Physics Help Forum Electromagnetic pumps

 Electricity and Magnetism Electricity and Magnetism Physics Help Forum

 Apr 21st 2010, 02:20 AM #1 Junior Member   Join Date: Mar 2010 Posts: 3 Electromagnetic pumps "Electromagnetic pumping is a non-invasive method of moving liquid used in situations where the moving parts of traditional pump are not practical. Pumping liquid metal is an example. Electromagnetic pumping involves applying an electric field and a magnetic field at the same time. Consider a pipe oriented perpendicular to the screen, a magnetic field applied from left to right, and an electric field directed upward. Determine the direction of the force on an ion in the liquid. Does the direction of the force depend on the charge of the particle?" Don't even know where to start with this one. Looked and looked for appropriate formulae but the only one I could find was F = IL + B. Is this right? If so, what does the L stand for? Thanks
 Apr 21st 2010, 07:06 AM #2 Senior Member   Join Date: Dec 2008 Location: Udupi, Karnataka, India Posts: 292 Force due to magnetic field is perpendicular to the flow of liquid and the direction of the magnetic field. Depending on the nature of the charge on the ion, this force will be up or down. Force due to electric field will be in the direction of the field or the in the opposite direction of the field, depending on the charge on the ion.
Apr 22nd 2010, 05:30 AM   #3
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Join Date: Dec 2009
Posts: 209
Yup

 Originally Posted by satxer "Electromagnetic pumping is a non-invasive method of moving liquid used in situations where the moving parts of traditional pump are not practical. Pumping liquid metal is an example. Electromagnetic pumping involves applying an electric field and a magnetic field at the same time. Consider a pipe oriented perpendicular to the screen, a magnetic field applied from left to right, and an electric field directed upward. Determine the direction of the force on an ion in the liquid. Does the direction of the force depend on the charge of the particle?" Don't even know where to start with this one. Looked and looked for appropriate formulae but the only one I could find was F = IL + B. Is this right? If so, what does the L stand for? Thanks
The previous reply is right on the money. The only thing I might add is that the B field does indeed apply a force upon the ion that is at a right angle to the B field and the direction of the metallic liquid within the pipe as long as the liquid is in motion.

In other words, while the E field will apply a force upon the ions regardless of whether they are moving or not, the B field only applies a force upon the ions if they are moving through the B field in a direction that is not parallel to the B field lines.

This is basically what the previous reply stated, but I just thought that while I have no doubt that person knows the charge must be in motion for a B field to apply a force upon the charges, you may not, so I thought I'd throw that in there.

What the previous reply states can be written as a mathematical function known as the Lorentz force law, and it is given by:

F = q(E + vXB)

This function first is a vector, or yields a vector answer. It states that the force applied upon some charge, q, is directly related to the E field it may find itself in (a vector quantity) summed with the cross product of the vector velocity of the charge and the vector B field it finds itself passing through.

The final force vector obtained from using this formula will tell you in what direction the force is accelerating the charge towards.

Lastly, the formula you found has problems. You can tell just by looking at it. It states the force on something is the current times some length summed with a B field. Ignoring the first term and looking at the second, a B field has units of Tesla's or Webbers per square meter, those units don't match with anything used as a force, i.e. Newtons, pounds, whatever. Every separate term in a function that yields some value must have units that match that value. If separate terms in a function have units then the only way to sum them is if their units are the same, if not you are trying to add "apples with oranges", and you cannot as they are not the same.

I believe the force on a conductor due to it having a current flowing through it while being within a B field is given by F = ILxB, so that may be where you found that function, but the B field is crossed and not summed with the IL term.

