Are Eddy Current Forces Always Opposing Motion?

[Rikki Lee]

Hello, I'm not sure where to post this so I hope this is the right spot. It's not a homework question, just a concept I need to get straight before an exam ;)

If I'm pulling a sheet of metal through a uniform magnetic field (which is perpendicular to the sheets motion), Eddy currents are created - does the net force due to the eddy currents have to be opposing the motion?

Every example I can find about eddy currents says it's a force that acts like a break, and tries to oppose the motion, but I want to be 100% sure this is true :)

Thanks!

 

[Bob S]

If you are pulling a (non-magnetic) metal sheet in a uniform magnetic field, there are no eddy currents, and there is no force opposing the motion.

A while back, I was in a very large magnet (4 meter diameter pole tip, 1-meter gap) running at 1.5 tesla, and somebody handed me a 40 cm long section of aluminum I-beam (30 cm high by 10 cm wide) to put in the center of the magnet. Pulling the aluminum I-beam through the fringe field (large gradient) was like pulling it through molasses. It didn't want to move.

 

[Dadface]

Hello, I'm not sure where to post this so I hope this is the right spot. It's not a homework question, just a concept I need to get straight before an exam ;)

If I'm pulling a sheet of metal through a uniform magnetic field (which is perpendicular to the sheets motion), Eddy currents are created - does the net force due to the eddy currents have to be opposing the motion?

Every example I can find about eddy currents says it's a force that acts like a break, and tries to oppose the motion, but I want to be 100% sure this is true :)

Thanks!

The opposition is known as Lenz's law which is an expression of the conservation of energy.The work done against the braking force is converted to electrical energy.

 

[jmatejka]

If you are pulling a (non-magnetic) metal sheet in a uniform magnetic field, there are no eddy currents, and there is no force opposing the motion.

A while back, I was in a very large magnet (4 meter diameter pole tip, 1-meter gap) running at 1.5 tesla, and somebody handed me a 40 cm long section of aluminum I-beam (30 cm high by 10 cm wide) to put in the center of the magnet. Pulling the aluminum I-beam through the fringe field (large gradient) was like pulling it through molasses. It didn't want to move.

Are you sure about this? I believe a current is induced in the (non-magnetic) aluminum, the force comes from the field of the magnet and the induced field in the aluminum.

The link shows moving magnets, but "relatively speaking" the magnets could be fixed and the aluminum moving.

Link:

http://www.physics.umd.edu/deptinfo/facilities/lecdem/services/demos/demosk2/k2-42.htm

 

[Bob S]

From Bob S
If you are pulling a (non-magnetic) metal sheet in a uniform magnetic field, there are no eddy currents, and there is no force opposing the motion.

A while back, I was in a very large magnet (4 meter diameter pole tip, 1-meter gap) running at 1.5 tesla, and somebody handed me a 40 cm long section of aluminum I-beam (30 cm high by 10 cm wide) to put in the center of the magnet. Pulling the aluminum I-beam through the fringe field (large gradient) was like pulling it through molasses. It didn't want to move.

Are you sure about this? I believe a current is induced in the (non-magnetic) aluminum, the force comes from the field of the magnet and the induced field in the aluminum.]

Agreed. I thought this is what I said or implied. But the B field either has to be nonuniform or varying with time [ -d/dt ∫B·n dA <>0] to get eddy currents. When I got the aluminum I-beam into the uniform central field of the magnet, it easily spun around a vertical axis. Getting it to lay down flat took some effort.
By the way, do you know how a homopolar generator works? A very large one was built at Canberra, AU. See
http://en.wikipedia.org/wiki/Homopolar_generator

 

[Dadface]

From Bob S
If you are pulling a (non-magnetic) metal sheet in a uniform magnetic field, there are no eddy currents, and there is no force opposing the motion.

