Question

A magnet with contant force passing through a copper pipe?

Answer 1

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.

Answer 2

@.Sam. @UnkleRhaukus

Answer 3

@countonme123 @Callisto

Answer 4

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

Answer 5

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.

Answer 6

@amistre64 @ghass1978 @ParthKohli

Answer 7

srry, not my area of expertise :)

Answer 8

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

Answer 9

@amistre64 I sent you a clarification message. :)

Answer 10

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.

Answer 11

Fair enough.

Answer 12

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.

Answer 13

@.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.

Answer 14

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.

Answer 15

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

Answer 16

is nobody else answering questions?

Answer 17

The force would increase just to keep the velocity constant.

Answer 18

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?

Answer 19

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

Answer 20

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.

Answer 21

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

Answer 22

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?

Answer 23

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.

Answer 24

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

Answer 25

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

Answer 26

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.

Answer 27

@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?

Answer 28

It depends on how strong the magnet is.

Answer 29

Strong enough to pull 15 lb = almost 67N

Answer 30

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:

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:


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).