When I measure it with a meter, it shows AC current. How is it possible to charge a DC battery with AC current?

    If I understand your question correctly you have been trying to measure the charger current with a multi-meter, apparently set to AC.

    In fact, it is possible to measure a DC current with a meter set to AC because it measures the average current going through, no matter which direction the current is flowing. It averages out an AC current in both directions and only measures its absolute value, not telling the direction. In summary, an AC meter measures both AC and DC currents.

    However, a DC meter can only measure DC current, and you need to connect it the right way, or the needle hits the bottom. Therefore, you have probably just measured the DC current from your charger into the battery.

    Dr. Eberhard Moebius
    (August 2005)

  6. Electromagnetic Effects on Water

    Can electric and/or magnetic fields have an effect on the flow of water?

    Water is a conductor of electrical currents. You know that if you have ever watched a murder mystery on TV and someone has dropped a toaster into someone's bath (It kills every time!). In the same manner, you don't walk into a puddle if the electrical lines are down. So water carries electrical currents and currents are nothing more than moving, electrically charged particles.

    Magnetic fields are another manifestation of electrical currents. Currents flow because of electric fields, or electric potential, but once they flow, the current creates a magnetic field. So if an electric current flows through water, it is created by an electric field and it creates a magnetic field.

    When a current flows in a magnetic field it creates a force on that current or on whatever physical object is supporting the current (a wire, for instance). So if you can get a current to flow in water and you have a magnetic field that passes through the water, it is possible to impart a force on the water and alter the way it flows. This isn't easy. Water is heavy, and it takes a lot of battery power to create that much current and that much magnetic field.

    Did you read the book or see the movie "Hunt for Red October"? Do you remember the caterpillar drive? This is also known as a magneto-hydrodynamic drive. The idea is to pass an electrical current through the water in the presence of a strong magnetic field and push the water backwards, thereby propelling the submarine forward. The problem is getting the magnetic field to penetrate deep enough into the water that you move more than the small amount of water on the surface. That's called skin depth.

    So the answer to your question is "Yes, electric and magnetic fields can have an affect on the ability of water to flow." However, it takes a lot of power. It's interesting to consider doing it on a small scale, like through a soda straw. How would you do that? You would definitely need your teacher's help - mixing electricity and water is difficult to do safely. If your teacher is interested, have them contact us and we can talk.

    Dr. Charles Smith
    (March 2005)

  7. Compass and Magnetic Poles

    What do compasses do around the equator? Would they vacillate? Would they be useless? Below the equator, do they point to the South Pole? On one of the magnetic poles, does a compass point in random directions? Could it spin?

    The North Pole is the North Pole is the North Pole. On Earth, a compass points to the North Pole, and it doesn't matter which hemisphere you're in.

    Dr. Louis Barbier
    (January 2001)

  8. Magnetic Fields

    Why are things magnetic? Is there a connection between the Earth being made predominently of iron, and magnets being iron? What's the Curie point of iron, and how does the Earth produce a magnetic field if the molten iron in the Earth is above iron's Curie point?

    The Earth generates its magnetic field more the way an electromagnet does, rather than a permanent magnet. It is electric currents in the molten mantle and core that create the field. Iron is one of the few elements that can have its individual atomic magnetic moments lined up and they'll stay that way, but it's also conductive, which is what's important for the Earth's magnetic field.

    The Curie point is where the thermal motions break up the alignment of the magnetic moments, but that is not important for electromagnetism.

    Dr. Eric Christian

  9. Magnetic Field Lines

    What are magnetic field lines made up of?

    Too often in physics we assume that everything we study and talk about is material and tangible. Magnetic field lines are neither.

    Here is where the study of magnetism begins: Two currents directed in the same or in opposite directions exert a force on one another. If you hang two wires near each other and attach each to a battery that drives sufficient current, you will observe that when you complete the circuit and the current flows the two wires jump either toward or away from one another. Which way they jump depends on which way the current flows. That's it, and everything else about magnetic fields comes from that.