Many Smiles,
Craig

 Apr 22nd 2010, 10:10 AM #4 Senior Member   Join Date: Dec 2009 Posts: 209 Yup, that's what I thought Hello again, I just checked to be sure and yes the following formula is the force on a (straight not curved) current carrying wire within a magnetic field , B. where I is the current flowing through the wire. B is the magnetic field the wire is within. L is a vector, whose magnitude is the length of wire (measured in meters), and whose direction is along the wire, aligned with the direction of conventional current flow. Although an alternative formula is equivalent, and that is: Where the vector direction is now associated with the current variable, instead of the length variable. The two forms are equivalent. If a curved wire is immersed in a B field, the force acting upon it can be found by integrating each infinitesimal segment of wire dℓ, then adding up all these forces through integration, which yields: This is for a curved wire in a B field. If the wire is straight then the integral just comes out to be I times the total length, L crossed with the B field. So this is the formula I believe you were referring to, except that you were summing the B field with the product of the current and the length and not crossing that product with the B field. These equations are known as the Lorentz force and they yield the force on a wire itself due to the charges flowing in the wire being displaced by the B field which acts to push them to one surface of the wire which creates a real force on the wire trying to push it out in the direction the charges are pushing against the surface of the wire they are traveling through. It can be derived from the Lorentz force law and the definition of electrical current. So you do not want to use this function as you want to know the displacement force due to both an E field and a B field that charges are passing through with some velocity, v. So you want to use: where q is the charge, both in value and in sign, of the ions or electrons that are passing through these fields. E is the electric field in volts per meter v is the velocity vector of the charge that is moving through these fields in meters per second. B is the magnetic field in Tesla's or Webber's per squared meter. I believe this is all you need to answer your own question now. Many smiles, Craig
 Apr 22nd 2010, 05:32 PM #5 Junior Member   Join Date: Mar 2010 Posts: 3 Thank you both so much, especially Craig. Couldn't have asked for better help!
 Apr 22nd 2010, 08:26 PM #6 Senior Member   Join Date: Dec 2008 Location: Udupi, Karnataka, India Posts: 292 Hi Craig, "This is basically what the previous reply stated, but I just thought that while I have no doubt that person knows the charge must be in motion for a B field to apply a force upon the charges, you may not, so I thought I'd throw that in there." In such pumps there is no gate valves. When you switch on the electric field, the ions start moving. And when you switch on the magnetic field, the moving ions are further accelerated. So I need not specify that the liquid is in motion.
Apr 22nd 2010, 11:35 PM   #7
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Posts: 209
Relax

 Originally Posted by sa-ri-ga-ma Hi Craig, "This is basically what the previous reply stated, but I just thought that while I have no doubt that person knows the charge must be in motion for a B field to apply a force upon the charges, you may not, so I thought I'd throw that in there." In such pumps there is no gate valves. When you switch on the electric field, the ions start moving. And when you switch on the magnetic field, the moving ions are further accelerated. So I need not specify that the liquid is in motion.
Relax,
I never said you had to, in fact I said I had no doubt you knew this, but that the original poster may not. I only put that in there for him, not for you. Just because he asked the question by no means makes it certain that he knows how such a pump operates and may well not know that there are "no gate valves" so ions are always in motion due to the E field first getting them moving before they even see a B field hence you not stating the need for them to be in motion in order to be affected by a B field force seems logical. So why post a reply trying to justify something that you need not even have to do.

I mean read the quote you took from me, I said I had no doubt that while you knew the ions need be in motion to be affected by a B field force, that he may not and that was the only reason why I put that in there. Why would you care about something so trivial to post such a defensive reply? All I did was expand upon what you had written, and only after first stating that what you wrote was "right on the money" giving you due credit for your contribution before even adding anything to the answer.

We've been swapping conversions for sometime and I have always seemed to like you, and I still do, but can't you see that all I said was that while you no doubt knew the B field only exerts a force on moving charged particles that that the original poster may not and I had stated that for no other reason? The only justification for making the statement that you "need not specify that the liquid is in motion" is if you knew the original poster of the question had knowledge that the ion flow is always moving due to the E field getting them moving before a B field is even seen, so you need not mention it. You knowing does not justify you not having to mention it, as you are the one trying to help some one else; so it is important that they know, if you knew they had such knowledge that justifies your statement for defending why you need not mention it. I'm saying you need not even mention it because it's implied in the function itself, so who cares? I mean we both know if its in an E field it will be moving anyway, but do you know that the original poster knows that?

So I never once said you failed to mention this, nor that you should have, but that I only was, because he may not know the B field only acts upon moving charged particles. Besides, don't you think the question was given to the original poster for reasons of using and getting to know how to use the Lorentz force laws? The EM pump is just a means to convey the question that makes the person think about charge in motion through those two important fields. So why the overreaction?

Craig

Last edited by clombard1973; Apr 22nd 2010 at 11:45 PM.

 May 13th 2010, 02:14 AM #8 Banned   Join Date: May 2010 Posts: 6 A pump in which a conductive liquid is made to move through a pipe by sending a large current transversely through the liquid; this current reacts with a magnetic field that is at right angles to the pipe and to current flow, to move the current-carrying liquid conductor.

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