Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is there a certain limit of force to a magnetic field?

  1. Apr 26, 2013 #1

    Okay, I always assumed when two "magnetic" objects attract/repel there is always a limit to how much force! Each magnetic object applies.

    Say you have a ceramic magnet and a electromagnet.
    You build a classic old motor. Now there is a force between the two, As you increase the (Power) fed to the electromagnet it increases the force between them.

    But the magnet on the other hand is constant. It has a certain value that can not change.

    I noticed that all permanent magnets have a "limit" to the amount of force it can apply to push/pull/flip any magnetic object.

    Each pole of a permanent magnet, has a certain value (x) I call the limit.
    The limit to the amount of FORCE that pole can apply.

    Have I comprehended this correctly?
    There is always a limit value of force a magnet's pole can apply to attract/repel another magnetic object?
  2. jcsd
  3. Apr 26, 2013 #2


    User Avatar
    2015 Award

    Staff: Mentor

    The magnetic field in permanent magnets is limited, and with realistic magnets the force between magnets is limited, too. This is not a fixed value for each magnet, however - it depends on the attracted objects and the environment.
  4. Apr 26, 2013 #3
    Indeed, THUS! The magnetic force generated by that limited field is aslo limited at a constant value.
    I tend to know the rate of the magnetic field of most of my magnets. However, The amount of force the poles apply is unknown... But I do know it depends on the "orientation" of the project. Many factors are involved would determine how high/low that force is applied. But I do know, that a magnet has a limited amount of force it's field can apply Im trying to figure out how much... Funky business!

    I could not figure out this though...
    What do you mean by the force between magnets being limited :O?

    I ment that each magnet has a fixed limit to the amount of force it can apply.
    For example:
    A Neodymium disc magnet attracting a iron nail = force is weak, the effort needed to split them apart is low.

    Now, lets replace that nail with a HUGE plate x 5 times larger than the magnet's dimensions.
    The attraction force (x) is strong! The effort needed increased! (x) can go lower as you decrease the size of the plate but can never be increased as you go x5 + It stay's at that value, and that (x) is the limit of force the field can apply :)

    Stunning thing really!
  5. Apr 27, 2013 #4


    User Avatar
    2015 Award

    Staff: Mentor

    If you have two objects, there is a maximal force you will have between them.

    That is what I meant: The same magnet can lead to different forces, depending on the other object (even if that is not a magnet on its own).
    How do you mean that?
  6. Apr 27, 2013 #5
    The thing is, when two magnetic objects attract/repel...
    The force is due to both of them, I understand that point. But I assume that each object exerts a certain force on the other and vice versa.

    So that amount of force lets say from a magnet. Is limited at a certain value(x)
    However, an electromagnet's value is not limited you can add more power and get a higher force easier. The magnet can't increase its value higher than (x).
    A magnet will have a limit to how much push/pull it can apply to another object always :)

    FOR example:
    A motor has a electromagnet + magnet.
    The magnet is set at the value(x) and can not change.
    However, the electromagnet can increase its value thus the motion is increased! The cause? The electromagnet's force of repulsion/attraction increased and can change and has a limit but HIGHER than the magnet. While the permanent magnet is limited at a value(x) and can not change in the case of the motor.

    Hope that helped :)
  7. Apr 27, 2013 #6
    Indeed, just like having a two stick's hitting each other, if you increase the force they break!
  8. Apr 28, 2013 #7
    Clever example,however not that correct,beacause when you have a two sticks system, you can hit them with any force size you want and as you mentioned there is a case of breaking.On the other side in a two magnets system there is a maximum force that your system can produce...and not bigger
  9. Apr 28, 2013 #8
    Say whaaaaat?!
  10. Apr 28, 2013 #9
    How come no discussion of the distance between the two magnetic objects? It would seem that would be a vital consideration. Both when 'like' poles are set against each other, and when 'opposite' poles are set against each other? In both cases a logarithmic function needs to be elucidated, or am I wrong in this? In the case of repulsion, in magnets that are perfectly aligned, the repulsive force would increase in a curve that would 'peak,' ever higher, as the distance between the two 'like' poles, where reduced. Or, the repulsion would become stronger as the distance decreased.
    Last edited: Apr 28, 2013
  11. Apr 29, 2013 #10
    As you want. What i wanted to say is that the example of the two sticks it is not the appropriate to describe the two magnets case. Because as it was told before there is a maximum force that the system can suffer.And the force depends on the system features.However in the two sticks system the force depents on how strong WE hit them, and there is not any kind of a determination about the force size!Even if the system breaks and this is why the force reason is external.In the two magnets we are not allowed to have bigger force from the one that system can stand.Any different view accepted.
  12. Apr 29, 2013 #11
    Of cousre the strengh depends on the distance <r> of the two magnetic poles and more specifically F=(1/μ)*((P1*P2)/(r^2))

    F is the force size,
    where P1 and P2 are the magnitudes of magnetic poles ,r is the distance and μ is the permeability of the intervening medium.No logarithmic function!If we guess stable all those factors except from <r> Force is a hyperbolic function.
  13. Apr 29, 2013 #12
    F= Km1m2/r^2

    Will not be appropriate if the distance is for example zero...
    The point of this thread is to somewhat bring awareness that if you have a permanent magnet for example
    and a electromagnet that has a greater capability to apply a force.
    A magnet has a certain limit to as how much force it's pole can apply a force lets put a value for example:
    A magnet's pole's(N/S) Has a limit of force = 5N(Newtons, is fit for this example).
    While an electromagnet's pole's(N/S) that is interacting with the magnet has a limit too of = 50N

    Assuming that the distance between them is 0 so the force is at it's MAX.
  14. May 4, 2013 #13
    Couple of comments - the attractive force between two magnets, assuming only the field strengths but not the geometry changes, is proportional to the product of the field strengths. If one is constant (as in a permanent magnet), then the force is linearly dependent on the other's field strength.

    I may be misunderstanding this limit thing, but one of the limits for a permanent magnet is associated with its remagnetisation levels and hysteresis - under high enough external fields a permanent magnet will be remagnetised to align with the external fields. This is how they are magnetised in the first place. This can be quite subtle if the alignment reinforces the existing magnetisation, when it can be just a few percent. I am not familiar with any other limit.
  15. May 4, 2013 #14
    Knowing that you would have one permeant magnet simply means there is a certain amount of force it's poles can apply. That limit can't be exceeded.
    But you're right, exposing the magnet to a strong magnetic field would mean that it could change the magnetization of the magnet.

    An electromagnet also has a limit. But by changing the flow of current you can control that amount of "force" the poles can apply.
    With a permanent magnet, its more complicated to "control" that force. By changing the distance, or the contact area the force of a magnet's pole could be controlled. But very very very complicated business here!
  16. May 4, 2013 #15


    User Avatar

  17. May 5, 2013 #16
  18. May 5, 2013 #17
    I think the limit on field strength is a bit arbitrary. As the current through the coils of an electromagnet increases the relative permeability of the core drops. You can either choose that as a limit or keep increasing the current, when the magnet's field strength will continue to increase but more slowly. The only query is how hot the windings get - for pulsed use or with superconductive coils the thermal limits here are as lot higher than for steady state use.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted
Similar Discussions: Is there a certain limit of force to a magnetic field?
  1. Magnetic field force (Replies: 2)