    So what are magnetic field lines? They are nothing more than a mathematical way of expressing that force. Some people mistakenly call them the magnetic lines of force, but actually the magnetic field lines are at right angles to the force they represent.

    And so we have a simple answer with a complicated understanding. Magnetic field lines are really no different in their origin than the lines of force we use to represent gravity. What is the gravity force field made of? Particle physics attempts to explain both forces as the exchange of elementary particles, but that is another discussion. The source of both is the knowledge that two objects some distance apart exert a force on one another either due to their masses or their currents. Fields, whether they are magnetic field lines or gravitational field lines, are simply a mathematical way of expressing those forces.

    Dr. Charles Smith
    (January 2003)

  10. Magnetic Field Lines vs. Magnetic Force

    Regarding the nature of magnetism...

    1. In a previous answer, you mentioned that magnetic force is perpendicular to the magnetic field. If this is the case, when a magnetic bar is hanging in the air, it will be deflected by another magnetic bar close to it. What is the direction of the magnetic force acting on the hanging bar? In books, the direction is always labeled based on magnetic field B but not magnetic force F. Isn't F parallel with B in this case?

    2. When a compass is placed to the right of an external north pole, the south pole of the needle moves toward the external north pole, but the north pole of the needle is pushed away from the external north. Again, the direction of the deflected needle is always labeled as B, but what is the direction of magnetic force F acting on the needle? You may say the magnetic force pushes the needle and provides a torque for it to turn. But in that case, the magnetic force that is providing the torque is parallel with B. Also, why must the magnetic force push the south pole of the needle toward the external pole but not the north pole needle? This comes back to the same question: What is the direction of the magnetic force that acts on the needle?

    3. Neither of the cases above involve any moving electric charge. From my reading, magnetic force is generated whenever an electric charge is moving in a magnetic field. But where does this magnetic field originate? Is it self-originated by the moving electric charge, or must we have another external magnetic field, such as a magnetic bar, to generate the magnetic force? If an external magnetic field is needed, where does it originate? In the case of a magnetic bar, there is no moving electric charge observed. Some books claim that the electron spins are the source of the moving electric charge. However, these are random localized magnetic fields. They can only be aligned when an external magnetic field (such as the Earth's) is applied. If this is true, then there must be some other gigantic magnetic field that causes the Earth's magnetic field to align. Otherwise, the Earth would be just like a normal piece of iron, but not a magnetic bar. Where is this gigantic magnetic field?

    Let's start by separating two entirely different concepts: magnetic field lines and magnetic force. Newton's classical mechanics tells us that force is proportional to acceleration, so the movement of the magnetic bar or of the compass needle indicates a force on the object. That does not mean that the magnetic field is in the direction of the motion. Quite the contrary!

    A magnetic field exists as a mathematical convention that relates a moving charge to the force on it. If you have seen the vector notation for this, it is F = q V x B, where F is the vector force on the object, "x" is the vector cross product, q is the electrical charge on the object, V is the vector representing the charged object's velocity, and B is the vector representing the magnetic field. (At a higher level, magnetic fields are represented as tensors, but that is another subject.) The vector resulting from a cross product is always perpendicular to the 2 vectors that form the cross product, so F is perpendicular to both V and B. That is the nature of magnetism.

    So how do we interpret your observations?

    When the hanging bar moves, the direction of movement will indicate the direction of the force on the bar. When it comes to rest and there is no more force on the bar, the direction of the bar will give the direction of the magnetic field (perpendicular to the motion of the bar). The same is true of compass needles, since they are just very small magnetic bars. Remember, these bars can only rotate, so the net force on them (magnetic, plus the pin that holds it in place) results in purely rotational motion about the pin. The net force is at right angles to the direction the bar points.

