Confusion about the nature of magnetic forces.

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Discussion Overview

The discussion revolves around the nature of magnetic forces, particularly in relation to magnetic dipoles and their interaction with magnetic fields. Participants explore concepts such as torque, net force, and the implications of magnetic field gradients, with references to historical experiments.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions how a bar magnet can be attracted to a refrigerator door if only torque acts on a magnetic dipole in a magnetic field, suggesting that magnetization of the door creates a magnetic field that influences the magnet.
  • Another participant asserts that there is no net force on a magnetic dipole in a uniform magnetic field.
  • A different participant references the Stern Gerlach experiment, noting that it demonstrated a force on neutral atoms in a magnetic field gradient, which led to a Nobel Prize due to the quantized nature of the force.
  • One participant discusses the dipole potential energy in a magnetic field and introduces the concept of force arising from a spatially varying magnetic field, indicating that the gradient of potential energy relates to the force that does work.
  • Another participant counters that the magnetic field in the Stern Gerlach experiment was not uniform, emphasizing that the field's non-uniformity in a certain direction is crucial for the observed forces.

Areas of Agreement / Disagreement

Participants express differing views on the nature of forces acting on magnetic dipoles, with some asserting that only torque is present in uniform fields, while others highlight the significance of field gradients in producing forces. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference specific conditions under which forces act on magnetic dipoles, such as uniform versus non-uniform magnetic fields, and the implications of these conditions on the understanding of magnetic interactions.

siddharth5129
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Seeing as there is no net force on a magnetic dipole placed in a magnetic field ( only torque ) , and that magnetism in say a bar magnet is caused by nothing more than tiny atomic current loops ( magnetic dipoles) oriented in the same direction , i don't understand how a bar magnet would get attracted to say a refrigerator door, as this would require a net force as opposed to a net torque (Am i right in assuming that the bar magnet or refrigerator magnet magnetizes a certain area of the refrigerator door , and this area then sets up a magnetic field which influences the refrigerator magnet)
 
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There is no net force on a magnetic dipole placed in a uniform magnetic field.
 
The Stern Gerlach experiment with a beam of neutral silver atoms in a magnetic field (~1923) with a gradient won the Nobel Prize in physics, because there was a force on the atoms..

[Edit] They won the Nobel Prize because there was a quantized force on the neutral atoms.
Bob S
 
Last edited:
The dipole potential energy U(r) in a magnetic field is -µB. If there is a space-dependence of B, i.e., B(r), then the -grad(U) is the force that makes work.
 
Last edited:
No, magnetic field was only uniform along on plane. It was in fact placed in a way that magnetic field was in fact non uniform in certain direction. Otherwise gradient of field will be zero.
 

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