## OneOwOne 2 years ago A magnet with contant force passing through a copper pipe?

1. OneOwOne

Lets say we applied 180 N of constant force to move a magnet through a copper pipe, the induced current will resist the magnet's force with 180 N so total force is 360N, however, in opposite directions: ---> <--- Input Resisting Force Force from the copper pipe 180N 180N And lets say the initial velocity of the magnet before entering the coil was 100m/s after going through the coil with a constant force of 180 N still pushing on the magnet. What would the velocity be?(Please solve this problem). In this case, the force is still being applied to push the magnet through the coil, would that force be enough to maintain the speed of 100 m/s? Or would it decrease? @Vincent-Lyon.Fr @Mashy Its better to answer here.

2. OneOwOne

@.Sam. @UnkleRhaukus

3. OneOwOne

@countonme123 @Callisto

4. OneOwOne

I think the velocity will stay constant at 100 m/s because there is no acceleration. What do you all think?

5. OneOwOne

I believe the magnet will stay at 100 m/s before and after passing the copper pipe. Even when the induced current will oppose the magnet with the same force.

6. OneOwOne

@amistre64 @ghass1978 @ParthKohli

7. amistre64

srry, not my area of expertise :)

8. amistre64

at the moment, i cant even understand what the question is to even make an educated guess

9. OneOwOne

@amistre64 I sent you a clarification message. :)

10. amistre64

our definitions of clarity might not be the same :) We did not get to magnets and their properties in the physics class i took so I am not too sure how they behave.

11. OneOwOne

Fair enough.

12. .Sam.

I'm not 100% sure but, based from the induced current, the resisting force would reduce the input force when moving through the pipe. So, $F=qvB \\ \\F ∝ v$ If the input force decreases, then velocity decreases as well. This is just some idea, I'm not so sure about it.

13. OneOwOne

@.Sam. Agreed, this is right if the input force was not constant. In the example I've stated the input force is constant at 180N. So I think the velocity would be constant too.

14. .Sam.

If the magnetic flux density doesn't change then I'd say its constant velocity, but there's another thing, If the force before entering the field is 180N, then when it is 360N in the field, the input force increased by 180N just to keep the velocity constant.

15. OneOwOne

So the magnet will pass through the copper pipe with a constant speed. Not changing at all.

16. Fosterthepeople

17. .Sam.

The force would increase just to keep the velocity constant.

18. OneOwOne

But if the force is constant at 180N that is pushing the magnet, and its initial speed is at 100 m/s would it stay that way, or...? Decrease/Increase?

19. OneOwOne

@Vincent-Lyon.Fr could you please clarify your point on this again using the example provided?

20. Visionary01

I'd say since the magnet will travel at a constant speed,because the forces cancels out? I mean, there is a force pushing the magnet at 180N, and another resisting force generated from the current pushing AGAINST it with 180N. So I think! I think! It will be constant at 100 m/s.

21. Visionary01

@Jemurray3 I am I right here? You tend to correct me when I'm wrong :p so...

22. Jemurray3

I am not sure about the premise of the question ... a magnet moving through a copper pipe will feel a resistive force due to the induced eddy currents, and the magnitude of that force will depend on the strength of the magnet as well as how fast the magnet is moving through the pipe at any given instant. What makes you say the forces will be equal and cancel each other out?

23. Visionary01

Well, thats WHY I MENTIONED YOU! lol, I just guessed. When you have to equal forces as stated in the example... That are opposite. I felt they won't effect the magnet's speed that is at 100 m/s.

24. Visionary01

I tired figuring this out. But... I just had a wild guess.

25. Visionary01

Come to think of it, I doubt the eddy currents will be able to create the same opposite force... Hmmm...

26. Jemurray3

I mean, the person who asked the question said they were equal, so I thought I'd ask... I will try to work this out and estimate it but I'll say immediately that the resistive force does not automatically cancel any applied force And don't count the eddy currents out, they can be powerful influences. There will be a terminal velocity, it's just a question of what it is.

27. OneOwOne

@Visionary01 I firstly assumed the forces cancel out but no they dont because: 1- The input force is constant. 2- The resistive force is dependent on the input... That was my logic. @Jemurray3 Generally I thought as a magnet passes through a copper pipe it would have a certain force. That force will equally be generated by the eddy currents. I assumed them to be equal. But what do you think the terminal velocity will be? More or less than 100m/s?

28. Jemurray3

It depends on how strong the magnet is.

29. OneOwOne

Strong enough to pull 15 lb = almost 67N

30. Jemurray3

The resistive force that the magnetic experiences will be proportional to its velocity and to its magnetic moment, or roughly speaking proportional to its velocity and the strength of the magnetic. If you apply a constant force to the magnet as it travels through the copper pipe, there is going to be some terminal velocity where the resistive force is exactly equal to the applied force and there's no acceleration. If the magnet is sufficiently weak, then this terminal velocity will be quite high, so the applied force will accelerate the magnet from 100 m/s to its terminal velocity in an exponential way:|dw:1361508172951:dw| If the magnet is sufficiently strong, then the terminal velocity may be less than 100 m/s, so the applied force will decelerate it in the same way: |dw:1361508299756:dw| In your case, your magnet is definitely strong enough that the terminal velocity will be less than 100 m/s, so the second case will happen. However, its velocity upon leaving the tube will depend on how long the tube is (which should be obvious by now).