    As for your third question, what kind of world would we live in if electrical charges could move in response to their own electromagnetic field? That would be a perpetual motion machine, and we can't have any of that! So the magnetic field is always external to the charge.

    The Earth's magnetic field originates from two sources. First, the rocks and minerals in the crust are magnetized in much the same manner that your magnetic bar is a magnet. They were within the field of a strong magnet when they were moltent, and the magnetic moments of the charges within the rocks aligned with the external field. What produced that external field? That's the second source. It's called a dynamo and is a means by which the movement of electrically conducting fluids create new forms of magnetic fields. The Earth's dynamo changes over very long time scales, so the magnetized rocks in the Earth's crust are not all magnetized in the same direction. The discovery of this fact is what lead to the theory of plate tectonics.

    Dr. Charles Smith
    (August 2008)

  11. Reversing a Magnetic Field

    Suppose you wanted to design magnetic boots with a reversible magnetic field, where you could flip the poles of the magnetic field in the boots in order to cause them to attract or repel a magnetic surface. How could you make the poles switch? Could this involve dynamos and their magnetic fields?

    Dynamos, as they are most often discussed today, relate to the way that an electrically conducting fluid can self-generate a magnetic field. It's true that the traditional definition relates to how to generate electricity by using magnets, but electric motors and electric generators are now well understood. So, let's look at your basic question, and that is how to reverse a magnetic field.

    Magnetic fields arise from currents. Magnetic force in classical physics describes the attraction or repulsion between two currents. That might seem odd, since you can touch a bar magnet without getting a shock and there aren't any currents there that you can easily measure, but the currents in a piece of magnetized iron lie within the atoms themselves. When all the atoms align correctly, there is a hidden, but very real, current circulating within the metal.

    So now the answer to your question is easy - to reverse the magnetic polarity of your magnetized boots, you simply reverse the currents in the boots. That means either you have currents in wires within your boots and you reverse a switch, or you have magnetized material in the boots and you reverse the material. You can, instead, place the boots with their magnets in place on a stronger magnet that will reverse the orientation of all those little atoms and that's how you magnetize a piece of iron in the first place.

    You should know that as far as I know the astronauts do not wear magnetized boots. The material that spacecraft are made of tends not to be magnetic. They use things like Velcro straps to hold their feet to the floor when they are working. Magnetized boots are still a thing of the future.

    Dr. Charles Smith
    (April 2004)

  12. Can You Stop a Magnetic Field?

    Is there any way a magnetic field can be stopped or broken?

    You can't cut magnetic fields, but you can slow them down.

    There are a couple of ways to describe why you can't cut magnetic field lines in the sense that you would create two "ends" of a string when you cut it, but it all comes back to the assumption that there are no magnetic monopoles. If there were magnetic monopoles, then at that point in space there would be more magnetic field lines exiting a region than entering it (or vice versa), the same way you CAN have electric field lines entering or exiting a region where there is a net charge like a single electron. So far, in spite of many experiments looking for magnetic monopoles, none hav ebeen found. If they are found, then we know how to revise the theory of magnetism to allow for "cut" field lines, but so far those changes have not been needed.

    So how do you slow it down? If you carry a magnet toward an object or toward a new region of space, the magnetic field will attempt to penetrate that object and fill that space. If the object is a good conductor of electrical current (that is, electrons are free to move), then the changing magnetic field will result in electrical currents that create additional magnetic fields that oppose the penetration of magnetic field into the object. We call this "permeability". Eventually, the magnetic field will get into the object, but if it's a good conductor, it will be slowed and take longer.

    Stopping the penetration of a magnetic field into an object is tough to do, but the near-perfect conduction of superconductors does a good job of excluding (or holding) magnetic fields for a long time. In the end, however, the magnetic field will move across the boundary, because nothing in the physical world is actually perfect (except, maybe, for your question).

    Then, if you want to take the next step, Google "magnetic reconnection" and open a whole new world of questions regarding the cutting of magnetic field lines.

