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Work/Energy in a Magnetic Field

Homework Statement
A compass needle has a magnetic moment of 𝜇. At its location, the magnitude of the Earth’s magnetic field is 𝑩 northward at 𝜃 below the horizontal. Identify the orientations of the needle that represent (a) the minimum potential energy and (b) the maximum potential energy of the needle–field system. (c) How much work must be done on the system to move the needle from the minimum to the maximum potential energy orientation?
Homework Equations
ΔU = -μBcos(θ)
Sorry if I am asking in the wrong fashion as I am new.

The above questions are easily solvable:
1) U = -μBcos(0)
2) U = -μBcos(180)
3) W = ΔU = 2) - 1)

My question is more related to some theory: where is this work/energy coming from since a magnetic force, to my knowledge can't do "work"?
What force is doing -W (minus W), is it simply the magnetic field, or is there another concrete equation to demonstrate what exactly is happening.

Thank you for anyone that can explain or possibly ask me directional questions
 

Delta2

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It's one of those bizarre situations in physics where a force is responsible for the change in momentum (angular momentum in this case since the needle rotates) but is not responsible for the change in the (rotational) kinetic energy (of the needle). For the latter are responsible internal electrostatic forces that appear inside the needle.
Another example of such a force is the friction between a car's wheels and the road. We all know that a car cannot move without friction, however we all also know that the work isn't done by friction but by the car's engine. So friction is responsible for the change in the momentum of the car, but internal forces at car's engine are responsible for the work done that increases the kinetic energy of the car.
 
Last edited:
It's one of those bizarre situations in physics where a force is responsible for the change in momentum (angular momentum in this case since the needle rotates) but is not responsible for the change in the (rotational) kinetic energy (of the needle). For the latter are responsible internal electrostatic forces that appear inside the needle.
Another example of such a force is the friction between a car's wheels and the road. We all know that a car cannot move without friction, however we all also know that the work isn't done by friction but by the car's engine. So friction is responsible for the change in the momentum of the car, but internal forces at car's engine are responsible for the work done that increases the kinetic energy of the car.
Thank you for taking the time, great answer!
 

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