31. Jemurray3

Strength of the magnetic field*

32. OneOwOne

@Jemurray3 Wait of the terminal velocity is less, whats the point of applying a constant force of 180N to push the magnet through the pipe? Or what would happen to that force? Your explanation fits perfectly if 180N was applied not "constantly" applied.

33. Jemurray3

What do you mean, what's the point? You said a constant force is being applied to a magnet moving through a pipe. That's what will happen.

34. Vincent-Lyon.Fr

You misunderstood my answer in your previous question. I said that you would reach a terminal velocity and that the magnetic force will balance the operator's force AFTER a transitory phase. So forces are NOT instantly opposite as you seem to assume in your new post. Jemurray3 drew the correct diagram as to how velocity evolves with time. If you do the experiment by dropping a small magnet in a standard copper pipe (12 to 16 mm diameter), you will barely notice the transitory phase: the magnet will move at terminal velocity almost instantly, whether you drop it with or without initial velocity.

35. OneOwOne

@Jemurray3 What I meant, is... If the velocity is eventually going to decrease I thought to myself whats the point of the constant input force. I was trying to imagine what would happen and lost myself. But I guess, the magnet will indeed slow down.

36. OneOwOne

@Vincent-Lyon.Fr When I tested a free fall experiment, the magnet drops 2 - 3 times slower than a normal object similar to its size. Side question: The current is not being drawn out of the coil, so its basically trapped there. What if I connected a load to the copper pipe. Do you think that would reduce the force being applied on the magnet or increase it?

37. OneOwOne

drawn out of the copper pipe* The eddy current are due to the induced currents... Why not direct some of that current elsewhere :)

38. OneOwOne

So the resistive force would be less and the magnet won't lose much of its speed. < Crazy ideas are developing!

39. Visionary01

Perfectly said @Jemurray3 . Based on the diagram alone you'd figure it out. Why didn't I think of this before!

40. gleem

The original problem should have included more information as the length of the magnet, its mass, its magnetic pole strength, and the length of the copper tube. As it is the most you can say is that as the magnet is pushed into the tube on entering the flux increases to induce a current in the tube so as to create a field opposing that change,. the strength of that field is directly related to rate of change of the flux entering which depends on the magnets strength and the velocity of the magnet as it is entering. When the magnet is about half way into the tube the flux becomes constant and the induced current and opposing field go to zero. As it continues to enter the flux changes again this time decreasing until the magnet is totally in the tube ( if the tube is long enough or the magnet short enough for this to occur) where again there is no flux change and no opposing force. the applied field accelerates it to the other end where there is another retarding force , then a brief period of no retarding force followed by a retarding force as it exits. the retarding force only depends on the velocity and magnet strength and not the applied force. I'm wondering if you can consider the retarding force which occurs four time twice entering and twice exiting as an impulse. If you timed the dropped magnet through the tube and timed its fall totally through the tube and see if you can estimate the impulse compare the calculated time to the actual time of the fall with four impulse. You probably nee a slo mo video cam and a stop watch.

41. OneOwOne

@Jemurray3 What if I had the first force pushing the magnet through the pipe stop at a certain point. And apply another force that would push the magnet closer to it. illustration: |dw:1362254921839:dw| Where F1 pushed the magnet through the pipe with 180N at 100 m/s Now half way through the pipe, the force decreased to 0 and the speed too. Then F2 started to pull the magnet closer and closer to 180N Would the velocity increase back to 100M/S in the copper pipe? @Vincent-Lyon.Fr

42. OneOwOne

The right side of the diagram there is F2 and 180 N.

43. Jemurray3

If I understand your question properly, then yes. Here's the general idea -- while moving through the pipe, the magnet experiences a velocity dependent force $F = b v$ where b is some constant that depends on the magnetization and all that stuff. If some force is being applied, then the terminal velocity of the magnet will be $\frac{F_{applied}}{b}$ The magnet will exponentially approach this terminal velocity as I drew in my diagram a while ago. If the force changes, the terminal velocity will change, and the magnet will exponentially approach the new terminal velocity. As a special case, with no applied force, the terminal velocity will be zero.

44. OneOwOne

Basically there is a force of 180 that pushes the magnet, the magnet goes through the pipe, half way through we notice the force drops from 180 and approches zero. Then another force equal to F1 is applied but its near to the direction of the magnet -->, Thus! Increasing the velocity back! To 100m/s I tried this using magnets... Very interesting results. Used a magnet to repel the main magnet through the copper pipe, then used another magnet at the end of the copper pipe. What I noticed was at the beginning the magnet was repelled with strong force, then was at high speeds. Then started to decrease gradually, then added another magnet at the bottom then all of the sudden that magnet increased speed significantly!

45. OneOwOne

Very interesting results indeed.

46. OneOwOne

what do you think @Jemurray3?

47. Jemurray3

The closer the magnets are to one another, the stronger the force, so it shouldn't come as a surprise that adding another magnet increases the speed.

48. OneOwOne

Another thing is... By decreasing the length of the copper wire you can maintain that velocity or something close to that number 90 - 100m/s

49. OneOwOne

@Jemurray3 So by decreasing the distance. Would that help? Lets say the copper pipe is now a coil to draw out power. What is best? To make the distance less?

50. Jemurray3

Best for what? I haven't the slightest clue what you're trying to do or why.