    Dr. Charles W. Smith
    (June 2007)

  13. Magnetic Fields and Light

    I recently read that the magnetic fields found in pulsars are strong enough to change the vacuum index of refraction. Does this mean that it is possible to use magnetic fields to bend light? I thought that the vacuum index of refraction couldn't be changed. How does this mechanism work?

    Yes, the vacuum index of refraction can change. This is a consequence of quantum field theory, where virtual electron-positron pairs result from vacuum fluctuations. These e-p pairs cause a virtual polarization of the vacuum, which can then change the index of refraction.

    Recent work also shows that a strong magnetic field can affect the vacuum polarization through its effect on the e-p pairs. This then changes the vacuum index of refraction. This of course then implies that light rays can be bent by the influence of the magnetic field.

    I do not know of any experimental verification of this effect.

    Dr. Randy Jokipii
    (November 2003)

  14. Motion and Magnetic Fields

    Dynamo theory proposes a mechanism of convection or rotation of a conductor to produce electric currents which produce magnetic fields. I don't understand how a purely mechanical motion can cause an electrical current, especially if all electric charges cancel out in large bodies, such as galaxies (which have huge magnetic fields). Electric currents require a voltage, an electromotive force. Without it an electric current cannot overcome resistance in a conductor. How does a rotating body, or a convective motion, produce an electromotive force? If it doesn't, is dynamo theory accommodated by the theory of electromagnetism?

    Magnetic fields are created by currents and currents consist of moving charges. Any given point in space may be electrically neutral with no net charge, but if the charges are moving there is a current. Let's see if we can't create new currents by mechanical motion.

    Think about how an electric motor works. There is a magnet that supplies a DC magnetic field. A loop of wire is suspended in the field of the magnet. When current flows through the wire, the current loop is forced to spin. If the current loop is attached to a rod of some kind, mechanical motion is taken off and the spinning current loop can be used to drive a machine. That's current producing a force in a magnetic field resulting in motion. Now run the motor in reverse - use mechanical force to spin the loop of wire in the magnetic field. A current is driven in the wire. Now you have a generator. Lead the wires away from the generator and with the current flowing in the wires you generate a new magnetic field away from the generator that's created by mechanical motion and that's a dynamo. Charge never builds up in the system, but current flows.

    Let's take one more step into the realm of suns, galaxies and the center of our own Earth. How does mechanical motion create magnetic fields when there are no wires? In each of these cases there is a fluid called a magneto-fluid made up of electrically charged particles. Think of water, but then separate the ions and electrons so they are free to move independently. (Mercury is a real magneto-fluid, but remember the dangers of handling it.) Imagine there is already a magnetic field and that the magneto-fluid is flowing across the field. That means that both the ions and the electrons are moving across the magnetic field in the same direction. You know from your studies that the ions and electrons try to move in different directions. They separate and that creates a current - oppositely charged particles moving in opposite directions. That current creates a new magnetic field by way of mechanical motion and you have a dynamo. Now the system is off and running. The trick, and I can't tell you how to do it, is to create a new magnetic field that persists by continuing to drive the system through mechanical motion.

    Keep thinking about difficult problems like this. Keep reading whatever you can find. You'll do very well in physics if you stay with it. You think about things in the right way to build understanding.

    Dr. Charles Smith
    (January 2004)

  15. Two Magnets

    If you took two magnets of equal field strength but unequal mass, attached them together with their opposite poles facing each other (with some space between), and placed them in zero gravity, would this create a form of perpetual motion? The magnet with greater mass would push the magnet with lesser mass away, and since they are attached, the magnet with lower mass would pull the other one back! This seems like it would be perpetual, and increase in velocity over time. Is this correct, or a stupid question?

    Interesting question. Let's see if we can't kill 2 or 3 birds with one stone. First, there aren't any stupid questions in physics. This is hard stuff and asking questions, putting ideas together, and searching out the answer is how we learn. We often learn more from our mistakes than our successes because we work harder on those, so making mistakes or putting 2 things together incorrectly isn't a problem -- it's an opportunity. So put that worry aside.