A while back, I was in a very large magnet (4 meter diameter pole tip, 1-meter gap) running at 1.5 tesla, and somebody handed me a 40 cm long section of aluminum I-beam (30 cm high by 10 cm wide) to put in the center of the magnet. Pulling the aluminum I-beam through the fringe field (large gradient) was like pulling it through molasses. It didn't want to move.

Agreed. I thought this is what I said or implied. But the B field either has to be nonuniform or varying with time [ -d/dt ∫B·n dA <>0] to get eddy currents. When I got the aluminum I-beam into the uniform central field of the magnet, it easily spun around a vertical axis. Getting it to lay down flat took some effort.
By the way, do you know how a homopolar generator works? A very large one was built at Canberra, AU. See
http://en.wikipedia.org/wiki/Homopolar_generator

I think the field can be uniform the flux change resulting from the relative motion.

 

[Bob S]

I think the field can be uniform the flux change resulting from the relative motion.

If you are pulling an aluminum sheet in a uniform magnetic field, -d/dt ∫B·n dA =0, so there are no eddy currents. Visualize a 1-meter square aluminum sheet in the middle of a 4-meter diameter magnet. There will be eddy currents in a loop (circumference of area A) only if the magnetic field inside (linked by) that loop changes.

 

[Dadface]

If you are pulling an aluminum sheet in a uniform magnetic field, -d/dt ∫B·n dA =0, so there are no eddy currents. Visualize a 1-meter square aluminum sheet in the middle of a 4-meter diameter magnet. There will be eddy currents in a loop (circumference of area A) only if the magnetic field inside (linked by) that loop changes.

Yes I agree and I should have read your post more carefully and noticed the huge diameter of your magnetI was thinking in terms of the aluminium entering or leaving the field.

 

[jmatejka]

From Bob S
If you are pulling a (non-magnetic) metal sheet in a uniform magnetic field, there are no eddy currents, and there is no force opposing the motion.

A while back, I was in a very large magnet (4 meter diameter pole tip, 1-meter gap) running at 1.5 tesla, and somebody handed me a 40 cm long section of aluminum I-beam (30 cm high by 10 cm wide) to put in the center of the magnet. Pulling the aluminum I-beam through the fringe field (large gradient) was like pulling it through molasses. It didn't want to move.

Agreed. I thought this is what I said or implied. But the B field either has to be nonuniform or varying with time [ -d/dt ∫B·n dA <>0] to get eddy currents. When I got the aluminum I-beam into the uniform central field of the magnet, it easily spun around a vertical axis. Getting it to lay down flat took some effort.
By the way, do you know how a homopolar generator works? A very large one was built at Canberra, AU. See
http://en.wikipedia.org/wiki/Homopolar_generator

Interesting! Thanks for the link. I have also seen some bad/misunderstood science applied to the "aluminum repulsive magnet" usually a solenoid moving a piece of aluminum. Solenoid must be AC or variable frequency for this to work,(induce current).

A friend once told me his neighbor was going to get rich, because he invented an aluminum magnet ;)

On another note, I think? aluminum is paramagnetic and will be attracted to a powerful enough field, and almost all aluminum is an alloy containing other ferromagnetic materials.

Thanks again for the link, I may have to build this generator, (on a very small desktop scale ;)).

 

[Rikki Lee]

Okay I think I understand this now.

Eddy currents are only created when the magnetic flux is changing, correct?

And when the magnetic flux is changing, will the Eddy currents always have a net force in the direction opposing motion? (That's what I initially wanted to know, and I don't think this question has been answered yet :P)

 

[Bob S]

Eddy currents are only created when the magnetic flux is changing, correct?

Yes, whenever the magnetic flux through the eddy current loop is changing.

And when the magnetic flux is changing, will the Eddy currents always have a net force in the direction opposing motion? (That's what I initially wanted to know, and I don't think this question has been answered yet :P)

I think I did answer it. When I tried to pull an aluminum object through the fringe field of a magnet, it felt like I was pulling it through molasses. This is equivalent to saying that there was an opposing force whenever I tried to move the object.
Bob S