Are Eddy Current Forces Always Opposing Motion?

Click For Summary

Discussion Overview

The discussion centers on the behavior of eddy currents in relation to motion through a magnetic field, specifically whether the forces generated by eddy currents always oppose that motion. Participants explore the conditions under which eddy currents are produced and their effects on moving conductive materials, with a focus on conceptual understanding rather than practical applications.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that eddy currents always oppose motion, referencing Lenz's law as a principle of energy conservation.
  • Others argue that if a non-magnetic metal sheet is moved through a uniform magnetic field, no eddy currents are generated, and thus no opposing force exists.
  • A participant shares a personal experience of moving an aluminum I-beam in a strong magnetic field, describing significant resistance, which they attribute to induced currents.
  • There is a discussion about the necessity of a changing magnetic field or a non-uniform field to generate eddy currents, with some participants emphasizing that uniform fields do not induce currents.
  • Some participants express uncertainty about the conditions under which eddy currents are produced and whether they always result in opposing forces when they do occur.
  • One participant questions the understanding of aluminum's magnetic properties, suggesting that aluminum may be attracted to strong magnetic fields, complicating the discussion of eddy currents.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There are competing views on whether eddy currents always oppose motion, with some asserting that they do under certain conditions, while others maintain that no eddy currents are present in uniform magnetic fields, leading to no opposing force.

Contextual Notes

Participants note that the generation of eddy currents depends on the changing magnetic flux and the specific configuration of the magnetic field. The discussion highlights the complexity of the topic, with various assumptions and conditions affecting the outcomes.

Who May Find This Useful

This discussion may be of interest to students and professionals in physics, engineering, and related fields who are exploring the principles of electromagnetism and the behavior of conductive materials in magnetic fields.

Rikki Lee
Messages
2
Reaction score
0
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!
 
Physics news on Phys.org
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.
 
Rikki Lee said:
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.
 
Bob S said:
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
 
Last edited by a moderator:
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.
jmatejka said:
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
 
Bob S said:
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.
 
Dadface said:
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.
 
Bob S said:
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 magnet:rolleyes:I was thinking in terms of the aluminium entering or leaving the field.
 
Bob S said:
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 ;)).
 
Last edited:
  • #10
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)
 
  • #11
Rikki Lee said:
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
 

Similar threads

Undergrad How do I calculate brake force for an eddy current brake?
  • · Replies 1 ·
Replies
1
Views
1K
Undergrad Power Loss Due to An Eddy Current
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Why does metal moving though a magnetic field slow down?
  • · Replies 17 ·
Replies
17
Views
4K
Undergrad Generating eddy currents, what is the best way?
  • · Replies 1 ·
Replies
1
Views
1K
Undergrad How Do Eddy Current Dampers Work in Space Applications?
  • · Replies 4 ·
Replies
4
Views
3K
Undergrad Charge movement relative to magnetic field frame dependence/invariance
  • · Replies 5 ·
Replies
5
Views
2K
How does an eddy current brake work exactly?
  • · Replies 2 ·
Replies
2
Views
3K
Undergrad Eddy currents, and induced current, can they both co-exist?
  • · Replies 11 ·
Replies
11
Views
4K
Undergrad Motional EMF in Faraday disc, co-rotating magnet axial mean flux
  • · Replies 5 ·
Replies
5
Views
976
Graduate Eddy Current Brake Behavior (Accelerating From Rest)
  • · Replies 3 ·
Replies
3
Views
2K