    Before I answer your question, let me tell you about a couple of physics principles. Isaac Newton said that for every action there is an equal and opposite reaction. That means you (or the big magnet) can't push on something smaller without feeling an equal push on yourself in the opposite direction. Imagine you are on skates and you push someone away from you -- it makes you move backwards until you brace your skate against the ice or the ground. This is going to happen with the magnets. The big one pushes the little one, but is pushed itself in the opposite direction. The little one pulls the big one, but it is pulled itself toward the big one.

    Mass just means that force is converted into acceleration and motion disproportionately. The little magnet accelerates faster and moves farther than the big magnet, but they both move in opposite directions as they exert force on one another.

    So pushing and pulling, pulling and pushing, the two magnets never actually get anywhere and in the end they settle into whatever configuration eliminates the forces acting on them. This brings us to the second physics principle: You can't get something for nothing. It's the second law of thermodynamics, but it shows up all the time. If someone tells you they have a perpetual motion machine, doubt them. Everything you do costs energy in some form. Two magnets that push themselves apart lose potential energy within their combined magnetic field as they gain the kinetic energy associated with motion.

    They can keep moving in whatever direction they start out in, but they can't reverse themselves without tapping into some other energy supply. But where will they find it?

    So for every action there is a reaction, and you can't get something for nothing. Pretty good rules. They seem to work in life, too.

    Dr. Charles Smith
    (February 2003)

  16. Testing Magnets

    Why is repulsion the only sure test that a metal bar is a magnet?

    Thank you very much for your question, which provides an opportunity to clarify a few issues about magnets.

    First, of course, the fact that two metal bars, when brought together in the correct orientation, repel each other tells us immediately that both have to be magnets. Both have to be magnets, because a simple iron bar would be attracted by either side of the magnet and not repelled.

    No other known force does that, EXCEPT perhaps two metal objects that we have charged up electrically with the same charge, i.e. both positive or both negative. Well, that throws a wrinkle into the test, does it? Of course, it is very hard to keep two metal bars charged. And if you touch them with anything, the charge is gone, and the repulsion won't happen again. This way you can differentiate your magnets.

    Second, let me say that I wouldn't state your question that categorically in the sense that "repulsion is the only sure test". We have three different macroscopic forces in our everyday world to deal with (I mean without going to the microscopic scale of nuclei of atoms, which feature two more forces): gravitation, electrostatic, and magnetic.

    I am separating electrostatic and magnetic for clarity here, although in physics they are usually combined into "electromagnetic". From the perspective that we can charge up an object electrically and that we can build a bar magnet, the separation into electrostatic (what you see for a charged object) and magnetic (what you see for a bar magnet) forces is well justified.

    If you have a bar magnet (unknown to you at that point) and several regular iron bars, you can still find out that the one bar is a magnet. It attracts each other iron bar, while they don't do this with each other. Now why is it magnetic and not gravitational or electrostatic force?

    • Gravitation is so weak that you don't see an effect between two small objects like metal bars. Only Earth.s gravity matters here.

    • Electrostatic charge is easy to bleed off a metal object, if you just touch it with anything. So, if you see first attraction between two objects, then you touch them, nothing moves again afterwards, you know that you had electrostatic forces in the play.

    In summary, you see that repulsion is a good test for magnetism, but not the only way, and you could even be fooled by the electrostatic force. However, touching an electrically charged object will always reveal its nature.

    Dr. Eberhard Moebius
    (April 2005)

  17. Magnets and Heat

    How does heat affect the strength of a magnet?

    The short answer is that as a magnetic material is heated, it loses its magnetization. This happens at different temperatures, according to the material, but eventually the kinetic energy of the atom from the heat wins, the magnetic alignment of the material is lost, and the material "demagnetizes".

    The details: Let's start from where magnetic fields originate and that means electrical currents. In classical physics, if a particle has electric charge (protons are positively charged, and electrons are negatively charged), then that charge produces an electric field. Opposite charges attract, like charges repel, and a force is transmitted between the particles.

    When an electric charge is set in motion, a current is created and a magnetic field results. In classical physics, magnetic fields can always be traced to currents. The currents may not be seen (they may exist within the bar magnets), but this is the source of the field. The magnetic field possesses a spatial dependence, so that its strength depends on location relative to the current. We characterize this by defining the magnetic moment of the current distribution.

    The world of atomic physics is governed by quantum mechanics, where particles are small, energy is quantized, and the classical concept of arbitrarily dividing an object breaks down, and things work a little differently. Quantum mechanics assigns a magnetic moment to every atom, and there is no current associated with it. Classical physics might try to associate an equivalent current to that moment, but such an association becomes secondary and the magnetic moment is the fundamental idea. Every atom then possesses a magnetic field that depends on location relative to the atom as defined by its magnetic moment.

    When a material is magnetized, those local magnetic moments all align to produce a large-scale magnetic field that we observe. As the material is heated, the atoms become more mobile and the alignment is lost, thereby causing the loss of the material.s magnetic properties.

    Dr. Charles Smith
    (March 2004)

  18. Electromagnets

    If I had 100 volts and 1 amp (which equals 100 watts) in a circuit with an electromagnet in it, would it have the same magnetic field as a circuit with 200 volts and 0.5 amps (also 100 watts) in it?

    According to the Biot-Savart Law (equivalent of Coulomb's Law for electric fields), the magnetic field is entirely induced by the changing electric field (i.e. current). In other words, the magnitude of the magnetic field depends on the current (not the power). So in your example above, the magnitude of the magnetic field will be effectively twice as large for the one amp case as for the 0.5 amp case. Of course, in both cases, the magnetic field intensity drops off as one over the square root of the distance from the current source.

    There is a nifty website that explains electromagnets and also how to build your own (or test your theories!): How Electromagnets Work

    Dr. Georgia de Nolfo
    (December 2003)

  19. Catching a Space Ship with an Electromagnet

    I'm writing a screenplay that takes place in the near-to-distant future, and I have a physics question. If a space ship were rocketing toward a sports arena, could you make a giant electromagnetic ring out of the iron bar at the top of the arena by wrapping it with copper wire and attaching it to a battery, and use the electromagnet to halt the ship and suspend it off the ground? If not, why not? And would there be any way for this idea to work?

    I'm betting it's more like the distant future than the near future.

    The short answer to your question is "yes" and the long answer is "not easily."

    It is true that an iron bar will help to concentrate the magnetic field of the copper wire and increase the force resulting from the current in the wire. I don't know where the ship will fit after the copper loops are filled with an iron bar, but this is the least of your worries.

    Likewise, you say "space ship" as opposed to "spacecraft", so I assume there are people onboard? Deceleration could be very unpleasant.

    Here's the real problem: The magnetic field "spills" out the ends of the copper coil in such a way that the ship (which I assume is also made of iron?) will more easily be deflected than stopped. It will be necessary to get the space ship to enter the coil when the magnetic force is trying to push it to either side. This makes a near miss very likely.

    You could address this in a way that you seem to want to do: Wrap the space ship in wire and drive a current in such a direction as to draw the craft into the solenoid over the arena. This would require coordinating the polarities of the two coils, but would also accelerate the ship so that you need to reverse polarity once inside the earthly coil to stop the space ship's motion.

    The last possibility I can think of would be to NOT drive a current through one of the 2 coils (earth-bound or space ship) and allow the passage of the charged coil to drive a current in the other. If that current is dissipated it would provide some breaking for the space ship's motion, but it would take a lot of induced current to slow the space ship. All told, I'd rather be on the ground.

    I trust that if this script is sold that Cosmicopia will be given an on-screen credit? Good luck with your screenplay and thanks for your question.

    Dr. Charles Smith
    (November 2005)

  20. Magnetic Fields Harmful?

    Is there is a difference between magnetic fields? Why is the natural magnetic field of Earth harming no one, but magnetic fields from electrical things do so?

    First, let me say that scientific evidence does not support the claim that magnetic fields from things like power lines harm humans. There is also no scientific evidence that strap-on magnets or any type of magnetic therapy can help your health in any way. Weak magnetic fields just don't do much. Extremely strong magnetic fields (that are created in laboratories) may cause currents in your blood and brain, but I've never heard of anyone being harmed by this. The medical procedure "Magnetic Resonance Imaging" (MRI) uses magnetic fields much stronger than those generated by electrical wires or microwave ovens or whatever, and thousands of people a day safely undergo MRI. So I don't really think that there's much of a problem.

    Dr. Eric Christian
    (February 2001)

  21. Magnetic Shielding

    Is there a material that does not allow magnetic fields to pass through it?

    There is such a material. It is called "MetGlass", and it is a metallic glass manufactured for this and similar problems. It is manufactured by combining the right metallic elements in liquid form and solidifying them in an instant, so that the natural lattice formed by metals is avoided. Instead, the atoms show no repeatable pattern in their arrangement, so that the material is said to be amorphous. An amorphous solid is also referred to as a glass. But how does it work and how do you use it?

    It works because the right disordered combination of elements will form a material with a magnetic susceptibility that actually traps the magnetic field line within the material, providing a new path that closes the field line on itself without allowing it to escape into the surrounding space.

    To use it, you need to realize that magnetic fields have no start or end. They are envisioned as continuous lines that close on themselves. Magnetic materials emit field lines in all directions and in varying strength. Therefore, if you only place the shielding between the source and the object you wish to protect, the field lines emanating from the other side of the magnet will find their way over to the half-protected object. So, you use the material to shield all sides of the magnetic object, capturing the field lines in all directions and containing them close to the source.

    If you want to get fancy, a superconductor placed around the source would do the same thing. It would trap the magnetic field and provide a closed pathway back to the source.

    We used MetGlass to shield magnetic objects on the ACE spacecraft when they were needed to make the spacecraft work properly. The valves that control the flow of fuel to the thrusters are especially magnetic. Had they not been shielded, the magnetometer would have measured the spacecraft field instead of the interplanetary field. Once shielded, we no longer measure the valves to any discernable degree.

    Dr. Charles Smith
    (October 2003)

  22. Propagation of Gravitational and Electromagnetic Fields

    What is the classical perception of "how fast" an electric field, which extends to all points in space around a charge, actually reaches to all points in the universe?

    Changes in gravity and electromagnetic fields propagate at the speed of light. Despite thought experiments to the contrary, you can't instantaneously change either. Everything, matter/energy and electric charges, moves at the speed of light (energy only) or less and the subsequent changes move out at the speed of light. The field lines are already there.

    Dr. Eric Christian
    (May 2002)

  23. Does Charge Affect Gravity?

    A few years ago, I heard the president of a company (I think it was Boeing) say that if you drop two objects off a building, they fall at the same rate... except if you charge one. He said there would be more information coming, but I never heard about it again. Is this true?

    We could not find any quotes from Boeing personnel similar to the one you describe, but the force of gravity does not depend on an object's electrostatic charge. If two objects, one charged and one not, are dropped off a building, then they will fall at the same rate. The only way that the two objects would fall at different rates is if there is another charged object on the ground, which would affect the charged falling object but not the uncharged one.

    Dr. Nick Sterling
    (February 2007)

  24. Magnet-Powered Travel?

    Why don't we just attach a magnet to a fixed point and allow it to be attracted to another object in space? This seems like it would derive a thrust without anything other than magnetism. A practical application could be a spaceship with a magnet on the end, and a metal plate on a nearby space station to attract it. Why isn't this done?

    You've asked an interesting question. There are several things we need to consider as we construct an answer for you: conservation of momentum, the nature of force in general, magnetism, and gravity. So let's get started.

    Force acts between 2 objects to impart motion. One object cannot exert a force on itself to make itself move. Consider trying to lift yourself off the ground by pulling on your own hair. You can pull until it comes out by the roots, but you aren't going anywhere. All you may accomplish is to create motion between two parts of your own body by removing hair from your scalp, but so long as the roots hold, your feet will remain on the ground.

    It is therefore necessary to remove either the magnet or the metal plate to another location. So let's do it.

    When two objects pull on one another (and Newton told us that for every action there is an equal and opposite reaction, meaning that the magnet pulls on the plate in the opposite sense that the plate pulls on the magnet) both move. We express this as conservation of momentum. Momentum is the object's mass times its velocity. When two objects exert a force between them, the heavier object develops less movement than the lighter object, but they both begin to move, so long as both have finite mass. A rocket propels itself by forcing a small amount of gas out the back that is moving at tremendous velocity. The rocket then moves forward at a smaller velocity because the rocket has more mass than the gas. If you want to move a spacecraft (the metal plate), you need to use a heavier mass to anchor the magnet. Otherwise, you just move the magnet instead of moving the metal plate (the spacecraft) unless the magnet is tremendously heavy, and getting such a massive magnet into space would cost more than moving the rocket.

    There aren't a lot of things out there that are heavier than a spacecraft, and those that are tend not to come close to the Earth (thankfully). So you have to use moons and such to anchor your magnet. Massive objects such as moons and spacecraft attract each other by gravitational forces, and gravitational forces are very long range. You can build a magnet that is strong enough to raise the object off the ground if the magnet is close to the object, but if the magnet and the Moon are both far away, the force of the Moon's gravity will be greater than the force of the magnet. Gravity wins. This means that the massive object you use to anchor the magnet will have a stronger pull on the spacecraft than the magnet, and the only place the spacecraft will go is down to the moon's surface.

    That's the basic problem. An object that can "lift" itself is getting something for nothing, and that never works. An object that tries to lift another object, such as a magnet anchored to a moon that attempts to move a spacecraft, has to make sure that it can project enough force to overcome all others. So far we don't know how to do that. So long as the dominant force is the gravity of the moon instead of the magnetic force of the magnet, the magnet will lose every time.

    Dr. Charles Smith
    (July 2003)

  25. Perpetual Magnetic Flywheel

    Does a perpetual magnetic flywheel exist? A series would create a generator capable of sustaining itself and generating electricity.

    What you describe is a type of perpetual motion machine. See Wikipedia for a discussion about machines of this type. However, the short answer to your question is, "No". See the "Patents" section on the cited Wikipedia web page for discussions of machines similar to the one you proposed. In order for these to work the "generators" must have efficiencies of over 100%. And 100% efficiency, plus no friction, would be needed just to keep the generator running itself!

    Dr. Ed Tedesco
    (March 2005)

  26. Levitating Superconductor

    I have been reading about gravity shields and anti-gravity devices using a levitating superconductor. Would shooting plasma around the superconductor make a better shield against gravity?

    A levitating superconductor is a fascinating object. But it is better thought of as using a balance of forces rather than anti-gravity. To me, anti-gravity means that you cancel gravity, which implies that gravity goes away for the object.

    It is better to think of the levitating superconductor as using magnetic forces (from the electric current in the superconductor) to oppose the force of gravity. In this case, the two forces cancel and the object sits at one point. Hence gravity is not gone, it is just balanced by another force.

    More information on this phenomenon can be found in the answer to another magnetic levitation question on our site.

    Depending on its intensity, the plasma can change the magnetic field, and so disrupt the levitation. It is not likely to help.

    Dr. Randy Jokipii and Beth Barbier
    (November 2002)

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